Cleanup & backportclinit

The phase rework, made new phases explicit and add them to the CodeGen driver.  Fix the flag-spilling on X86 and ARM.  Fix the GCM conservative anti-deps and backport.

47 files changed, 832 insertions, 599 deletions

diff --git a/chapter21/README.md b/chapter21/README.md
index 80304c1a..a3ae6cb8 100644
--- a/chapter21/README.md
+++ b/chapter21/README.md
@@ -17,6 +17,18 @@ in the linear revision history and
 
 Here is the [complete language grammar](docs/21-grammar.md) for this chapter.
 
+Two phases extend the pipeline: `Encoding` turns allocated nodes into bytes,
+and `Export` resolves relocations in memory or writes an ELF object. For example:
+
+```java
+code.driver(CodeGen.Phase.Encoding, "x86_64_v2", "SystemV");
+code.exportELF("program.o");
+```
+
+The CLI uses the same driver. `--dump-after-iter`, the other phase dump options,
+and `--dump-time` expose the stages without duplicating their ordering in the
+command-line frontend.
+
 ## From machine nodes to bytes
 
 Instruction selection chooses the operation and register constraints. Register
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/codegen/CodeGen.java b/chapter21/src/main/java/com/seaofnodes/simple/codegen/CodeGen.java
index 2c44cb05..95e30d12 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/codegen/CodeGen.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/codegen/CodeGen.java
@@ -9,7 +9,6 @@ import com.seaofnodes.simple.type.*;
 import java.io.IOException;
 import java.util.BitSet;
 import java.util.HashMap;
-import java.util.IdentityHashMap;
 
 @SuppressWarnings("unchecked")
 public class CodeGen {
@@ -20,19 +19,61 @@ public class CodeGen {
     public static CodeGen CODE;
 
     public enum Phase {
-        Parse,                  // Parse ASCII text into Sea-of-Nodes IR
-        Opto,                   // Run ideal optimizations
-        TypeCheck,              // Last check for bad programs
-        LoopTree,               // Build a loop tree; break infinite loops
-        InstSelect,             // Convert to target hardware nodes
-        Schedule,               // Global schedule (code motion) nodes
-        LocalSched,             // Local schedule
-        RegAlloc,               // Register allocation
-        Encoding,               // Encoding
-        Export,                 // Export
+        Parse,                  // Build ideal IR from source
+        Iter,                   // Pessimistic peepholes
+        TypeCheck,              // Check the surviving ideal graph
+        LoopTree,               // Discover loops and connect nonreturning paths
+        InstSelect,             // Select machine instructions
+        Schedule,               // Global code motion
+        LocalSched,             // Order instructions within blocks
+        RegAlloc,               // Assign registers and stack slots
+        Encoding,               // Encode instructions
+        Export;                 // Link in memory or write ELF
+
     }
     public Phase _phase;
 
+    // Run through target, skipping phases already completed.
+    public CodeGen driver(Phase target) { return driver(target,null,null); }
+    public CodeGen driver(Phase target, String cpu, String callingConv) { return driver(target,cpu,callingConv,0); }
+    public CodeGen driver(Phase target, String cpu, String callingConv, int dump) {
+        if( target.ordinal()>=Phase.RegAlloc.ordinal() && cpu==null && _mach==null )
+            throw new IllegalArgumentException("Machine code requires a CPU target");
+        if( target==Phase.Export ) throw new IllegalArgumentException("Use exportELF with an output file or null for memory linking");
+        step(target, Phase.Parse, this::parse, dump);
+        step(target, Phase.Iter, this::iter, dump);
+        step(target, Phase.TypeCheck, this::typeCheck, dump);
+        step(target, Phase.LoopTree, this::loopTree, dump);
+        step(target, Phase.InstSelect, () -> instSelect(cpu,callingConv), dump);
+        step(target, Phase.Schedule, this::GCM, dump);
+        step(target, Phase.LocalSched, this::localSched, dump);
+        step(target, Phase.RegAlloc, this::regAlloc, dump);
+        step(target, Phase.Encoding, this::encode, dump);
+        return this;
+    }
+
+    private void step(Phase target, Phase next, Runnable work, int dump) {
+        if( next.ordinal()>target.ordinal() || (_phase!=null && _phase.ordinal()>=next.ordinal()) ) return;
+        Phase prior = _phase;
+        work.run();
+        if( _phase!=prior && (dump & (1<<_phase.ordinal()))!=0 ) {
+            if( (dump & (1<<30))!=0 ) System.err.println("After "+_phase+":");
+            System.err.println(this);
+        }
+    }
+
+    public final long[] _times = new long[Phase.values().length];
+    private CodeGen phase(Phase next, boolean ordered, Runnable work) {
+        assert ordered : "Cannot run "+next+" after "+_phase;
+        _phase = next;
+        long t0 = System.currentTimeMillis();
+        try { work.run(); }
+        finally { _times[next.ordinal()] = System.currentTimeMillis()-t0; }
+        if( _obs!=null ) _obs.phase(next.name());
+        return this;
+    }
+
+
     // ---------------------------
     // Compilation source code
     public final String _src;
@@ -53,21 +94,6 @@ public class CodeGen {
     }
 
 
-    // All passes up to Phase, except ELF
-    public CodeGen driver( Phase phase ) { return driver(phase,null,null); }
-    public CodeGen driver( Phase phase, String cpu, String callingConv ) {
-        if( _phase==null )                       parse();
-        if( _phase.ordinal() < phase.ordinal() ) opto();
-        if( _phase.ordinal() < phase.ordinal() ) typeCheck();
-        if( _phase.ordinal() < phase.ordinal() ) loopTree();
-        if( _phase.ordinal() < phase.ordinal() && cpu != null ) instSelect(cpu,callingConv);
-        if( _phase.ordinal() < phase.ordinal() ) GCM();
-        if( _phase.ordinal() < phase.ordinal() ) localSched();
-        if( _phase.ordinal() < phase.ordinal() ) regAlloc();
-        if( _phase.ordinal() < phase.ordinal() ) encode();
-        return this;
-    }
-
     // Run all the phases through final ELF emission
     public CodeGen driver( String cpu, String callingConv, String obj ) throws IOException {
         return driver(Phase.Encoding,cpu,callingConv).exportELF(obj);
@@ -143,16 +169,10 @@ public class CodeGen {
     public final Parser P;
 
     // Parse ASCII text into Sea-of-Nodes IR
-    public int _tParse;
     public CodeGen parse() {
-        assert _phase == null;
-        _phase = Phase.Parse;
-        long t0 = System.currentTimeMillis();
-
-        P.parse();
-        _tParse = (int)(System.currentTimeMillis() - t0);
-        if( _obs != null ) _obs.phase("Parse");
-        return this;
+        return phase(Phase.Parse, _phase==null, () -> {
+            P.parse();
+        });
     }
 
 
@@ -167,61 +187,23 @@ public class CodeGen {
     public void iterCnt() { if( !_midAssert ) _iter_cnt++; }
     public void iterNop() { if( !_midAssert ) _iter_nop_cnt++; }
 
-    // Run ideal optimizations
-    public int _tOpto;
-    public CodeGen opto() {
-        assert _phase == Phase.Parse;
-        _phase = Phase.Opto;
-        long t0 = System.currentTimeMillis();
-
-        // Pessimistic peephole optimization on a worklist
-        _iter.iterate(this);
-        _tOpto = (int)(System.currentTimeMillis() - t0);
-
-        // TODO:
-        // Optimistic
-        // TODO:
-        // loop unroll, peel, RCE, etc
-        if( _obs != null ) _obs.phase("Opto");
-        return this;
-    }
+    // Pessimistic peepholes reach a fixed point after parsing.
+    public CodeGen iter() { return phase(Phase.Iter, _phase==Phase.Parse, () -> _iter.iterate(this)); }
     public <N extends Node> N add( N n ) { return (N)_iter.add(n); }
     public void addAll( Ary<Node> ary ) { _iter.addAll(ary); }
 
 
     // ---------------------------
     // Last check for bad programs
-    public int _tTypeCheck;
-    public CodeGen typeCheck() {
-        // Demand phase Opto for cleaning up dead control flow at least,
-        // required for the following GCM.
-        assert _phase.ordinal() <= Phase.Opto.ordinal();
-        _phase = Phase.TypeCheck;
-        long t0 = System.currentTimeMillis();
-
-        Parser.ParseException err = _stop.walk(n -> {
-            Parser.ParseException msg = n.err();
-            if( msg != null && _obs != null ) _obs.error(n, msg.getMessage());
-            return msg;
-        });
-        _tTypeCheck = (int)(System.currentTimeMillis() - t0);
-        if( err != null )
-            throw err;
-        if( _obs != null ) _obs.phase("TypeCheck");
-        return this;
-    }
+    public CodeGen typeCheck() { return phase(Phase.TypeCheck, _phase==Phase.Iter, () -> TypeCheck.run(this)); }
 
     // ---------------------------
     // Build the loop tree; break never-exit loops
-    public int _tLoopTree;
     public CodeGen loopTree() {
-        assert _phase.ordinal() <= Phase.TypeCheck.ordinal();
-        _phase = Phase.LoopTree;
-        long t0 = System.currentTimeMillis();
-        // Build the loop tree, fix never-exit loops
-        _start.buildLoopTree(_stop);
-        _tLoopTree = (int)(System.currentTimeMillis() - t0);
-        return this;
+        return phase(Phase.LoopTree, _phase==Phase.TypeCheck, () -> {
+            // Build the loop tree, fix never-exit loops
+            _start.buildLoopTree(_stop);
+        });
     }
 
     // ---------------------------
@@ -243,138 +225,41 @@ public class CodeGen {
     public RegMask _rpcMask;
 
     // Convert to target hardware nodes
-    public int _tInsSel;
     public CodeGen instSelect( String cpu, String callingConv ) { return instSelect(cpu,callingConv,PORTS); }
     public CodeGen instSelect( String cpu, String callingConv, String base ) {
-        assert _phase.ordinal() == Phase.LoopTree.ordinal();
-        _phase = Phase.InstSelect;
-
-        _callingConv = callingConv;
-
-        // Not timing the class load...
-        // Look for CPU in fixed named place:
-        //   com.seaofnodes.simple.node.cpus."cpu"."cpu.class"
-        String clzFile = base+"."+cpu+"."+cpu;
-        try { _mach = ((Class<Machine>) Class.forName( clzFile )).getDeclaredConstructor(new Class[]{CodeGen.class}).newInstance(this); }
-        catch( Exception e ) { throw new RuntimeException(e); }
-
-        // Build global copies of common register masks.
-        long callerSave = _mach.callerSave();
-        long  neverSave = _mach. neverSave();
-        int maxReg = Math.min(64,_mach.regs().length);
-        assert maxReg>=64 || (-1L << maxReg & callerSave)==0; // No stack slots in callerSave
-        _callerSave = new RegMask(callerSave);
-
-        // Build a Return RegMask array.  All returns have the following inputs:
-        // 0 - ctrl
-        // 1 - memory
-        // 2 - varies with returning GPR,FPR
-        // 3 - RPC
-        // 4+- Caller save registers
-        _retMasks = new RegMask[(maxReg-_callerSave.size()-Long.bitCount( neverSave ))+4];
-        for( int reg=0, i=4; reg<maxReg; reg++ )
-            if( !_callerSave.test(reg) && ((1L<<reg)&neverSave)==0 )
-                _retMasks[i++] = new RegMask(reg);
-        _rpcMask = new RegMask(_mach.rpc());
-        _retMasks[3] = _rpcMask;
-
-
-        // Convert to machine ops
-        long t0 = System.currentTimeMillis();
-        _uid = 1;               // All new machine nodes reset numbering
-        var map = new IdentityHashMap<Node,Node>();
-        _instSelect( _stop, map );
-        _stop  = ( StopNode)map.get(_stop );
-        _start = (StartNode)map.get(_start);
-        _instOuts(_stop,visit());
-        _visit.clear();
-        _tInsSel = (int)(System.currentTimeMillis() - t0);
-        return this;
-    }
-
-    // Walk all ideal nodes, recursively mapping ideal to machine nodes, then
-    // make a machine node for "this".
-    private Node _instSelect( Node n, IdentityHashMap<Node,Node> map ) {
-        if( n==null ) return null;
-        Node x = map.get(n);
-        if( x !=null ) return x; // Been there, done that
-
-        // If n is a MachNode already, then its part of a multi-node expansion.
-        // It does not need instruction selection (already selected!)
-        // but it does need its inputs walked.
-        if( n instanceof MachNode ) {
-            for( int i=0; i < n.nIns(); i++ )
-                n._inputs.set(i, _instSelect(n.in(i),map) );
-            return n;
-        }
-
-        // Produce a machine node from n; map it to flag as done so stops cycles.
-        map.put(n, x=_mach.instSelect(n) );
-        // Walk machine op and replace inputs with mapped inputs
-        for( int i=0; i < x.nIns(); i++ )
-            x._inputs.set(i, _instSelect(x.in(i),map) );
-        if( x instanceof MachNode mach )
-            mach.postSelect(this);  // Post selection action
-
-        // Updates forward edges only.
-        n._outputs.clear();
-        return x;
-    }
-
-    // Walk all machine Nodes, and set their output edges
-    private void _instOuts( Node n, BitSet visit ) {
-        if( visit.get(n._nid) ) return;
-        visit.set(n._nid);
-        for( Node in : n._inputs )
-            if( in!=null ) {
-                in._outputs.push(n);
-                _instOuts(in,visit);
-            }
+        if( cpu==null ) return this;
+        return phase(Phase.InstSelect, _phase==Phase.LoopTree,
+                     () -> new InstSelect(this).run(cpu,callingConv,base));
     }
 
 
     // ---------------------------
     // Control Flow Graph in Reverse Post Order.
-    public int _tGCM;
     public Ary<CFGNode> _cfg = new Ary<>(CFGNode.class);
 
     // Global schedule (code motion) nodes
     public CodeGen GCM() {
-        assert _phase.ordinal() <= Phase.InstSelect.ordinal();
-        _phase = Phase.Schedule;
-        long t0 = System.currentTimeMillis();
-
-        GlobalCodeMotion.buildCFG(this);
-        _tGCM = (int)(System.currentTimeMillis() - t0);
+        return phase(Phase.Schedule, (_phase==Phase.LoopTree || _phase==Phase.InstSelect), () -> {
+            GlobalCodeMotion.buildCFG(this);
 
-        return this;
+        });
     }
 
     // ---------------------------
     // Local (basic block) scheduler phase, a classic list scheduler
-    public int _tLocal;
     public CodeGen localSched() {
-        assert _phase == Phase.Schedule;
-        _phase = Phase.LocalSched;
-        long t0 = System.currentTimeMillis();
-        ListScheduler.sched(this);
-        _tLocal = (int)(System.currentTimeMillis() - t0);
-        return this;
-     }
+        return phase(Phase.LocalSched, _phase==Phase.Schedule, () -> ListScheduler.sched(this));
+    }
 
 
     // ---------------------------
     // Register Allocation
-    public int _tRegAlloc;
     public RegAlloc _regAlloc;
     public CodeGen regAlloc() {
-        assert _phase == Phase.LocalSched;
-        _phase = Phase.RegAlloc;
-        long t0 = System.currentTimeMillis();
-        _regAlloc = new RegAlloc(this);
-        _regAlloc.regAlloc();
-        _tRegAlloc = (int)(System.currentTimeMillis() - t0);
-        return this;
+        return phase(Phase.RegAlloc, _phase==Phase.LocalSched, () -> {
+            _regAlloc = new RegAlloc(this);
+            _regAlloc.regAlloc();
+        });
     }
 
     // Human readable register name
@@ -390,27 +275,27 @@ public class CodeGen {
 
     // ---------------------------
     // Encoding
-    public int _tEncode;
     public Encoding _encoding;   // Encoding object
     public void preEncode() {  } // overridden by alternative ports
     public CodeGen encode() {
-        assert _phase == Phase.RegAlloc;
-        _phase = Phase.Encoding;
-        long t0 = System.currentTimeMillis();
-        _encoding = new Encoding(this);
-        preEncode();
-        _encoding.encode();
-        _tEncode = (int)(System.currentTimeMillis() - t0);
-        return this;
+        return phase(Phase.Encoding, _phase==Phase.RegAlloc, () -> {
+            _encoding = new Encoding(this);
+            preEncode();
+            _encoding.encode();
+        });
     }
 
     // ---------------------------
     // Exporting to external formats
     public CodeGen exportELF(String fname) throws IOException {
-        assert _phase == Phase.Encoding;
-        _phase = Phase.Export;
-        if( fname == null ) new LinkMem(this).link(); // In memory patching
-        else new ElfFile(this).export(fname); // External ELF file
+        assert _phase==Phase.Encoding;
+        _phase=Phase.Export;
+        long t0=System.currentTimeMillis();
+        try {
+            if( fname == null ) new LinkMem(this).link(); // In memory patching
+            else new ElfFile(this).export(fname); // External ELF file
+        } finally { _times[Phase.Export.ordinal()]=System.currentTimeMillis()-t0; }
+        if( _obs!=null ) _obs.phase(Phase.Export.name());
         return this;
     }
 
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/codegen/GlobalCodeMotion.java b/chapter21/src/main/java/com/seaofnodes/simple/codegen/GlobalCodeMotion.java
index 447a6df1..d4db558a 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/codegen/GlobalCodeMotion.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/codegen/GlobalCodeMotion.java
@@ -274,10 +274,11 @@ public abstract class GlobalCodeMotion {
                 lca = anti_dep(load,late[st._nid],st.cfg0(),lca,st,anti);
                 break;
             case PhiNode phi:
-                // Repeat anti-dep for matching Phi inputs.
-                // No anti-dep edges but may raise the LCA.
+                // Inputs carrying different memory might overwrite this alias.
+                // Raise the LCA for those paths; unchanged memory does not
+                // clobber the load. No scheduling edge to the Phi.
                 for( int i=1; i<phi.nIns(); i++ )
-                    if( phi.in(i)==load.mem() )
+                    if( phi.in(i)!=load.mem() )
                         lca = anti_dep(load,phi.region().cfg(i),load.mem().cfg0(),lca,null,anti);
                 break;
             case NewNode st:
@@ -294,7 +295,9 @@ public abstract class GlobalCodeMotion {
     private static CFGNode anti_dep( MemOpNode load, CFGNode stblk, CFGNode defblk, CFGNode lca, Node st, int[] anti ) {
         // A conditional writer can precede the load without dominating its late block.
         // Walk back to the first overlap with the load's placement range.
-        for( ; stblk != defblk.idom(); stblk = stblk.idom() ) {
+        // Another Phi input need not be dominated by this memory definition.
+        // Include its depth, and retain the sentinel for an entry's null idom.
+        for( ; stblk != defblk.idom() && stblk.idepth() >= defblk.idepth(); stblk = stblk.idom() ) {
             // Store and Load overlap, need anti-dependence
             if( anti[stblk._nid]==load._nid ) {
                 lca = stblk.domLCA(lca,null); // Raise Loads LCA
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/codegen/InstSelect.java b/chapter21/src/main/java/com/seaofnodes/simple/codegen/InstSelect.java
new file mode 100644
index 00000000..4d67be4b
--- /dev/null
+++ b/chapter21/src/main/java/com/seaofnodes/simple/codegen/InstSelect.java
@@ -0,0 +1,90 @@
+package com.seaofnodes.simple.codegen;
+
+import com.seaofnodes.simple.node.*;
+import java.util.BitSet;
+import java.util.IdentityHashMap;
+
+// Replace ideal nodes with machine nodes, then rebuild the copied graph's uses.
+class InstSelect {
+    private final CodeGen _code;
+    InstSelect(CodeGen code) { _code=code; }
+
+    @SuppressWarnings("unchecked")
+    void run(String cpu, String callingConv, String base) {
+        _code._callingConv = callingConv;
+
+        // Look for CPU in fixed named place:
+        //   com.seaofnodes.simple.node.cpus."cpu"."cpu.class"
+        String clzFile = base+"."+cpu+"."+cpu;
+        try { _code._mach = ((Class<Machine>) Class.forName( clzFile )).getDeclaredConstructor(new Class[]{CodeGen.class}).newInstance(_code); }
+        catch( Exception e ) { throw new RuntimeException(e); }
+
+        // Build global copies of common register masks.
+        long callerSave = _code._mach.callerSave();
+        long  neverSave = _code._mach. neverSave();
+        int maxReg = Math.min(64,_code._mach.regs().length);
+        assert maxReg>=64 || (-1L << maxReg & callerSave)==0; // No stack slots in callerSave
+        _code._callerSave = new RegMask(callerSave);
+
+        // Build a Return RegMask array.  All returns have the following inputs:
+        // 0 - ctrl
+        // 1 - memory
+        // 2 - varies with returning GPR,FPR
+        // 3 - RPC
+        // 4+- Caller save registers
+        _code._retMasks = new RegMask[(maxReg-_code._callerSave.size()-Long.bitCount( neverSave ))+4];
+        for( int reg=0, i=4; reg<maxReg; reg++ )
+            if( !_code._callerSave.test(reg) && ((1L<<reg)&neverSave)==0 )
+                _code._retMasks[i++] = new RegMask(reg);
+        _code._rpcMask = new RegMask(_code._mach.rpc());
+        _code._retMasks[3] = _code._rpcMask;
+
+
+        // Convert to machine ops
+        _code._uid = 1;               // All new machine nodes reset numbering
+        var map = new IdentityHashMap<Node,Node>();
+        _instSelect( _code._stop, map );
+        _code._stop  = ( StopNode)map.get(_code._stop );
+        _code._start = (StartNode)map.get(_code._start);
+        _instOuts(_code._stop,_code.visit());
+        _code._visit.clear();
+    }
+
+    private Node _instSelect( Node n, IdentityHashMap<Node,Node> map ) {
+        if( n==null ) return null;
+        Node x = map.get(n);
+        if( x !=null ) return x; // Been there, done that
+
+        // If n is a MachNode already, then its part of a multi-node expansion.
+        // It does not need instruction selection (already selected!)
+        // but it does need its inputs walked.
+        if( n instanceof MachNode ) {
+            for( int i=0; i < n.nIns(); i++ )
+                n._inputs.set(i, _instSelect(n.in(i),map) );
+            return n;
+        }
+
+        // Produce a machine node from n; map it to flag as done so stops cycles.
+        map.put(n, x=_code._mach.instSelect(n) );
+        // Walk machine op and replace inputs with mapped inputs
+        for( int i=0; i < x.nIns(); i++ )
+            x._inputs.set(i, _instSelect(x.in(i),map) );
+        if( x instanceof MachNode mach )
+            mach.postSelect(_code);  // Post selection action
+
+        // Updates forward edges only.
+        n._outputs.clear();
+        return x;
+    }
+
+    private void _instOuts( Node n, BitSet visit ) {
+        if( visit.get(n._nid) ) return;
+        visit.set(n._nid);
+        for( Node in : n._inputs )
+            if( in!=null ) {
+                in._outputs.push(n);
+                _instOuts(in,visit);
+            }
+    }
+
+}
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/codegen/LRG.java b/chapter21/src/main/java/com/seaofnodes/simple/codegen/LRG.java
index ec254a2e..6ece27a2 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/codegen/LRG.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/codegen/LRG.java
@@ -59,6 +59,8 @@ public class LRG {
 
     public int nadj() { return _adj==null ? 0 : _adj._len; }
 
+    public RegMask useMask() { return _machUse==null ? null : _machUse.regmap(_uidx); }
+
     void addNeighbor(LRG lrg) {
         if( _adj==null ) _adj = new Ary<>(LRG.class);
         _adj.push(lrg);
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java b/chapter21/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
index dd3d2acb..85b3d283 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
@@ -361,7 +361,7 @@ public class RegAlloc {
         // If the minLoopDepth is less than the maxLoopDepth: for-all defs and
         // uses, if at minLoopDepth or lower, split after def and before use.
         for( Node n : _ns ) {
-            if( n instanceof SplitNode ) continue; // Ignoring splits; since spilling need to split in a deeper loop
+            if( n instanceof SplitNode split && !split.isNonStackSplit() ) continue; // Ordinary splits already reach stack storage.
             if( n.isDead() ) continue; // Some Clonable went dead by other spill changes
             // If this is a 2-address commutable op (e.g. AddX86, MulX86) and the rhs has only a single user,
             // commute the inputs... which chops the LHS live ranges' upper bound to just the RHS.
@@ -414,8 +414,8 @@ public class RegAlloc {
     }
 
     private static long ldepth( long ld, Node n, CFGNode cfg ) {
-        // Do not count splits
-        if( n instanceof SplitNode ) return ld;
+        // Ordinary splits already reach stack storage; register-only transfers do not.
+        if( n instanceof SplitNode split && !split.isNonStackSplit() ) return ld;
         // Collect min/max loop depth
         int min = (int)ld;
         int max = (int)(ld>>32);
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/codegen/TypeCheck.java b/chapter21/src/main/java/com/seaofnodes/simple/codegen/TypeCheck.java
new file mode 100644
index 00000000..0997fd2f
--- /dev/null
+++ b/chapter21/src/main/java/com/seaofnodes/simple/codegen/TypeCheck.java
@@ -0,0 +1,15 @@
+package com.seaofnodes.simple.codegen;
+
+import com.seaofnodes.simple.Parser;
+
+abstract class TypeCheck {
+    static void run(CodeGen code) {
+        Parser.ParseException err = code._stop.walk(n -> {
+            Parser.ParseException msg = n.err();
+            if( msg != null && code._obs != null ) code._obs.error(n, msg.getMessage());
+            return msg;
+        });
+        if( err != null )
+            throw err;
+    }
+}
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/SplitNode.java b/chapter21/src/main/java/com/seaofnodes/simple/node/SplitNode.java
index ff18b292..437a700a 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/node/SplitNode.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/SplitNode.java
@@ -10,6 +10,8 @@ public abstract class SplitNode extends MachConcreteNode {
     public final String _kind;  // Kind of split
     public final byte _round;
     public SplitNode(String kind, byte round, Node[] nodes) { super(nodes); _kind = kind; _round = round; }
+    // A register-only transfer may need another split to reach stack storage.
+    public boolean isNonStackSplit() { return false; }
     @Override public String op() { return "mov"; }
     @Override protected ExprPrinter<Node> _print1(ExprPrinter<Node> p) {
         p.p("mov(");
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/SplitFlagsARM.java b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/SplitFlagsARM.java
new file mode 100644
index 00000000..3e045a6f
--- /dev/null
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/SplitFlagsARM.java
@@ -0,0 +1,21 @@
+package com.seaofnodes.simple.node.cpus.arm;
+
+import com.seaofnodes.isa.Arm64;
+import com.seaofnodes.simple.codegen.*;
+
+// NZCV transfers require a GPR. Further splits can spill that GPR to the stack.
+public class SplitFlagsARM extends SplitARM {
+    private static final RegMask MASK = new RegMask(arm.WR_BITS,1L);
+    SplitFlagsARM(String kind,byte round) { super(kind,round); }
+    @Override public boolean isNonStackSplit() { return true; }
+    @Override public RegMask regmap(int i) { return MASK; }
+    @Override public RegMask outregmap() { return MASK; }
+
+    @Override public void encoding(Encoding enc) {
+        short dst=enc.reg(this), src=enc.reg(in(1));
+        if( dst==src ) return;
+        if( src==arm.FLAGS ) enc.add4(Arm64.flags(dst,false));
+        else if( dst==arm.FLAGS ) enc.add4(Arm64.flags(src,true));
+        else super.encoding(enc);
+    }
+}
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/SplitGPRARM.java b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/SplitGPRARM.java
new file mode 100644
index 00000000..46eff4ca
--- /dev/null
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/SplitGPRARM.java
@@ -0,0 +1,11 @@
+package com.seaofnodes.simple.node.cpus.arm;
+
+import com.seaofnodes.simple.codegen.*;
+
+// Keep a saved NZCV value in GPRs and stack slots through subsequent spills.
+public class SplitGPRARM extends SplitARM {
+    private static final RegMask MASK = new RegMask(arm.WR_BITS,-2L);
+    SplitGPRARM(String kind,byte round) { super(kind,round); }
+    @Override public RegMask regmap(int i) { return MASK; }
+    @Override public RegMask outregmap() { return MASK; }
+}
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/arm.java b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/arm.java
index 66a59fde..9910795d 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/arm.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/arm.java
@@ -267,7 +267,16 @@ public class arm extends Machine {
     @Override public int rpc() { return X30; }
 
     // Create a split op; any register to any register, including stack slots
-    @Override public SplitNode split(String kind, byte round, LRG lrg) {  return new SplitARM(kind,round);  }
+    @Override public SplitNode split(String kind, byte round, LRG lrg) {
+        RegMask def = lrg._machDef==null ? null : lrg._machDef.outregmap(), use = lrg.useMask();
+        if( (def!=null && def.size1() && def.test(FLAGS)) ||
+            (use!=null && use.size1() && use.test(FLAGS)) )
+            return new SplitFlagsARM(kind,round);
+        // Spilling the saved flags requires a GPR between NZCV and the stack.
+        if( lrg._machDef instanceof SplitFlagsARM || lrg._machDef instanceof SplitGPRARM )
+            return new SplitGPRARM(kind,round);
+        return new SplitARM(kind,round);
+    }
 
     // Return a MachNode unconditional branch
     @Override public CFGNode jump() {
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/SplitFlagsX86.java b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/SplitFlagsX86.java
new file mode 100644
index 00000000..4650b535
--- /dev/null
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/SplitFlagsX86.java
@@ -0,0 +1,25 @@
+package com.seaofnodes.simple.node.cpus.x86_64_v2;
+
+import com.seaofnodes.isa.X86;
+import com.seaofnodes.simple.codegen.*;
+
+// Flag snapshots can use GPRs or stack slots, but never XMM registers.
+public class SplitFlagsX86 extends SplitX86 {
+    private static final RegMask MASK = new RegMask(x86_64_v2.WR_BITS | (1L<<x86_64_v2.FLAGS) | x86_64_v2.SPILLS,-1L);
+    SplitFlagsX86(String kind,byte round) { super(kind,round); }
+    @Override public RegMask regmap(int i) { return MASK; }
+    @Override public RegMask outregmap() { return MASK; }
+
+    @Override public void encoding(Encoding enc) {
+        short dst=enc.reg(this), src=enc.reg(in(1));
+        if( dst==src ) return;
+        if( dst==x86_64_v2.FLAGS || src==x86_64_v2.FLAGS ) {
+            boolean toFlags = dst==x86_64_v2.FLAGS;
+            int reg = toFlags ? src : dst;
+            if( reg>=x86_64_v2.MAX_REG )
+                X86.flagsStack(enc,enc._fun.computeStackOffset(enc._code,reg),toFlags);
+            else
+                X86.flags(enc,reg,toFlags);
+        } else super.encoding(enc);
+    }
+}
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/SplitX86.java b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/SplitX86.java
index 5cf668d1..3c4dde17 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/SplitX86.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/SplitX86.java
@@ -17,12 +17,10 @@ public class SplitX86 extends SplitNode {
     @Override public RegMask regmap(int i) { return x86_64_v2.SPLIT_MASK; }
     @Override public RegMask outregmap() { return x86_64_v2.SPLIT_MASK; }
 
-    // Need to handle 8 cases: . reg->reg, reg->xmm, reg->flags, xmm->reg, xmm->xmm, xmm->flags, flags->reg, flags->xmm,
-    // flags->flags.
+    // Ordinary GPR, XMM, and stack moves. Flags use SplitFlagsX86.
     @Override public void encoding( Encoding enc ) {
         short dst=enc.reg(this), src=enc.reg(in(1));
-        if( dst==x86_64_v2.FLAGS ) { X86.flags(enc,src,true); return; }
-        if( src==x86_64_v2.FLAGS ) { X86.flags(enc,dst,false); return; }
+        assert dst!=x86_64_v2.FLAGS && src!=x86_64_v2.FLAGS;
         boolean dstX=dst>=x86_64_v2.XMM_OFFSET, srcX=src>=x86_64_v2.XMM_OFFSET;
         // Stack spills
         if( dst >= x86_64_v2.MAX_REG ) {
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/x86_64_v2.java b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/x86_64_v2.java
index 1b7cf775..1dc47188 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/x86_64_v2.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/x86_64_v2/x86_64_v2.java
@@ -45,7 +45,7 @@ public class x86_64_v2 extends Machine {
     static RegMask RPC_MASK = new RegMask(RPC);
 
     static final long SPILLS = -(1L << MAX_REG);
-    static final RegMask SPLIT_MASK = new RegMask(WR_BITS | FP_BITS /* | (1L<<FLAGS)*/ | SPILLS, -1L );
+    static final RegMask SPLIT_MASK = new RegMask(WR_BITS | FP_BITS | SPILLS, -1L );
 
     // Load/store mask; both GPR and FPR
     static RegMask MEM_MASK = new RegMask(WR_BITS | FP_BITS);
@@ -235,7 +235,9 @@ public class x86_64_v2 extends Machine {
 
     // Create a split op; any register to any register, including stack slots
     @Override public SplitNode split(String kind, byte round, LRG lrg) {
-        return new SplitX86(kind, round);
+        return lrg._machDef!=null && lrg._machDef.outregmap().test(FLAGS)
+            ? new SplitFlagsX86(kind,round)
+            : new SplitX86(kind,round);
     }
 
     // Return a MachNode unconditional branch
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java b/chapter21/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
index d8d0e216..99b3d190 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
@@ -17,7 +17,7 @@ public class SimpleGraphObserver extends GraphCapture<Node> {
     @Override protected void compile(String src) {
         _code = new CodeGen(src);
         _code._obs = this;
-        _code.parse().opto().typeCheck();
+        _code.driver(CodeGen.Phase.TypeCheck);
     }
     @Override protected String phase() { return _code._phase.name(); }
     @Override protected int pos() { return _code._phase == CodeGen.Phase.Parse ? _code.P.pos() : -1; }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/BrainFuckTest.java b/chapter21/src/test/java/com/seaofnodes/simple/BrainFuckTest.java
index aa6fada0..97d33b70 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/BrainFuckTest.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/BrainFuckTest.java
@@ -20,7 +20,7 @@ public class BrainFuckTest {
         String brain_fuck = "Hello World!\n";
         TestC.run("brain_fuck", brain_fuck, 40);
 
-        EvalRisc5 R5 = TestRisc5.build("brain_fuck", 0, 146, false);
+        EvalRisc5 R5 = TestRisc5.build("brain_fuck", 0, 42, false);
         int trap = R5.step(100000);
         assertEquals(0,trap);
         int ptr = (int)R5.regs[riscv.A0];
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter05Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter05Test.java
index 6ff4ac95..c809f72f 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter05Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter05Test.java
@@ -20,7 +20,7 @@ else {
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,(arg+2),(arg-3));", code.print());
     }
 
@@ -36,7 +36,7 @@ if (arg == 1) {
 }
 return c;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,4,3);", code.print());
     }
 
@@ -64,7 +64,7 @@ if( arg==1 )
 else
     b=a+1;
 return a+b;""");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return ((arg<<1)+Phi(Region,2,3));", code.print());
     }
 
@@ -79,7 +79,7 @@ if( arg==1 )
 else
     a=b+1;
 return a+b;""");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return ((Phi(Region,(arg<<1),arg)+arg)+Phi(Region,4,5));", code.print());
     }
 
@@ -99,7 +99,7 @@ else
     a=5;
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,3,5,2,4);", code.print());
     }
 
@@ -115,7 +115,7 @@ if( arg==0 )
     b=2;
 return arg+a+b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return ((arg+Phi(Region,1,0))+Phi(Region,2,0));", code.print());
     }
 
@@ -130,7 +130,7 @@ if( arg==1 )
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (arg==Phi(Region,3,2));", code.print());
     }
 
@@ -145,7 +145,7 @@ else
     a=3;
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3;", code.print());
     }
 
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter06Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter06Test.java
index b6bc2bf3..2c97b9c2 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter06Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter06Test.java
@@ -15,7 +15,7 @@ public class Chapter06Test {
 if( true ) return 2;
 return 1;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 2;", code.print());
         assertTrue(code.ctrl() instanceof FunNode);
     }
@@ -29,7 +29,7 @@ if (arg)
     a = 2;
 return (arg < a) < 3; // Because (arg < a) is a bool/uint1/[0-1], its always less than 3
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
     }
 
@@ -44,7 +44,7 @@ else
   a=3;
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 2;", code.print());
         assertTrue(code.ctrl() instanceof FunNode);
     }
@@ -65,7 +65,7 @@ else
     b=5;
 return b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,42,5);", code.print());
     }
 
@@ -84,7 +84,7 @@ if( arg==2 )
 else
     b=5;
 return b;""");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,2,5);", code.print());
     }
 
@@ -104,7 +104,7 @@ else if( arg==3 )
 else
     a=5;
 return a;
-""", TypeInteger.constant(2), 123L).parse().opto();
+""", TypeInteger.constant(2), 123L).parse().iter();
         assertEquals("return 5;", code.print());
     }
 
@@ -124,7 +124,7 @@ else if( arg==3 )
 else
     a=5;
 return a;
-""", TypeInteger.constant(1), 123L).parse().opto();
+""", TypeInteger.constant(1), 123L).parse().iter();
         assertEquals("return 3;", code.print());
     }
 
@@ -144,7 +144,7 @@ else
     a=5;
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,3,5,4);", code.print());
     }
 
@@ -165,7 +165,7 @@ else
     a=5;
 return a;
 """, TypeInteger.constant(1), 123L);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3;", code.print());
     }
 
@@ -186,7 +186,7 @@ else
     a=5;
 return a;
 """, TypeInteger.constant(3), 123L);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 4;", code.print());
     }
 
@@ -202,7 +202,7 @@ if( arg ) {
 }
 return a+b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,4,1);", code.print());
     }
 
@@ -221,7 +221,7 @@ if( arg ) {
 }
 return a+b;
 """, TypeInteger.constant(1), 123L);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 4;", code.print());
     }
 
@@ -239,7 +239,7 @@ if( arg ) {
 }
 return a+b;
 """, TypeInteger.constant(0), 123L);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
     }
 
@@ -257,7 +257,7 @@ if( arg ) {
 }
 return a+b+c;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,6,1,5);", code.print());
     }
 
@@ -278,7 +278,7 @@ if( arg ) {
 }
 return a+b+c;
 """, TypeInteger.constant(1), 123L);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 6;", code.print());
     }
 
@@ -298,7 +298,7 @@ if( arg ) {
 }
 return a+b+c;
 """, TypeInteger.constant(2), 123L);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 5;", code.print());
     }
 
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter07Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter07Test.java
index 88554323..b0526eb1 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter07Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter07Test.java
@@ -34,7 +34,7 @@ if(arg){}else{
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,1,Phi(Loop,1,(Phi_a+1)));", code.print());
     }
 
@@ -54,7 +54,7 @@ while(i < arg) {
 }
 return sum;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,Phi(Loop,Phi_sum,(Phi_sum+Phi(Loop,0,(Phi_j+1)))));", code.print());
     }
 
@@ -89,7 +89,7 @@ while(a < 10) {
 }
 return b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,2,(Phi(Region,Phi_b,4)+1));", code.print());
         assertTrue(code.ctrl() instanceof CProjNode);
     }
@@ -122,7 +122,7 @@ while(a < 10) {
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,1,(Phi_a+3));", code.print());
         assertTrue(code.ctrl() instanceof CProjNode);
     }
@@ -148,7 +148,7 @@ int a = 1;
 while(arg) a = 2;
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,1,2);", code.print());
         assertTrue(code.ctrl() instanceof CProjNode);
     }
@@ -180,7 +180,7 @@ while(a < 10) {
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,1,(Phi_a+3));", code.print());
         assertTrue(code.ctrl() instanceof CProjNode);
     }
@@ -214,7 +214,7 @@ while(a < 10) {
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,1,(Phi_a+3));", code.print());
         assertTrue(code.ctrl() instanceof CProjNode);
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter08Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter08Test.java
index 09669cc7..487a2c7b 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter08Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter08Test.java
@@ -22,7 +22,7 @@ while(arg < 10) {
 }
 return arg;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,(Phi(Loop,arg,Add)+1),Add,Phi_arg);", code.print());
         assertTrue(code.ctrl() instanceof RegionNode);
         Assert.assertEquals( "5", Eval2.eval(code, 1));
@@ -45,7 +45,7 @@ while(arg < 10) {
 }
 return a;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,1,Phi(Region,Phi_a,(Phi_a+1),Phi_a));", code.print());
         assertTrue(code.ctrl() instanceof CProjNode);
     }
@@ -64,7 +64,7 @@ while(arg < 10) {
 }
 return arg;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,Phi(Loop,arg,(Phi_arg+1)),Add);", code.print());
         assertTrue(code.ctrl() instanceof RegionNode);
     }
@@ -80,7 +80,7 @@ while(arg < 10) {
 }
 return arg;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,Phi(Loop,arg,(Phi_arg+1)),Add);", code.print());
         assertTrue(code.ctrl() instanceof RegionNode);
     }
@@ -99,7 +99,7 @@ while(arg < 10) {
 }
 return arg;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,arg,(Phi_arg+1));", code.print());
         assertTrue(code.ctrl() instanceof CProjNode);
     }
@@ -116,7 +116,7 @@ while(arg < 10) {
 }
 return arg;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,arg,(Phi_arg+1));", code.print());
         assertTrue(code.ctrl() instanceof CProjNode);
     }
@@ -132,14 +132,14 @@ while( arg < 10 ) {
 }
 return arg;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return arg;", code.print());
     }
 
     @Test
     public void testRegress2() {
         CodeGen code = new CodeGen("if(1) return 0;  else while(arg>- -arg) arg=arg+1; return 0;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -167,7 +167,7 @@ while(arg < 10) {
 }
 return arg;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return arg;", code.print());
     }
 
@@ -184,7 +184,7 @@ while(arg < 10) {
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,Phi(Loop,1,(Phi_a+1)),Add);", code.print());
         assertTrue(code.ctrl() instanceof RegionNode);
     }
@@ -200,7 +200,7 @@ while(1) {
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (Phi(Loop,1,Add)+1);", code.print());
         assertTrue(code.ctrl() instanceof CProjNode prj && prj._idx==1 );
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter09Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter09Test.java
index 401f346f..0a3049e6 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter09Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter09Test.java
@@ -24,7 +24,7 @@ while(v0) {
 }
 return 0;
                 """);
-        code.parse().opto();
+        code.parse().iter();
     }
 
     @Test
@@ -40,7 +40,7 @@ else {
 }
 return x;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,(arg<<1),(Shl+1));", code.print());
         Assert.assertEquals( "2", Eval2.eval(code,  1));
         Assert.assertEquals("23", Eval2.eval(code, 11));
@@ -49,7 +49,7 @@ return x;
     @Test
     public void testGVN2() {
         CodeGen code = new CodeGen("return arg*arg-arg*arg;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         Assert.assertEquals("0", Eval2.eval(code,  1, 1));
     }
@@ -63,7 +63,7 @@ while (arg < 10) {
 }
 return arg;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,arg,(Phi_arg+2));", code.print());
         Assert.assertEquals("11", Eval2.eval(code,  1));
     }
@@ -80,7 +80,7 @@ while (arg < 10) {
 }
 return arg;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,arg,(Phi_arg+4));", code.print());
         Assert.assertEquals("13", Eval2.eval(code,  1));
     }
@@ -109,7 +109,7 @@ while (arg) {
 }
 return arg;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -127,14 +127,14 @@ while(v1+arg) {
     v1=v2;
 }
                 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
     @Test
     public void testWhile0() {
         CodeGen code = new CodeGen("while(0) continue; if(0) arg=0;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -146,7 +146,7 @@ if(0) while(0) {
     while(0) {}
 }
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -154,14 +154,14 @@ if(0) while(0) {
     @Test
     public void testPrecedence() {
         CodeGen code = new CodeGen("return 3-1+2;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 4;", code.print());
     }
 
     @Test
     public void testSwap2() {
         CodeGen code = new CodeGen("return 1+(1+1);");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3;", code.print());
     }
 
@@ -179,7 +179,7 @@ while(arg) {
 }
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,(-(Phi_a+3)));", code.print());
     }
 
@@ -193,7 +193,7 @@ arg=0;
 int v0=0!=0<-0;
 return -0+0+0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Top;", code.print());
     }
 
@@ -201,14 +201,14 @@ return -0+0+0;
     @Test
     public void testFuzz2() {
         CodeGen code = new CodeGen("return 0+-0;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
     @Test
     public void testFuzz3() {
         CodeGen code = new CodeGen("int v0=0; while(0==69) while(v0) return 0;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -220,7 +220,7 @@ while(1) {
     if(1<0) while(arg==-0) arg=arg-arg;
 }
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Top;", code.print());
     }
 
@@ -242,7 +242,7 @@ while(1) {
 }
 return 0!=0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Top;", code.print());
     }
 
@@ -254,7 +254,7 @@ int v0=0;
 while(0==1) while(v0)
         v0=1+v0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -270,14 +270,14 @@ while(1)
         v1=1;
 return v1+v0;
                                    """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Top;", code.print());
     }
 
     @Test
     public void testFuzz8() {
         CodeGen code = new CodeGen("while(arg) arg = arg - 1;  return arg;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter10Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter10Test.java
index 91505f41..8af1c71d 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter10Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter10Test.java
@@ -14,7 +14,7 @@ public class Chapter10Test {
             "if(1) return new S; else return 0;"
         } ) {
             String src = "struct S { int x; }; " + body;
-            var code = new CodeGen(src).parse().opto().typeCheck();
+            var code = new CodeGen(src).parse().iter().typeCheck();
             org.junit.Assert.assertTrue(body,
                 code.expr()._type instanceof com.seaofnodes.simple.type.TypeMemPtr);
         }
@@ -39,7 +39,7 @@ public class Chapter10Test {
             "if (!!!p) return -1; return p.x;",
             "int b = !!p; if (b) return p.x + b - 1; return -1;"
         } ) {
-            CodeGen code = new CodeGen(NULLABLE_POINT_SOURCE+body).parse().opto().typeCheck();
+            CodeGen code = new CodeGen(NULLABLE_POINT_SOURCE+body).parse().iter().typeCheck();
             assertEquals(body,"-1",Eval2.eval(code,0));
             assertEquals(body,"42",Eval2.eval(code,1));
         }
@@ -54,7 +54,7 @@ public class Chapter10Test {
             "if (!!p) { int x = p.x; } return p.x;"
         } ) {
             try {
-                new CodeGen(NULLABLE_POINT_SOURCE+body).parse().opto().typeCheck();
+                new CodeGen(NULLABLE_POINT_SOURCE+body).parse().iter().typeCheck();
                 fail(body);
             } catch( RuntimeException e ) {
                 // Known null is rejected during parsing in these chapters.
@@ -86,7 +86,7 @@ if( (arg/13)==0 ) {
 int r = g+h;
 return p-r;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -106,7 +106,7 @@ bar.a = 1;
 bar.a = 2;
 return bar.a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 2;", code.print());
     }
 
@@ -122,7 +122,7 @@ else
     v.y = 3;
 return v;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Vector2D;", code.print());
     }
 
@@ -160,7 +160,7 @@ while (arg) {
 }
 return bar.a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,(Phi_a+2));", code.print());
     }
 
@@ -173,7 +173,7 @@ if (arg) bar = null;
 bar.a = 1;
 return bar.a;
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Type null is not of declared type *Bar",e.getMessage()); }
     }
 
@@ -186,7 +186,7 @@ if (arg) bar = new Bar;
 bar.a = 1;
 return bar.a;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Might be null accessing 'a'",e.getMessage()); }
     }
 
@@ -212,7 +212,7 @@ if (arg) bar = null;
 if( bar ) bar.a = 1;
 return bar;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,null,Bar);", code.print());
     }
 
@@ -228,7 +228,7 @@ if( !bar ) rez=4;
 else bar.a = 1;
 return rez;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,4,3);", code.print());
     }
 
@@ -245,7 +245,7 @@ while(arg) {
 }
 return ret;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) {
             assertEquals("Might be null accessing 'v0'", e.getMessage()); }
     }
@@ -282,7 +282,7 @@ while( i.x < i.len ) {
 }
 return sum;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,(Phi(Loop,0,(Phi_x+1))+Phi_sum));", code.print());
     }
 
@@ -301,7 +301,7 @@ while(arg) {
 }
 return ret;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,s0,Phi(Region,s0,Phi_ret));", code.print());
     }
 
@@ -319,7 +319,7 @@ while(arg) {
 }
 return ret;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,s0,Phi(Region,s0,Phi_ret));", code.print());
     }
 
@@ -336,7 +336,7 @@ while(arg < 10) {
 }
 return ret;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,s0,Phi(Region,s0,Phi_ret));", code.print());
     }
 
@@ -363,7 +363,7 @@ if(0) {
     }
 }
    """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -377,7 +377,7 @@ if(0) return 0;
 else return new s0;
 if(new s0.f0) return 0;
     """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return s0;", code.print());
     }
 
@@ -393,7 +393,7 @@ while(0<arg) {
 }
 return 0;
     """);
-        code.parse().opto();
+        code.parse().iter();
     }
 
     @Test
@@ -405,7 +405,7 @@ while(v0.f0) {}
 s0 v1 = v0;
 return v1;
     """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (const)s0;", code.print());
     }
 
@@ -437,7 +437,7 @@ while(0) {}
         }    }
 }
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -448,7 +448,7 @@ int v0=arg==0;
 while(v0) continue;
 return 0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -462,7 +462,7 @@ return 0;
             a.y = 55; b.y = 66;
             a.x = 33; b.x = 44;
             return before*100 + p.x;
-            """).parse().opto();
+            """).parse().iter();
         StopNode stop = code._stop;
         assertEquals("1133",Eval2.eval(code,0));
         assertEquals("2244",Eval2.eval(code,1));
@@ -488,7 +488,7 @@ return 0;
             """;
 
     @Test public void testMemoryAcrossNestedLoops() {
-        var code = new CodeGen(NESTED_MEMORY).parse().opto();
+        var code = new CodeGen(NESTED_MEMORY).parse().iter();
         StopNode stop = code._stop;
         for (int arg=0; arg<7; arg++) {
             assertEquals("S{x="+(5L+arg)+",y="+(7L+arg*(arg+1)-(arg>=2 ? 2 : 0))+
@@ -504,7 +504,7 @@ return 0;
             if (arg) { s.x = 11; s.z = 13; return s; }
             s.y = 17;
             return s;
-            """).parse().opto();
+            """).parse().iter();
         StopNode stop = code._stop;
         for (int arg=0; arg<2; arg++) {
             assertEquals(arg==0 ? "S{x=3,y=17,z=7}" : "S{x=11,y=5,z=13}",Eval2.eval(code,arg));
@@ -521,7 +521,7 @@ return 0;
             int v;
             if (arg>1) v=p.x; else v=q.x;
             return v;
-            """).parse().opto();
+            """).parse().iter();
         StopNode stop = code._stop;
         // The one-step safety check stops at the memory aggregate.
         assertEquals(2,countMemoryNodes(stop,LoadNode.class,new BitSet()));
@@ -536,7 +536,7 @@ return 0;
             !S !s = new S;
             if (arg) s.x=arg+1; else s.x=arg+2;
             return s;
-            """).parse().opto();
+            """).parse().iter();
         StopNode stop = code._stop;
         assertEquals(1,countMemoryNodes(stop,StoreNode.class,new BitSet()));
         assertEquals("S{x=2}",Eval2.eval(code,0));
@@ -554,7 +554,7 @@ return 0;
             if (arg>1) { v=p.x; p.x=41; p.y=5; }
             else       { v=q.x; q.x=42; }
             return v;
-            """).parse().opto();
+            """).parse().iter();
         StopNode stop = code._stop;
         assertEquals(2,countMemoryNodes(stop,LoadNode.class,new BitSet()));
         assertEquals("-1",Eval2.eval(code,-1));
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter11Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter11Test.java
index 439333ca..a74f8540 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter11Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter11Test.java
@@ -13,7 +13,7 @@ public class Chapter11Test {
             s.x = 42;
             while (arg > 0) { s.y = s.y + arg; arg = arg - 1; }
             return s.x;
-            """).parse().opto();
+            """).parse().iter();
         var stop = code._stop;
         assertEquals("return 42;",stop.toString());
         assertEquals("42",Eval2.eval(code,5));
@@ -32,7 +32,7 @@ public class Chapter11Test {
                 arg = arg - 1;
             }
             return a;
-            """).parse().opto();
+            """).parse().iter();
         var stop = code._stop;
         int[] phis = {0};
         stop.walk(n -> {
@@ -54,7 +54,7 @@ public class Chapter11Test {
         // it. Keep both the worklist assertion and the final heap check.
         for (int seed : new int[]{0,97,123,456}) {
             var code = new CodeGen(Chapter10Test.NESTED_MEMORY,com.seaofnodes.simple.type.TypeInteger.BOT,seed).parse();
-            code.opto();
+            code.iter();
             assertEquals("S{x=9,y=25,z=17}",Eval2.eval(code,4));
         }
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter12Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter12Test.java
index 0b4ad8ec..324b2a12 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter12Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter12Test.java
@@ -14,7 +14,7 @@ public class Chapter12Test {
 """
 return 3.14;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3.14;", code.print());
         assertEquals("3.14", Eval2.eval(code,  0));
     }
@@ -34,7 +34,7 @@ while( arg ) {
 }
 return head.next.i;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Might be null accessing 'next'",e.getMessage()); }
     }
 
@@ -56,7 +56,7 @@ LLI? next = head.next;
 if( next==null ) return 1;
 return next.i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,0,1,.i);", code.print());
         assertEquals("2", Eval2.eval(code,  3));
     }
@@ -75,7 +75,7 @@ i0.f = f0;
 f0.i = i0;
 return f0.i.f.i.i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 17;", code.print());
     }
 
@@ -87,7 +87,7 @@ struct N { !N? !next; int i; };
 N n = new N;
 return n.next;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return null;", code.print());
     }
 
@@ -100,7 +100,7 @@ struct N { !M !next; int i; };
 N n = new N { next = new M; };
 return n.next;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (const)M;", code.print());
     }
 
@@ -113,7 +113,7 @@ struct N { !M !next; int i; };
 N n = new N { next = null; }
 return n.next;
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Type null is not of declared type *M",e.getMessage()); }
     }
 
@@ -126,20 +126,20 @@ struct N { !N? !next; int i; };
 n.i = 3.14;
 return n.i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("3",Eval2.eval(code,0));
     }
 
     @Test
     public void testNullRef4() {
         CodeGen code = new CodeGen("-null-5/null-5;");
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot 'Add' null",e.getMessage()); }
     }
 
     @Test public void testNullRef5() {
         CodeGen code = new CodeGen("return null+42;");
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot 'Add' null",e.getMessage()); }
     }
 
@@ -149,7 +149,7 @@ return n.i;
 """
 struct S{};
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code,  0));
     }
@@ -161,7 +161,7 @@ struct S{};
 struct S1 { !S2? !s; };
 return new S2;
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Unknown struct type 'S2'",e.getMessage()); }
     }
 
@@ -173,7 +173,7 @@ struct S1 { !S2? !s; };
 struct S2 { int x; };
 return new S1.s=new S2;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return S2;", code.print());
     }
 
@@ -190,7 +190,7 @@ p2.pi.i = 2;
 if (arg) p1 = new P;
 return p1.pi.i + 1;
 """);
-        try { code.parse().opto().typeCheck();  fail(); }
+        try { code.parse().iter().typeCheck();  fail(); }
         catch( Exception e ) {  assertEquals("Might be null accessing 'i'",e.getMessage());  }
     }
 
@@ -255,7 +255,7 @@ z.a=k;  z.b=s; z.c=e;
 return a.b.c.a.b.c.a.b.c.a.b.c.a.b.c.a.b.c;
 
 """);
-        code.parse().opto().typeCheck().GCM().localSched();
+        code.driver(CodeGen.Phase.LocalSched);
         assertEquals("return R;", code._stop.toString());
         assertEquals("R{a=C{a=N{a=Y{a=J{a=U{a=F{a=Q{a=B{a=M{a=X{a=I{a=T{a=E{a=P{a=A{a=L{a=W{a=H{a=S{a=D{a=O{a=Z{a=K{a=V{a=G{a=$cyclic,b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic},b=$cyclic,c=$cyclic}", Eval2.eval(code,  0));
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter13Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter13Test.java
index 66e12e42..4e56b9de 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter13Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter13Test.java
@@ -290,8 +290,11 @@ else {
         }
 }
 """);
-        code.parse().opto().typeCheck().GCM();
+        code.driver(CodeGen.Phase.TypeCheck);
         assertEquals("return 9;", code.print());
+        code.driver(CodeGen.Phase.Schedule);
+        // LoopTree connects the nonreturning paths using synthetic Never exits.
+        assertEquals("return Phi(Region,9,Top,Top,Top,Top);", code.print());
     }
 
 
@@ -322,7 +325,7 @@ while( prime <= arg ) {
 }
 return primeCount;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,0,Phi(Loop,1,Phi(Region,Phi_primeCount,Phi_primeCount,(Phi_primeCount+1))));", code.print());
         assertEquals("0", Eval2.eval(code,  1)); // No primes 1 or below
         assertEquals("1", Eval2.eval(code,  2)); // 2
@@ -344,7 +347,7 @@ i=v.f;
 if (arg) v.f=1;
 return i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 2;", code.print());
         assertEquals("2", Eval2.eval(code, 0));
         assertEquals("2", Eval2.eval(code, 1));
@@ -367,7 +370,7 @@ if (arg) {
 }
 return i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,.f,0);", code.print());
     }
 
@@ -386,7 +389,7 @@ if (v1) {
 }
 return v0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return S;", code.print());
     }
 
@@ -404,7 +407,7 @@ if (arg+1) arg= 0;
 while (arg) v.f = 2;
 return i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return arg;", code.print());
     }
 
@@ -421,7 +424,7 @@ while(1) {
 }
 return v;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return S;", code.print());
     }
 
@@ -436,7 +439,7 @@ while( -arg )
 while(1)
   arg = arg+ptr.v;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Top;", code.print());
     }
 
@@ -454,7 +457,7 @@ while (arg) {
 }
 return i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -474,7 +477,7 @@ while(arg) {
 }
 return arg;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -492,7 +495,7 @@ if (arg) {
 }
 return v;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return S;", code.print());
     }
 
@@ -511,7 +514,7 @@ while (arg > 0) {
 }
 return v;
                 """);
-        code.parse().opto();
+        code.parse().iter();
         //assertEquals("return new S;", code.print());
     }
 
@@ -528,7 +531,7 @@ while(0>=0) {
 }
 return 0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Top;", code.print());
     }
 
@@ -546,7 +549,7 @@ u32 v0=0;
     return v1;
 }
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter14FloatTest.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter14FloatTest.java
index 6377bc14..cf7d3481 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter14FloatTest.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter14FloatTest.java
@@ -20,7 +20,7 @@ public class Chapter14FloatTest {
             {"f32 rounded=arg+16777216.0; return rounded;", "1.6777232E7", "1.6777199E7"}
         };
         for( String[] test : tests ) {
-            var code = new CodeGen(test[0]).parse().opto().typeCheck();
+            var code = new CodeGen(test[0]).parse().iter().typeCheck();
             assertEquals(test[0],test[1],Eval2.eval(code,17));
             assertEquals(test[0],test[2],Eval2.eval(code,-17));
         }
@@ -28,7 +28,7 @@ public class Chapter14FloatTest {
 
 
     @Test public void testSubZeroFloat() {
-        var code = new CodeGen("flt x = arg; return 0-x;").parse().opto().typeCheck();
+        var code = new CodeGen("flt x = arg; return 0-x;").parse().iter().typeCheck();
         // Compare strings so +0.0 and -0.0 remain distinct.
         assertEquals("0.0", Eval2.eval(code,0));
         assertEquals("-1.0", Eval2.eval(code,1));
@@ -42,7 +42,7 @@ public class Chapter14FloatTest {
             {"return 2.5; return 7;", "2.5"}
         } ) {
             String src = test[0];
-            var code = new CodeGen(src).parse().opto().typeCheck();
+            var code = new CodeGen(src).parse().iter().typeCheck();
             assertEquals(src,test[1],Eval2.eval(code,0));
         }
     }
@@ -55,7 +55,7 @@ public class Chapter14FloatTest {
 """
 return 3.14;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3.14;", code.print());
         assertEquals("3.14", Eval2.eval(code,  0));
     }
@@ -66,7 +66,7 @@ return 3.14;
 """
 return 3.14;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3.14;", code.print());
         assertEquals("3.14", Eval2.eval(code,  0));
     }
@@ -83,7 +83,7 @@ while( 1 ) {
 }
 return guess;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,(flt)arg,(((ToFloat/Phi_guess)+Phi_guess)*0.5f));", code.print());
         assertEquals("3.0", Eval2.eval(code,  9));
         assertEquals("1.414213562373095", Eval2.eval(code,  2));
@@ -96,7 +96,7 @@ return guess;
 flt x = arg;
 return x+1==x;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return ((flt)arg==(ToFloat+1.0f));", code.print());
         assertEquals("0", Eval2.eval(code, 1));
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter14Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter14Test.java
index f0fe7dcd..ebc94d21 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter14Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter14Test.java
@@ -16,7 +16,7 @@ public class Chapter14Test {
 """
 return 3.14;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3.14;", code.print());
         assertEquals("3.14", Eval2.eval(code,  0));
     }
@@ -32,7 +32,7 @@ if( b < 0 ) c = -1;
 if( b > 2 ) c =  1;
 return c;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 99;", code.print());
         assertEquals("99", Eval2.eval(code,  0));
     }
@@ -45,7 +45,7 @@ u8 b = 123;
 b = b + 456;// Truncate
 return b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 67;", code.print());
         assertEquals("67", Eval2.eval(code,  0));
     }
@@ -59,7 +59,7 @@ u8 b = 123;
 while( b ) b = b + 456;// Truncate
 return b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -72,7 +72,7 @@ b = b + 456;// Truncate
 u1 c = b;   // No more truncate needed
 return c;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
         assertEquals("1", Eval2.eval(code,  0));
     }
@@ -85,7 +85,7 @@ int b = 123;
 b = b+456 & 31;                 // Precedence
 return b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3;", code.print());
         assertEquals("3", Eval2.eval(code,  0));
     }
@@ -99,7 +99,7 @@ struct Foo { u1 b; };
 f.b = 123;
 return f.b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
         assertEquals("1", Eval2.eval(code,  0));
     }
@@ -111,7 +111,7 @@ return f.b;
 i8 b = 255;                     // Chopped
 return b;                       // Sign extend
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return -1;", code.print());
         assertEquals("-1", Eval2.eval(code,  0));
     }
@@ -124,7 +124,7 @@ i8 b = arg;
 b = b + 1;// Truncate
 return b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (((((arg<<56)>>56)+1)<<56)>>56);", code.print());
         assertEquals("1", Eval2.eval(code, 0));
         assertEquals("-128", Eval2.eval(code, 127));
@@ -138,7 +138,7 @@ u16 mask = (1<<16)-1;           // AND mask
 int c = 123456789 & mask;
 return c;                       //
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 52501;", code.print());
         assertEquals("52501", Eval2.eval(code,  0));
     }
@@ -149,7 +149,7 @@ return c;                       //
 """
 return (arg | 123 ^ 456) >>> 1;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (((arg|123)^456)>>>1);", code.print());
         assertEquals("217", Eval2.eval(code,  0));
     }
@@ -162,7 +162,7 @@ flt f = arg;
 arg = f & 1;
 return arg;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot '&' flt",e.getMessage()); }
     }
 
@@ -175,7 +175,7 @@ while(arg) v1=-v0;
 while(v1) break;
 return v1;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (Phi(Loop,((1<<arg)&1),0)&Phi(Loop,Shl,4294967295));", code.print());
         assertEquals("1", Eval2.eval(code,  0));
     }
@@ -194,7 +194,7 @@ if(0&0>>>0) {
 }
 return v0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code,  0));
     }
@@ -220,7 +220,7 @@ f32 ff32 = 3.141592653589793;  if( ff32 != 3.1415927410125732) return 5;
 
 return 0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code,  0));
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter15Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter15Test.java
index cb92277f..fbbb6c9d 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter15Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter15Test.java
@@ -17,7 +17,7 @@ public class Chapter15Test {
 """
 return 3.14;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3.14;", code.print());
         assertEquals("3.14", Eval2.eval(code,  0));
     }
@@ -31,7 +31,7 @@ struct C { !C? !l; };
 c.l = c;
 return c;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return C;", code.print());
         assertEquals("C{l=$cyclic}", Eval2.eval(code,  0));
     }
@@ -50,7 +50,7 @@ while (i < old#) {
 output[i] = 1;
 return output;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return [u8];", code.print());
         assertEquals("\u0001", Eval2.eval(code,  0));
     }
@@ -62,7 +62,7 @@ return output;
 int[] is = new int[2];
 return is[1];
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code,  0));
     }
@@ -75,7 +75,7 @@ int[] is = new int[2];
 int[] is2 = new int[2];
 return is[1];
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code,  0));
     }
@@ -89,7 +89,7 @@ a[0] = 1;
 a[1] = 2;
 return a[0];
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
         assertEquals("1", Eval2.eval(code,  0));
     }
@@ -102,7 +102,7 @@ struct A { int i; };
 !A?[] !a = new !A?[2];
 return a;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return [*A?];", code.print());
         assertEquals("*A {int !i; }?[ null,null]", Eval2.eval(code, 0));
     }
@@ -131,7 +131,7 @@ else {
 }
 return rez;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,1.2,.y,2.3);", code.print());
         assertEquals("3.14", Eval2.eval(code, 0));
         assertEquals("1.2", Eval2.eval(code, 1));
@@ -156,7 +156,7 @@ if( iss[arg] )
     if( iss[arg][2] )
         rez = iss[arg][2].y;
 return rez;""");
-        code.parse().opto().typeCheck();
+        code.parse().iter().typeCheck();
         assertEquals("return Phi(Region,1.2,1.2,.y);", code.print());
         assertEquals("3.14", Eval2.eval(code,  0));
         try { Eval2.eval(code, 10); fail(); } // Fails AIOOBE in Eval2, no range checks
@@ -174,7 +174,7 @@ root._kids = new !Tree?[2]; // NO BANG SO ARRAY IS OF IMMUTABLE TREES????
 root._kids[0] = new Tree;
 return root;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Tree;", code.print());
         assertEquals("Tree{_kids=*Tree?[ Tree{_kids=null},null]}", Eval2.eval(code,  0));
     }
@@ -186,7 +186,7 @@ return root;
 struct S { int a; int[] !b; };
 return 0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -209,7 +209,7 @@ while( i < ary# - 1 ) {
 }
 return ary[1] * 1000 + ary[3]; // 1 * 1000 + 6
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (.[]+(.[]*1000));", code.print());
         assertEquals("1006", Eval2.eval(code,  4));
     }
@@ -218,7 +218,7 @@ return ary[1] * 1000 + ary[3]; // 1 * 1000 + 6
     public void sieveOEratosthenes() throws IOException {
         String src = Files.readString(Path.of("src/test/java/com/seaofnodes/simple/progs/sieve.smp"));
         CodeGen code = new CodeGen(src+"return sieve(arg);");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return [u32];", code.print());
         assertEquals("u32[ 2,3,5,7,11,13,17,19]",Eval2.eval(code, 20));
     }
@@ -235,7 +235,7 @@ s2.f = 2.0;
 if (arg) s1 = new S;
 return s1.i + s1.f;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return ((flt).i+.f);", code.print());
         assertEquals("0.0", Eval2.eval(code,  0));
     }
@@ -247,7 +247,7 @@ return s1.i + s1.f;
 """
 return new flt;
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Cannot allocate a flt",e.getMessage()); }
     }
 
@@ -257,7 +257,7 @@ return new flt;
 """
 int is = new int[2];
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Type *[int] is not of declared type int",e.getMessage()); }
     }
 
@@ -268,7 +268,7 @@ int is = new int[2];
 int[] is = new int[3.14];
 return is[1];
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Cannot allocate an array with length 3.14",e.getMessage()); }
     }
 
@@ -278,7 +278,7 @@ return is[1];
 int[] is = new int[arg];
 return is[1];
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code,  4));
         try { Eval2.eval(code,-1); fail(); }
@@ -311,7 +311,7 @@ while(v5+(0&0)) {
 }
 return v1;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code,  0));
     }
@@ -326,7 +326,7 @@ u32 v7=0;
 int v8=0;
 while(0<--1>>>---(v7*0==v8)) {}
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 
@@ -350,7 +350,7 @@ else {
         }
 }
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
     }
 }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter16Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter16Test.java
index 362388d6..78e468cd 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter16Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter16Test.java
@@ -17,7 +17,7 @@ public class Chapter16Test {
 """
 return 3.14;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3.14;", code.print());
         assertEquals("3.14", Eval2.eval(code,  0));
     }
@@ -30,7 +30,7 @@ return 3.14;
 int x, y;
 return x+y;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code,  0));
     }
@@ -41,7 +41,7 @@ return x+y;
 int x=2, y=x+1;
 return x+y;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 5;", code.print());
         assertEquals("5", Eval2.eval(code,  0));
     }
@@ -54,7 +54,7 @@ struct X { int x=3; };
 X z = new X;
 return z.x;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3;", code.print());
         assertEquals("3", Eval2.eval(code,  0));
     }
@@ -66,7 +66,7 @@ struct X { int x=3; };
 X z = new X { x = 4; };
 return z.x;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 4;", code.print());
         assertEquals("4", Eval2.eval(code,  0));
     }
@@ -87,7 +87,7 @@ LLI? next = head.next;
 if( !next ) return 1;
 return next.i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,0,1,.i);", code.print());
         assertEquals("0", Eval2.eval(code,  0));
         assertEquals("1", Eval2.eval(code,  1));
@@ -118,7 +118,7 @@ LLI? next = head.next;
 if( !next ) return 1;
 return next.i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,0,1,.i);", code.print());
         assertEquals("0", Eval2.eval(code,  0));
         assertEquals("1", Eval2.eval(code,  1));
@@ -143,7 +143,7 @@ struct Square {
 };
 return new Square;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Square;", code.print());
         assertEquals("Square{side=3.0,diag=1.7320508075688772}", Eval2.eval(code,  3));
         assertEquals("Square{side=4.0,diag=2.0}", Eval2.eval(code, 4));
@@ -167,7 +167,7 @@ return new Square;
         """;
 
     @Test public void testConstructorMemory() {
-        var code = new CodeGen(CONSTRUCTOR_MEMORY).parse().opto();
+        var code = new CodeGen(CONSTRUCTOR_MEMORY).parse().iter();
         assertEquals("111249",Eval2.eval(code,0));
         assertEquals("222352",Eval2.eval(code,1));
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter17aTest.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter17aTest.java
index 16ffe967..4a69c33d 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter17aTest.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter17aTest.java
@@ -17,7 +17,7 @@ int x=2, y=3;
 if( arg ) { int x = y; x = x*x; y=x; } // Shadow final x
 return y;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,9,3);", code.print());
         assertEquals("3", Eval2.eval(code, 0));
         assertEquals("9", Eval2.eval(code, 1));
@@ -30,7 +30,7 @@ struct X { int ~x; };
 X z = new X { x=3; };
 return z.x;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 3;", code.print());
         assertEquals("3", Eval2.eval(code,  0));
     }
@@ -42,7 +42,7 @@ struct Point { int ~x, ~y; };
 Point p = new Point { x=3; y=4; };
 return p;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (const)Point;", code.print());
         assertEquals("Point{x=3,y=4}", Eval2.eval(code,  0));
     }
@@ -55,7 +55,7 @@ Point p = new Point { x=5; y=6; };
 p.x++;
 return p;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
     }
 
@@ -67,7 +67,7 @@ Point p = new Point;
 p.x++;
 return p;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
     }
 
@@ -75,7 +75,7 @@ return p;
     public void testStructFinal3() {
         for( String fields : new String[] { "var x; var y;", "int[] !x;", "int[] !x;" } ) {
             String src = "struct Point { "+fields+" }; Point p = new Point; p.x++; return p;";
-            try { new CodeGen(src).parse().opto(); fail(fields); }
+            try { new CodeGen(src).parse().iter(); fail(fields); }
             catch( Exception e ) { assertEquals("'Point' is not fully initialized, field 'x' needs to be set in a constructor",e.getMessage()); }
         }
     }
@@ -88,7 +88,7 @@ Point p = new Point;
 p.x++;
 return p;
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Cannot reassign final 'x'",e.getMessage()); }
     }
 
@@ -100,7 +100,7 @@ struct Point { var x=3; var y=4; };
 p.x++;
 return p;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Point;", code.print());
         assertEquals("Point{x=4,y=4}", Eval2.eval(code,  0));
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter17bTest.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter17bTest.java
index e8f6e0bd..8ea72c21 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter17bTest.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter17bTest.java
@@ -15,7 +15,7 @@ public class Chapter17bTest {
 """
 return 1;
 """);
-        code.parse().opto().typeCheck();
+        code.parse().iter().typeCheck();
         assertEquals("return 1;", code.print());
         assertEquals("1", Eval2.eval(code,  0));
     }
@@ -27,7 +27,7 @@ return 1;
         CodeGen code = new CodeGen("""
 return arg++;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return arg;", code.print());
         assertEquals("0", Eval2.eval(code, 0));
     }
@@ -37,7 +37,7 @@ return arg++;
         CodeGen code = new CodeGen("""
 return arg+++arg++;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return ((arg<<1)+1);", code.print());
         assertEquals("1", Eval2.eval(code, 0));
     }
@@ -48,7 +48,7 @@ return arg+++arg++;
 //   -(arg--)-(arg--)
 return -arg---arg--;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (-((arg<<1)+-1));", code.print());
         assertEquals("1", Eval2.eval(code, 0));
     }
@@ -61,7 +61,7 @@ xs[0]++;
 xs[1]++;
 return xs[0];
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
         assertEquals("1", Eval2.eval(code, 2));
     }
@@ -73,7 +73,7 @@ u8[] !xs = new u8[1];
 xs[0]--;
 return xs[0];
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 255;", code.print());
         assertEquals("255", Eval2.eval(code, 2));
     }
@@ -86,7 +86,7 @@ struct S { u16 x; };
 s.x--;
 return s.x;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 65535;", code.print());
         assertEquals("65535", Eval2.eval(code, 2));
     }
@@ -94,7 +94,7 @@ return s.x;
     @Test
     public void testInc6() {
         CodeGen code = new CodeGen("return --arg;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (arg+-1);", code.print());
         assertEquals("-1", Eval2.eval(code, 0));
     }
@@ -102,7 +102,7 @@ return s.x;
     @Test
     public void testInc7() {
         CodeGen code = new CodeGen("u8 x=0; return --x;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 255;", code.print());
         assertEquals("255", Eval2.eval(code, 0));
     }
@@ -110,7 +110,7 @@ return s.x;
     @Test
     public void testInc8() {
         CodeGen code = new CodeGen("int x; x+=2; return x+=3;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 5;", code.print());
         assertEquals("5", Eval2.eval(code, 0));
     }
@@ -118,14 +118,14 @@ return s.x;
     @Test
     public void testInc9() {
         CodeGen code = new CodeGen("flt x = 1.5; x++; return x;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 2.5f;", code.print());
         assertEquals("2.5", Eval2.eval(code, 0));
     }
 
     @Test public void testInt10() {
         CodeGen code = new CodeGen("flt x = 1.5; x++; int i = x; return i;");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("2",Eval2.eval(code,0));
     }
 
@@ -133,73 +133,73 @@ return s.x;
     // ---------------------------------------------------------------
     @Test public void testVar0() {
         CodeGen code = new CodeGen("var d; return d;");
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Syntax error, expected =expression: ;",e.getMessage()); }
     }
 
     @Test public void testVar1() {
         CodeGen code = new CodeGen("val d; return d;");
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Syntax error, expected =expression: ;",e.getMessage()); }
     }
 
     @Test public void testVar2() {
         CodeGen code = new CodeGen("int x; x=3; x++; return x; // Ok, no initializer so x is mutable ");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 4;", code.print());
         assertEquals("4", Eval2.eval(code, 0));
     }
 
     @Test public void testVar3() {
         CodeGen code = new CodeGen("int x=3; x++; return x; // Ok, primitive so x is mutable despite initializer ");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 4;", code.print());
         assertEquals("4", Eval2.eval(code, 0));
     }
 
     @Test public void testVar4() {
         CodeGen code = new CodeGen("struct S{int x;}; !S? !s; s=new S; s.x++; return s.x; // Ok, no initializer so x is mutable ");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
         assertEquals("1", Eval2.eval(code, 0));
     }
 
     @Test public void testVar5() {
         CodeGen code = new CodeGen("struct S{int x;}; !S? !s; s=new S{x=3;}; s.x++; return s.x; // Ok, no initializer so x is mutable ");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 4;", code.print());
         assertEquals("4", Eval2.eval(code, 0));
     }
 
     @Test public void testVar6() {
         CodeGen code = new CodeGen("struct S{int x;}; S s=new S; s.x++; return s.x; // Error initializer so x is immutable ");
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
     }
 
     @Test public void testVar7() {
         CodeGen code = new CodeGen("struct S{int x;}; S s=new S{x=3;}; s.x++; return s.x; // Error initializer so x is immutable ");
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
     }
 
     @Test public void testVar8() {
         CodeGen code = new CodeGen("struct S{int x;}; !S !s=new S; s.x++; return s.x; // Ok, has '!' so s.x is mutable ");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
         assertEquals("1", Eval2.eval(code, 0));
     }
 
     @Test public void testVar9() {
         CodeGen code = new CodeGen("struct S{int x;}; var s=new S; s.x++; return s.x; // Ok, has var so s.x is mutable ");
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 1;", code.print());
         assertEquals("1", Eval2.eval(code, 0));
     }
 
     @Test public void testVar10() {
         CodeGen code = new CodeGen("struct S{int x;}; ~S s=new S; s.x++; return s.x; // Error, has val so x is immutable ");
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
     }
 
@@ -217,7 +217,7 @@ foo.bar.x++;   // Ok foo and foo.bar and foo.bar.x are all mutable
 ~Foo xfoo = foo; // Throw away mutability
 xfoo.bar.x++;   // Error, cannot mutate through xfoo
  """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
     }
 
@@ -238,7 +238,7 @@ foo.bar.x++;           // Bumps to 5
 int x5 = xfoo.bar.x;   // Ok to read through xfoo, gets 5
 return x4*10+x5;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 45;", code.print());
         assertEquals("45", Eval2.eval(code, 0));
     }
@@ -248,7 +248,7 @@ return x4*10+x5;
         CodeGen code = new CodeGen("""
 int i,i++;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Redefining name 'i'",e.getMessage()); }
     }
 
@@ -263,7 +263,7 @@ A x = new A {
     b = new B;
 };
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Cannot read uninitialized field 'b'",e.getMessage()); }
     }
 
@@ -276,7 +276,7 @@ return new A {
     if (arg) b = new B; // Constructor ends with partial init of b
 }.b;
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("'A' is not fully initialized, field 'b' needs to be set in a constructor",e.getMessage()); }
     }
 
@@ -290,7 +290,7 @@ val s = new S{x = 2;};
 return x;
 """
 );
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Cannot reassign final 'x'",e.getMessage()); }
     }
 
@@ -309,7 +309,7 @@ focus(me);
 return me;
 """
 );
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Syntax error, expected =expression: ;",e.getMessage()); }
     }
 
@@ -319,7 +319,7 @@ return me;
         CodeGen code = new CodeGen("""
 return arg ? 1 : 2;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,1,2);", code.print());
         assertEquals("2", Eval2.eval(code, 0));
         assertEquals("1", Eval2.eval(code, 1));
@@ -330,7 +330,7 @@ return arg ? 1 : 2;
         CodeGen code = new CodeGen("""
 return arg ? 0 : arg;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 0;", code.print());
         assertEquals("0", Eval2.eval(code, 0));
     }
@@ -342,7 +342,7 @@ struct Bar { int x; };
 var b = arg ? new Bar : null;
 return b ? b.x++ + b.x++ : -1;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,((.x<<1)+1),-1);", code.print());
         assertEquals("-1", Eval2.eval(code, 0));
         assertEquals("1", Eval2.eval(code, 1));
@@ -355,7 +355,7 @@ struct Bar { int x; };
 var b = arg ? new Bar;
 return b ? b.x++ + b.x++ : -1;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,((.x<<1)+1),-1);", code.print());
         assertEquals("-1", Eval2.eval(code, 0));
         assertEquals("1", Eval2.eval(code, 1));
@@ -369,7 +369,7 @@ struct Bar { !Bar? !next; int x; };
 var b = arg ? new Bar { next = (arg==2) ? new Bar{x=2;}; x=1; };
 return b ? b.next ? b.next.x : b.x; // parses "b ? (b.next ? b.next.x : b.x) : 0"
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,.x,0,.x);", code.print());
         assertEquals("0", Eval2.eval(code, 0));
         assertEquals("1", Eval2.eval(code, 1));
@@ -382,7 +382,7 @@ return b ? b.next ? b.next.x : b.x; // parses "b ? (b.next ? b.next.x : b.x) : 0
 flt f=arg?1:1.2;
 return f;   // missing widening
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Region,1.0f,1.2);", code.print());
         assertEquals("1.2", Eval2.eval(code,  0));
     }
@@ -394,7 +394,7 @@ return f;   // missing widening
 struct S{};
 return arg ? 7 : new S;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("No common type amongst int and reference",e.getMessage()); }
     }
 
@@ -407,7 +407,7 @@ for( int i=0; i<arg; i++ )
     sum += i;
 return sum;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,(Phi_sum+Phi(Loop,0,(Phi_i+1))));", code.print());
         assertEquals("3", Eval2.eval(code, 3));
         assertEquals("45", Eval2.eval(code, 10));
@@ -420,7 +420,7 @@ for( ; i<arg; i++ )
     sum += i;
 return sum;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,(Phi_sum+Phi(Loop,0,(Phi_i+1))));", code.print());
         assertEquals("3", Eval2.eval(code, 3));
         assertEquals("45", Eval2.eval(code, 10));
@@ -435,7 +435,7 @@ for( int i=0; ; i++ ) {
 }
 return sum;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,(Phi_sum+Phi(Loop,0,(Phi_i+1))));", code.print());
         assertEquals("3", Eval2.eval(code, 3));
         assertEquals("45", Eval2.eval(code, 10));
@@ -448,7 +448,7 @@ for( int i=0; i<arg; )
     sum += i++;
 return sum;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,(Phi_sum+Phi(Loop,0,(Phi_i+1))));", code.print());
         assertEquals("3", Eval2.eval(code, 3));
         assertEquals("45", Eval2.eval(code, 10));
@@ -461,7 +461,7 @@ for( int i=0; i<arg; i++ )
     sum += i;
 return i;
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Undefined name 'i'",e.getMessage()); }
     }
     @Test
@@ -469,7 +469,7 @@ return i;
         CodeGen code = new CodeGen("""
 for(;;arg++;) {}
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Syntax error, expected Unexpected code after expression: ;",e.getMessage()); }
     }
 
@@ -485,7 +485,7 @@ struct A{
 };
 return new A;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return A;", code.print());
         assertEquals("A{f1=null,f2=null}", Eval2.eval(code,0));
     }
@@ -499,7 +499,7 @@ struct A{
 };
 return new A.not_array_of_b;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return [*B?];", code.print());
         assertEquals("*B?[]", Eval2.eval(code,0));
     }
@@ -518,7 +518,7 @@ for( ; ptr; ptr = ptr.next )
     sum += ptr.i;
 return sum;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return Phi(Loop,0,(Phi_sum+.i));", code.print());
         assertEquals("45", Eval2.eval(code, 10));
     }
@@ -549,7 +549,7 @@ for( int j=0; j<nprimes; j++ )
     rez[j] = primes[j];
 return rez;
 """);
-        code.parse().opto().typeCheck().GCM();
+        code.driver(CodeGen.Phase.Schedule);
         assertEquals("return [int];", code.print());
         assertEquals("int[ 2,3,5,7,11,13,17,19]",Eval2.eval(code, 20));
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter18Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter18Test.java
index 7c0af028..6282f26c 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter18Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter18Test.java
@@ -91,7 +91,7 @@ public class Chapter18Test {
              "No common type amongst int and reference"}
         } ) {
             try {
-                new CodeGen(test[0]).parse().opto().typeCheck();
+                new CodeGen(test[0]).parse().iter().typeCheck();
                 fail("Expected incompatible return types: " + test[0]);
             } catch( Parser.ParseException e ) {
                 assertEquals(test[1],e.getMessage());
@@ -140,7 +140,7 @@ while(arg--) {
 }
 return a;
 """);
-        code.parse().opto().typeCheck().GCM();
+        code.driver(CodeGen.Phase.Schedule);
         assertEquals("return Phi(Loop,1,Phi(Loop,2,Phi_a));", code._stop.toString());
         assertEquals("1", Eval2.eval(code,  0));
         assertEquals("2", Eval2.eval(code,  1));
@@ -157,7 +157,7 @@ return a;
 {int -> int}? x2 = null; // null function ptr
 return x2;
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return null;", code._stop.toString());
         assertEquals("null", Eval2.eval(code, 0 ) );
     }
@@ -170,7 +170,7 @@ return x2;
     x*x;
 };
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return (Parm_x(sq,int)*x);", code._stop.toString());
         //assertEquals("{ int -> int #1}", Eval2.eval(code, 3));
     }
@@ -197,7 +197,7 @@ return sq(arg)+sq(3);
 int cnt=1;
 return { -> cnt; };
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("Variable 'cnt' is out of function scope and must be a final constant",e.getMessage()); }
     }
 
@@ -209,7 +209,7 @@ return { -> cnt; };
 val cnt=2;
 return { -> cnt; }();
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("return 2;", code._stop.toString());
         assertEquals("2", Eval2.eval(code, 0));
     }
@@ -222,7 +222,7 @@ return { -> cnt; }();
 var fcn = arg ? { int x -> x*x; } : { int x -> x+x; };
 return fcn(3);
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("Stop[ return #2; return (Parm_x($fun1,int,3)*x); return (Parm_x($fun2,int,3)<<1); ]", code._stop.toString());
         assertEquals("6", Eval2.eval(code, 0));
         assertEquals("9", Eval2.eval(code, 1));
@@ -232,7 +232,7 @@ return fcn(3);
     @Test
     public void testFcn5() {
         CodeGen code = new CodeGen("val fact = { int x -> x <= 1 ? 1 : x*fact(x-1); }; return fact(arg);");
-        code.parse().opto().typeCheck();
+        code.parse().iter().typeCheck();
         assertEquals("Stop[ return #2; return Phi(Region,1,(Parm_x(fact,int,arg,(x-1))*#2)); ]", code._stop.toString());
         assertEquals( "1", Eval2.eval(code, 0));
         assertEquals( "1", Eval2.eval(code, 1));
@@ -249,7 +249,7 @@ struct S { int i; };
 val newS = { int x -> return new S { i=x; }; };
 return newS(1).i;
 """);
-        code.parse().opto().typeCheck().GCM();
+        code.driver(CodeGen.Phase.Schedule);
         assertEquals("return 1;", code._stop.toString());
         assertEquals("1", Eval2.eval(code,  0));
     }
@@ -264,7 +264,7 @@ val f = {->1;};
 val g = {->2;};
 return 2;
 """);
-        code.parse().opto().typeCheck().GCM();
+        code.driver(CodeGen.Phase.Schedule);
         assertEquals("Stop[ return 1; return 1; return 2; ]", code._stop.toString());
         assertEquals("1", Eval2.eval(code,  0));
     }
@@ -342,7 +342,7 @@ for(;;) {
 }
 return 1;
 """);
-        try { code.parse().opto(); fail(); }
+        try { code.parse().iter(); fail(); }
         catch( Exception e ) { assertEquals("No active loop for a break or continue",e.getMessage()); }
     }
 
@@ -357,7 +357,7 @@ for(;;) {
     i2i = null;
 }
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Might be null calling { int -> int #1}?",e.getMessage()); }
     }
 
@@ -369,7 +369,7 @@ for(;;) {
 val f = { int i, int j -> return i+j; };
 return f();
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Expecting 2 arguments, but found 0",e.getMessage()); }
     }
 
@@ -387,7 +387,7 @@ for(;;) {
     if (s) return s.x;
 }
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("Accessing unknown field 'x' from '*S'",e.getMessage()); }
     }
 
@@ -400,7 +400,7 @@ val g = { ->
 };
 return 0;
 """);
-        try { code.parse().opto().typeCheck(); fail(); }
+        try { code.parse().iter().typeCheck(); fail(); }
         catch( Exception e ) { assertEquals("No defined return type",e.getMessage()); }
     }
 
@@ -414,7 +414,7 @@ val is_even = { int x -> x ? is_odd (x-1) : true ; };
 val is_odd  = { int x -> x ? is_even(x-1) : false; };
 return is_even(arg);
 """);
-        code.parse().opto();
+        code.parse().iter();
         assertEquals("Stop[ return is_even( arg); return Phi(Region,Phi(Region,is_even( ((Parm_x(is_even,int,arg,Sub)-1)-1)),0),1); ]", code._stop.toString());
         assertEquals("1", Eval2.eval(code, 0));
         assertEquals("0", Eval2.eval(code, 1));
@@ -436,7 +436,7 @@ for(;;) {
     if (s) return s.f;
 }
 """);
-        code.parse().opto().typeCheck().GCM();
+        code.driver(CodeGen.Phase.Schedule);
         assertEquals("return 0;", code._stop.toString());
         assertEquals("0", Eval2.eval(code,  0));
     }
@@ -488,7 +488,7 @@ return p.coffee_count;
         """;
 
     @Test public void testCallMemory() {
-        var code = new CodeGen(CALL_MEMORY).parse().opto();
+        var code = new CodeGen(CALL_MEMORY).parse().iter();
         int[] calls={0};
         code._stop.walk(n -> { if(n instanceof CallNode) calls[0]++; return null; });
         assertEquals(2,calls[0]); // Keep real calls so this checks scheduling across them.
@@ -517,7 +517,7 @@ return p.coffee_count;
         """;
 
     @Test public void testRecursiveMemory() {
-        var code = new CodeGen(RECURSIVE_MEMORY).parse().opto();
+        var code = new CodeGen(RECURSIVE_MEMORY).parse().iter();
         for(int n=0; n<7; n++)
             assertEquals(Long.toString((10L*(n+1)+n*(n+1)/2)*100+10+n),Eval2.eval(code,n));
     }
@@ -535,7 +535,7 @@ return p.coffee_count;
         """;
 
     @Test public void testInlineAllocationMemory() {
-        var code = new CodeGen(INLINE_MEMORY).parse().opto();
+        var code = new CodeGen(INLINE_MEMORY).parse().iter();
         assertNull(code._stop.walk(n -> n instanceof CallNode ? n : null));
         assertEquals("400944",Eval2.eval(code,0));
         assertEquals("420946",Eval2.eval(code,2));
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter19Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter19Test.java
index c53a8e11..2027dbd6 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter19Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter19Test.java
@@ -39,7 +39,7 @@ public class Chapter19Test {
         CodeGen code = new CodeGen("""
 return 0;
 """);
-        code.parse().opto().typeCheck();
+        code.parse().iter().typeCheck();
         assertEquals("return 0;", code._stop.toString());
         assertEquals("0", Eval2.eval(code,  2));
     }
@@ -48,7 +48,7 @@ return 0;
     @Test
     public void testString() throws IOException {
         String src = Files.readString(Path.of("src/test/java/com/seaofnodes/simple/progs/stringHash.smp"));
-        CodeGen code = new CodeGen(src).parse().opto().typeCheck().GCM().localSched();
+        CodeGen code = new CodeGen(src).driver(CodeGen.Phase.LocalSched);
         assertEquals("Stop[ return Phi(Region,._hashCode,Phi(Region,123456789,Phi(Loop,0,(.[]+((Phi_hash<<5)-Phi_hash))))); return Phi(Region,1,0,0,1); ]", code._stop.toString());
         //assertEquals("-4898613127354160978", Eval2.eval(code,  2));
     }
@@ -357,11 +357,11 @@ return sq(arg) + sq(3);
         for( String cpu : new String[]{"x86_64_v2","riscv","arm"} )
             for( String src : new String[]{Chapter10Test.NESTED_MEMORY,Chapter16Test.CONSTRUCTOR_MEMORY,
                                           Chapter18Test.CALL_MEMORY,Chapter18Test.RECURSIVE_MEMORY} ) {
-                var code = new CodeGen(src).parse().opto().typeCheck();
+                var code = new CodeGen(src).parse().iter().typeCheck();
                 var aliases = memoryPhis(code._stop);
-                code.loopTree().instSelect(cpu,"SystemV");
+                code.loopTree().driver(CodeGen.Phase.InstSelect,cpu,"SystemV");
                 assertEquals(aliases,memoryPhis(code._stop));
-                code.GCM().localSched();
+                code.driver(CodeGen.Phase.Schedule).localSched();
                 code._stop.walk(n -> {
                     if( !(n instanceof CFGNode) && !(n instanceof ProjNode) )
                         assertTrue(n.in(0) instanceof CFGNode);
@@ -413,7 +413,7 @@ return sq(arg) + sq(3);
             int before = p.x + arg;
             a.x=33; b.x=44;
             return before;
-            """).parse().opto().typeCheck().loopTree().instSelect("x86_64_v2","SystemV").GCM().localSched();
+            """).driver(CodeGen.Phase.LocalSched,"x86_64_v2","SystemV");
         var read = (MemOpNode)code._stop.walk(n ->
             n instanceof com.seaofnodes.simple.node.cpus.x86_64_v2.AddMemX86 ? n : null);
         assertNotNull(read);
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter20Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter20Test.java
index e63fb32e..93d8caae 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter20Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter20Test.java
@@ -9,6 +9,10 @@ import org.junit.Test;
 import static org.junit.Assert.*;
 
 public class Chapter20Test {
+    @Test public void testFlagSpills() throws Exception {
+        com.seaofnodes.simple.codegen.FlagSpillTestSupport.run();
+    }
+
     @Test public void testAllocatorTopPhi() { com.seaofnodes.simple.codegen.RegAllocTestSupport.topPhi(); }
 
     @Test public void testNeverReturnAllocation() {
@@ -69,7 +73,7 @@ return f(s);
         // The entry of g can disappear before its Return is folded into f.
         // Exercise both worklist orders; deleting the entry does not kill 123.
         for( int seed=0; seed<64; seed++ ) {
-            CodeGen code = new CodeGen(src,com.seaofnodes.simple.type.TypeInteger.BOT,seed).parse().opto().typeCheck();
+            CodeGen code = new CodeGen(src,com.seaofnodes.simple.type.TypeInteger.BOT,seed).parse().iter().typeCheck();
             assertEquals("seed "+seed, "123", Eval2.eval(code,0));
         }
     }
@@ -93,7 +97,7 @@ return f(s);
     @Test
     public void testJig() {
         CodeGen code = new CodeGen("return 0;");
-        code.parse().opto().typeCheck();
+        code.parse().iter().typeCheck();
         assertEquals("return 0;", code._stop.toString());
         assertEquals("0", Eval2.eval(code,  2));
     }
@@ -339,7 +343,7 @@ for( int pc = 0; pc < program#; pc++ ) {
 return output;
 """;
         testTarget(src,"x86_64_v2", "SystemV",40,null);
-        testTarget(src,"riscv"    , "SystemV",146,null);
+        testTarget(src,"riscv"    , "SystemV",42,null);
         testTarget(src,"arm"      , "SystemV",34,null);
         //assertEquals("Hello World!\n", Eval2.eval(code, 0, 10000));
     }
@@ -407,8 +411,7 @@ return a;
             for( String src : new String[]{Chapter10Test.NESTED_MEMORY,Chapter16Test.CONSTRUCTOR_MEMORY,
                                           Chapter18Test.CALL_MEMORY,Chapter18Test.RECURSIVE_MEMORY,
                                           "int[] !a = new int[3]; a[arg]=arg; return a[0]+1;"} ) {
-                var code = new CodeGen(src).parse().opto().typeCheck().loopTree()
-                    .instSelect(cpu,"SystemV").GCM().localSched().regAlloc();
+                var code = new CodeGen(src).driver(CodeGen.Phase.RegAlloc,cpu,"SystemV");
                 com.seaofnodes.simple.codegen.RegAllocTestSupport.checkRegisters(code);
                 code._stop.walk(n -> {
                     // Whole memory, precise Phis, and memory projections need no register.
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Chapter21Test.java b/chapter21/src/test/java/com/seaofnodes/simple/Chapter21Test.java
index a51ae3bc..b19d1bb5 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Chapter21Test.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Chapter21Test.java
@@ -27,7 +27,7 @@ public class Chapter21Test {
                 else if(i==4) b.x=40;
             }
             return sum;
-            """).parse().opto();
+            """).parse().iter();
         // The i==2 read still sees 10: three reads of 10, then three of 30.
         assertEquals("120",Eval2.eval(code,0));
         assertEquals("60",Eval2.eval(code,1));
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Compile.java b/chapter21/src/test/java/com/seaofnodes/simple/Compile.java
index 088055f9..f3ca1905 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Compile.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Compile.java
@@ -16,7 +16,7 @@ public class Compile {
     private static Process run(String simple, String c) throws IOException, InterruptedException {
         boolean USE_WSL = System.getProperty("os.name").startsWith("Windows");
         CodeGen code = new CodeGen(simple);
-        code.parse().opto().typeCheck().instSelect("x86_64_v2", "SystemV").GCM().localSched().regAlloc().encode();
+        code.driver(CodeGen.Phase.Encoding,"x86_64_v2","SystemV");
         if (c != null) for (var n:code._start._outputs) if (n instanceof FunNode f && f._name.equals("main")) f._name = "simple_main";
         code.exportELF("test.o");
         var params = new ArrayList<String>();
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/CompoundAssignmentTest.java b/chapter21/src/test/java/com/seaofnodes/simple/CompoundAssignmentTest.java
index 48f739cf..8131c95f 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/CompoundAssignmentTest.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/CompoundAssignmentTest.java
@@ -88,7 +88,7 @@ public class CompoundAssignmentTest {
     }
 
     private static void check(String src, long arg, long expected) {
-        var code = new CodeGen(src).parse().opto().typeCheck();
+        var code = new CodeGen(src).parse().iter().typeCheck();
         assertEquals(src, Long.toString(expected), Eval2.eval(code, arg));
     }
 }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Eval2.java b/chapter21/src/test/java/com/seaofnodes/simple/Eval2.java
index a855350e..f39750c2 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Eval2.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Eval2.java
@@ -116,10 +116,7 @@ public abstract class Eval2 {
     public static String eval( CodeGen code, long arg, int timeout ) {
         SB trace = null; // new SB(); // TRACE, set to null for off, new SB() for on
         // Force local scheduling phase
-        if( code._phase.ordinal() < CodeGen.Phase.TypeCheck .ordinal() )  code.typeCheck();
-        if( code._phase.ordinal() < CodeGen.Phase.LoopTree  .ordinal() )  code.loopTree();
-        if( code._phase.ordinal() < CodeGen.Phase.Schedule  .ordinal() )  code.GCM();
-        if( code._phase.ordinal() < CodeGen.Phase.LocalSched.ordinal() )  code.localSched();
+        code.driver(CodeGen.Phase.LocalSched);
         // Set global, so don't have to pass everywhere
         CODE = code;
 
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Main.java b/chapter21/src/test/java/com/seaofnodes/simple/Main.java
index 589c043b..ca1d0b5b 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Main.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Main.java
@@ -11,7 +11,7 @@ public class Main {
         String src = Files.readString(Path.of(args[0]));
         int arg = Integer.valueOf(args[1]);
         CodeGen code = new CodeGen(src);
-        code.parse().opto().typeCheck().GCM().localSched();
+        code.driver(CodeGen.Phase.LocalSched);
         System.out.println(code._stop);
         System.out.println(Eval2.eval(code,arg,100000));
     }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/MemoryEncodingTest.java b/chapter21/src/test/java/com/seaofnodes/simple/MemoryEncodingTest.java
index 1ce2cd34..7cdf217b 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/MemoryEncodingTest.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/MemoryEncodingTest.java
@@ -65,7 +65,7 @@ public class MemoryEncodingTest {
             """
         };
         for( int p=0; p<sources.length; p++ ) {
-            CodeGen code=new CodeGen(sources[p]).parse().opto();
+            CodeGen code=new CodeGen(sources[p]).parse().iter();
             if( p==0 ) assertNull(code._stop.walk(n ->
                 n instanceof LoadNode ld && ld.ptr() instanceof ProjNode ptr &&
                 ptr.in(0) instanceof NewNode nn &&
@@ -158,7 +158,7 @@ public class MemoryEncodingTest {
                 }
                 return null;
             }));
-            code.instSelect(target,"SystemV").GCM().localSched().regAlloc();
+            code.driver(CodeGen.Phase.RegAlloc,target,"SystemV");
             RegAllocTestSupport.checkRegisters(code);
             code.encode();
         }
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/MutabilityTest.java b/chapter21/src/test/java/com/seaofnodes/simple/MutabilityTest.java
index 7ae80314..baf4e252 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/MutabilityTest.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/MutabilityTest.java
@@ -6,12 +6,12 @@ import com.seaofnodes.simple.codegen.CodeGen;
 
 public class MutabilityTest {
     private static void ok(String src, String result) {
-        var code = new CodeGen(src).parse().opto().typeCheck();
+        var code = new CodeGen(src).parse().iter().typeCheck();
         assertEquals(result,Eval2.eval(code,0));
     }
     private static void bad(String src) {
         try {
-            new CodeGen(src).parse().opto().typeCheck();
+            new CodeGen(src).parse().iter().typeCheck();
         } catch( RuntimeException expected ) {
             String msg = expected.getMessage();
             assertTrue("Expected a permission error, got: "+expected,
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/PhaseTest.java b/chapter21/src/test/java/com/seaofnodes/simple/PhaseTest.java
new file mode 100644
index 00000000..d890a537
--- /dev/null
+++ b/chapter21/src/test/java/com/seaofnodes/simple/PhaseTest.java
@@ -0,0 +1,42 @@
+package com.seaofnodes.simple;
+
+import com.seaofnodes.graph.GraphObserver;
+import com.seaofnodes.simple.codegen.CodeGen;
+import com.seaofnodes.simple.codegen.CodeGen.Phase;
+import com.seaofnodes.simple.node.Node;
+import java.util.ArrayList;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class PhaseTest {
+    private static final String SRC = "int sum=0; for(int i=0; i<arg; i++) sum+=i; return sum;";
+
+    @Test public void testStopAndResume() {
+        ArrayList<String> phases = new ArrayList<>();
+        CodeGen code = new CodeGen(SRC);
+        code._obs = new GraphObserver<Node>() {
+            @Override public void before(Node n) {}
+            @Override public void after(Node n, Node repl, boolean applied) {}
+            @Override public void phase(String phase) { phases.add(phase); }
+        };
+        code.driver(Phase.Iter);
+        assertEquals(Phase.Iter,code._phase);
+        assertEquals("Parse Iter",String.join(" ",phases));
+        code.driver(Phase.TypeCheck);
+        code.driver(Phase.TypeCheck); // Repeated requests do not rerun phases.
+        code.driver(Phase.LoopTree);
+        assertEquals(Phase.LoopTree,code._phase);
+        code.driver(Phase.LocalSched);
+        assertEquals("Parse Iter TypeCheck LoopTree Schedule LocalSched",String.join(" ",phases));
+        assertEquals("45",Eval2.eval(code,10));
+    }
+
+    @Test public void testResumeMachineCompilation() {
+        CodeGen staged = new CodeGen(SRC).driver(Phase.TypeCheck);
+        staged.driver(Phase.InstSelect,"x86_64_v2","SystemV");
+        staged.driver(Phase.Encoding,"x86_64_v2","SystemV");
+        byte[] bits = staged._encoding._bits.toByteArray();
+        CodeGen direct = new CodeGen(SRC).driver(Phase.Encoding,"x86_64_v2","SystemV");
+        assertArrayEquals(bits,direct._encoding._bits.toByteArray());
+    }
+}
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/SchedulerTest.java b/chapter21/src/test/java/com/seaofnodes/simple/SchedulerTest.java
index 3b19857a..a388d8c3 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/SchedulerTest.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/SchedulerTest.java
@@ -11,6 +11,26 @@ import static org.junit.Assert.assertEquals;
 
 public class SchedulerTest {
 
+    @Test public void testMemoryPhiAcrossSiblingBranch() {
+        // The inner memory Phi defines the load's memory. The outer Phi's
+        // other input comes from a branch not dominated by that definition.
+        String src = """
+            struct S { int x; }; !S !a=new S; !S !b=new S;
+            a.x=10; b.x=20; !S !p=a; if(arg>3) p=b;
+            int result=0;
+            if(arg>0) {
+                if(arg>2) p.x=30; else p.x=40;
+                result=a.x;
+            }
+            return result;
+            """;
+        var code = new CodeGen(src).driver(CodeGen.Phase.LocalSched);
+        long[] expected = {0,40,40,30,10,10};
+        for( int arg=0; arg<expected.length; arg++ )
+            assertEquals(Long.toString(expected[arg]),Eval2.eval(code,arg));
+    }
+
+
     private static void assertObj(Object obj, String name, Object... fields) {
         assertEquals(Evaluator.Obj.class, obj.getClass());
         var o = (Evaluator.Obj)obj;
@@ -31,7 +51,7 @@ int i=v.f;
 if (arg) v.f=1;
 return i;
 """);
-        code.parse();//.opto();
+        code.parse();//.iter();
         var eval = new Evaluator(code._stop);
         assertEquals(2L, eval.evaluate(1, 10));
     }
@@ -46,7 +66,7 @@ struct S {
 if(arg) v0.f=1;
 return v0;
 """);
-        code.parse().opto();
+        code.parse().iter();
         var eval = new Evaluator(code._stop);
         assertObj(eval.evaluate(0, 10), "S", 0L);
         assertObj(eval.evaluate(1, 10), "S", 1L);
@@ -65,7 +85,7 @@ i=v.f;
 if (arg) v.f=1;
 return i;
 """);
-        code.parse().opto();
+        code.parse().iter();
         var eval = new Evaluator(code._stop);
         assertEquals(2L, eval.evaluate(0, 10));
         assertEquals(2L, eval.evaluate(1, 10));
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/Simple.java b/chapter21/src/test/java/com/seaofnodes/simple/Simple.java
index 705b086a..c496ea90 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/Simple.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/Simple.java
@@ -1,6 +1,7 @@
 package com.seaofnodes.simple;
 
 import com.seaofnodes.simple.codegen.CodeGen;
+import com.seaofnodes.simple.codegen.CodeGen.Phase;
 import com.seaofnodes.simple.print.*;
 
 import java.nio.file.Files;
@@ -13,20 +14,20 @@ import java.util.stream.Collectors;
 public class Simple {
     public static final String PORTS = "com.seaofnodes.simple.node.cpus";
 
-    static final int DUMP_AFTER_PARSE        = 1<<0;
-    static final int DUMP_AFTER_OPTO         = 1<<1;
-    static final int DUMP_AFTER_TYPE_CHECK   = 1<<2;
-    static final int DUMP_AFTER_LOOP_TREE    = 1<<3;
-    static final int DUMP_AFTER_INSTR_SELECT = 1<<4;
-    static final int DUMP_AFTER_GCM          = 1<<5;
-    static final int DUMP_AFTER_LOCAL_SCHED  = 1<<6;
-    static final int DUMP_AFTER_REG_ALLOC    = 1<<7;
-    static final int DUMP_AFTER_ENCODE       = 1<<8;
+    static final int DUMP_AFTER_PARSE = 1<<Phase.Parse.ordinal();
+    static final int DUMP_AFTER_ITER = 1<<Phase.Iter.ordinal();
+    static final int DUMP_AFTER_TYPE_CHECK = 1<<Phase.TypeCheck.ordinal();
+    static final int DUMP_AFTER_LOOP_TREE = 1<<Phase.LoopTree.ordinal();
+    static final int DUMP_AFTER_INSTR_SELECT = 1<<Phase.InstSelect.ordinal();
+    static final int DUMP_AFTER_GCM = 1<<Phase.Schedule.ordinal();
+    static final int DUMP_AFTER_LOCAL_SCHED = 1<<Phase.LocalSched.ordinal();
+    static final int DUMP_AFTER_REG_ALLOC = 1<<Phase.RegAlloc.ordinal();
+    static final int DUMP_AFTER_ENCODE = 1<<Phase.Encoding.ordinal();
 
     static final int DUMP_FINAL              = 1<<16;
 
     static final int DUMP_PASS_NAME          = 1<<30;
-    static final int DUMP_AFTER_ALL          = 0xff | DUMP_PASS_NAME;
+    static final int DUMP_AFTER_ALL = ((1<<Phase.values().length)-1) | DUMP_PASS_NAME;
 
     static final int DUMP_DISASSEMBLE        = 1<<31;
 
@@ -39,15 +40,16 @@ public class Simple {
 simple <input-file> [options] [--eval|--run ...]
 Options:
   --dump                     - dump final intermediate representation
-  --dump-after-parse         - dump intermediate representation after parse
-  --dump-after-opto          - dump intermediate representation after opto pass
-  --dump-after-type-check    - dump intermediate representation after type check pass
-  --dump-after-instr-select  - dump intermediate representation after instrution selection pass
-  --dump-after-gcm           - dump intermediate representation after GCM pass
-  --dump-after-local-sched   - dump intermediate representation after local scheduling pass
-  --dump-after-reg-alloc     - dump intermediate representation after register allocation pass
-  --dump-after-encode        - dump intermediate representation after encoding pass
-  --dump-after-all           - dump intermediate representation after all passes
+  --dump-after-parse           - dump intermediate representation after parsing
+  --dump-after-iter            - dump intermediate representation after Iter
+  --dump-after-type-check      - dump intermediate representation after type checking
+  --dump-after-loop-tree       - dump intermediate representation after loop-tree construction
+  --dump-after-instr-select    - dump intermediate representation after instruction selection
+  --dump-after-gcm             - dump intermediate representation after GCM
+  --dump-after-local-sched     - dump intermediate representation after local scheduling
+  --dump-after-reg-alloc       - dump intermediate representation after register allocation
+  --dump-after-encode          - dump intermediate representation after encoding
+  --dump-after-all             - dump intermediate representation after all passes
   -S                         - dump generated assembler code
   --eval ...                 - evaluate the compiled code in emulator
   --run ...                  - run the compiled code natively
@@ -80,50 +82,19 @@ Options:
     }
 
     static void dump(CodeGen code, int dump, int pass) {
-        if ((dump & pass) != 0) {
-            if ((dump & DUMP_PASS_NAME) != 0) {
-                System.err.println(switch (pass) {
-                    case DUMP_AFTER_PARSE        -> "After Parse:";
-                    case DUMP_AFTER_OPTO         -> "After OPTO:";
-                    case DUMP_AFTER_TYPE_CHECK   -> "After Type Check:";
-                    case DUMP_AFTER_LOOP_TREE    -> "After Loop Tree:";
-                    case DUMP_AFTER_INSTR_SELECT -> "After Instruction Selection:";
-                    case DUMP_AFTER_GCM          -> "After GCM:";
-                    case DUMP_AFTER_LOCAL_SCHED  -> "After Local Scheduling:";
-                    case DUMP_AFTER_REG_ALLOC    -> "After Register Allocation:";
-                    case DUMP_AFTER_ENCODE       -> "After Instruction Encoding:";
-                    case DUMP_FINAL              -> "Final:";
-                    default                      -> throw Utils.TODO();
-                });
-            }
-
-            System.err.println(IRPrinter.prettyPrint(code._stop, 9999));
-
-        }
+        if( (dump & pass)==0 ) return;
+        if( (dump & DUMP_PASS_NAME)!=0 ) System.err.println("Final:");
+        System.err.println(code);
     }
 
     static void print_compilation_times(CodeGen code) {
-        double t, total = 0;
-
-        total += t = code._tParse / 1e3;
-        System.out.println(String.format("Parsing Time:               %.3f sec", t));
-        total += t = code._tOpto / 1e3;
-        System.out.println(String.format("Optimization Time:          %.3f sec", t));
-        total += t = code._tTypeCheck / 1e3;
-        System.out.println(String.format("Type Checking Time:         %.3f sec", t));
-        total += t = code._tLoopTree / 1e3;
-        System.out.println(String.format("Loop Tree Time:             %.3f sec", t));
-        total += t = code._tInsSel / 1e3;
-        System.out.println(String.format("Instruction Selection Time: %.3f sec", t));
-        total += t = code._tGCM / 1e3;
-        System.out.println(String.format("GCM Time:                   %.3f sec", t));
-        total += t = code._tLocal / 1e3;
-        System.out.println(String.format("Local Scheduling Time:      %.3f sec", t));
-        total += t = code._tRegAlloc / 1e3;
-        System.out.println(String.format("Register Allocation Time:   %.3f sec", t));
-        total += t = code._tEncode / 1e3;
-        System.out.println(String.format("Instruction Encoding Time:  %.3f sec", t));
-        System.out.println(String.format("TOTAL COMPILATION TIME:     %.3f sec", total));
+        long total=0;
+        for( Phase phase : Phase.values() ) {
+            long time=code._times[phase.ordinal()];
+            total+=time;
+            System.out.printf("%-24s %.3f sec%n",phase+" Time:",time/1e3);
+        }
+        System.out.printf("TOTAL COMPILATION TIME:   %.3f sec%n",total/1e3);
     }
 
     static String getInput() throws IOException {
@@ -157,16 +128,16 @@ loop:   for (int i = 0; i < args.length; i++) {
                 switch (arg) {
                     case "-":                         use_stdin = true; break;
                     case "--dump":                    dump |= DUMP_FINAL; break;
-                    case "--dump-after-parse":        dump |= DUMP_AFTER_PARSE; break;
-                    case "--dump-after-opto":         dump |= DUMP_AFTER_OPTO; break;
-                    case "--dump-after-type-check":   dump |= DUMP_AFTER_TYPE_CHECK; break;
-                    case "--dump-after-loop-tree":    dump |= DUMP_AFTER_LOOP_TREE; break;
-                    case "--dump-after-instr-select": dump |= DUMP_AFTER_INSTR_SELECT; break;
-                    case "--dump-after-gcm":          dump |= DUMP_AFTER_GCM; break;
-                    case "--dump-after-local-sched":  dump |= DUMP_AFTER_LOCAL_SCHED; break;
-                    case "--dump-after-reg-alloc":    dump |= DUMP_AFTER_REG_ALLOC; break;
-                    case "--dump-after-encode":       dump |= DUMP_AFTER_ENCODE; break;
-                    case "--dump-after-all":          dump |= DUMP_AFTER_ALL; break;
+                    case "--dump-after-parse":          dump |= DUMP_AFTER_PARSE; break;
+                    case "--dump-after-iter":           dump |= DUMP_AFTER_ITER; break;
+                    case "--dump-after-type-check":     dump |= DUMP_AFTER_TYPE_CHECK; break;
+                    case "--dump-after-loop-tree":      dump |= DUMP_AFTER_LOOP_TREE; break;
+                    case "--dump-after-instr-select":   dump |= DUMP_AFTER_INSTR_SELECT; break;
+                    case "--dump-after-gcm":            dump |= DUMP_AFTER_GCM; break;
+                    case "--dump-after-local-sched":    dump |= DUMP_AFTER_LOCAL_SCHED; break;
+                    case "--dump-after-reg-alloc":      dump |= DUMP_AFTER_REG_ALLOC; break;
+                    case "--dump-after-encode":         dump |= DUMP_AFTER_ENCODE; break;
+                    case "--dump-after-all":            dump |= DUMP_AFTER_ALL; break;
                     case "-S":                        dump |= DUMP_DISASSEMBLE; break;
                     case "--eval":                    do_eval = true; first_arg = i + 1; break loop;
                     case "--run":                     do_run = true; first_arg = i + 1; break loop;
@@ -221,36 +192,8 @@ loop:   for (int i = 0; i < args.length; i++) {
         // Compilation pipeline
         CodeGen code = new CodeGen(src);
 
-        code.parse();
-        dump(code, dump, DUMP_AFTER_PARSE);
-
-        code.opto();
-        dump(code, dump, DUMP_AFTER_OPTO);
-
-        code.typeCheck();
-        dump(code, dump, DUMP_AFTER_TYPE_CHECK);
-
-        code.loopTree();
-        dump(code, dump, DUMP_AFTER_LOOP_TREE);
-
-        if (do_codegen) {
-            code.instSelect(PORTS, cpu, abi);
-            dump(code, dump, DUMP_AFTER_INSTR_SELECT);
-        }
-
-        code.GCM();
-        dump(code, dump, DUMP_AFTER_GCM);
-
-        code.localSched();
-        dump(code, dump, DUMP_AFTER_LOCAL_SCHED);
-
-        if (do_codegen) {
-            code.regAlloc();
-            dump(code, dump, DUMP_AFTER_REG_ALLOC);
-
-            code.encode();
-            dump(code, dump, DUMP_AFTER_ENCODE);
-        }
+        code.driver(do_codegen ? Phase.Encoding : Phase.LocalSched,
+                    do_codegen ? cpu : null, abi, dump);
 
         dump(code, dump, DUMP_FINAL);
 
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/codegen/FlagSpillTestSupport.java b/chapter21/src/test/java/com/seaofnodes/simple/codegen/FlagSpillTestSupport.java
new file mode 100644
index 00000000..7054ade4
--- /dev/null
+++ b/chapter21/src/test/java/com/seaofnodes/simple/codegen/FlagSpillTestSupport.java
@@ -0,0 +1,145 @@
+package com.seaofnodes.simple.codegen;
+
+import com.seaofnodes.isa.X86;
+import com.seaofnodes.isa.eval.EvalArm64;
+import com.seaofnodes.simple.TestC;
+import com.seaofnodes.simple.node.*;
+import com.seaofnodes.simple.node.cpus.arm.arm;
+import com.seaofnodes.simple.node.cpus.x86_64_v2.x86_64_v2;
+import com.seaofnodes.simple.type.TypeInteger;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import static org.junit.Assert.*;
+
+// Force overlapping flags, then force every GPR unavailable across the gap.
+public class FlagSpillTestSupport {
+    private static class Op extends MachConcreteNode {
+        final RegMask _def, _use, _kill;
+        Op(RegMask def,RegMask use,RegMask kill,Node... ins) {
+            super(ins); _def=def; _use=use; _kill=kill; _type=TypeInteger.BOT;
+        }
+        @Override public String op() { return "flagsProbe"; }
+        @Override public RegMask outregmap() { return _def; }
+        @Override public RegMask regmap(int i) { return _use; }
+        @Override public RegMask killmap() { return _kill; }
+        @Override public void encoding(Encoding enc) { throw new AssertionError(); }
+    }
+
+    public static void run() throws Exception {
+        StringBuilder asm = new StringBuilder(".text\n");
+        for( boolean isArm : new boolean[]{false,true} )
+            for( boolean stack : new boolean[]{false,true} ) {
+                CodeGen code = new CodeGen("");
+                code._callingConv="SystemV";
+                code._mach=isArm ? new arm(code) : new x86_64_v2(code);
+                RegionNode block = new RegionNode(null,code._start,code._start);
+                code._cfg.add(block);
+                int flagsReg=isArm ? 64 : 32, firstStack=flagsReg+1;
+                RegMask flags = new RegMask(flagsReg);
+                Op def = new Op(flags,null,null,block);
+                RegMask killMask = stack ? (isArm ? new RegMask(0x7fffffffL,1L) : new RegMask(0x1ffffffefL)) : flags;
+                Op kill = new Op(null,null,killMask,block);
+                new Op(null,flags,null,block,def);
+                RegAlloc alloc = new RegAlloc(code);
+                alloc.regAlloc();
+
+                int splits=0, slots=0;
+                for( Node n : block._outputs ) if( n instanceof SplitNode split ) {
+                    int dst=alloc.regnum(n), src=alloc.regnum(n.in(1));
+                    assertTrue(split.outregmap().test(dst));
+                    assertTrue(split.regmap(1).test(src));
+                    for( int fp=isArm ? 32 : 16; fp<flagsReg; fp++ ) {
+                        assertFalse("FP output allowed",split.outregmap().test(fp));
+                        assertFalse("FP input allowed",split.regmap(1).test(fp));
+                    }
+                    if( dst>=firstStack || src>=firstStack ) slots++;
+                    splits++;
+                }
+                assertTrue(splits>=2);
+                assertEquals(stack,slots!=0);
+
+                code._regAlloc=alloc;
+                FunNode frame = new FunNode(null,null,(Node)null);
+                Encoding save = new Encoding(code), restore = new Encoding(code);
+                save._fun=restore._fun=frame;
+                Encoding enc = save;
+                for( Node n : block._outputs ) {
+                    if( n==kill ) enc=restore;
+                    if( n instanceof SplitNode split ) split.encoding(enc);
+                }
+                if( isArm ) {
+                    for( int bits=0; bits<16; bits++ ) {
+                        EvalArm64 cpu = new EvalArm64(new byte[1024],512);
+                        System.arraycopy(save._bits.buf(),0,cpu._buf,0,save._bits.size());
+                        System.arraycopy(restore._bits.buf(),0,cpu._buf,save._bits.size(),restore._bits.size());
+                        cpu.N=(bits&8)!=0; cpu.Z=(bits&4)!=0; cpu.C=(bits&2)!=0; cpu.V=(bits&1)!=0;
+                        assertEquals(0,cpu.step(save._bits.size()/4));
+                        cpu.N=cpu.Z=cpu.C=cpu.V=false;
+                        if( stack ) for( int r=0; r<31; r++ ) cpu.regs[r]=0;
+                        assertEquals(0,cpu.step(restore._bits.size()/4));
+                        assertEquals((bits&8)!=0,cpu.N); assertEquals((bits&4)!=0,cpu.Z);
+                        assertEquals((bits&2)!=0,cpu.C); assertEquals((bits&1)!=0,cpu.V);
+                        assertEquals(512,cpu.regs[31]);
+                    }
+                } else {
+                    Encoding body = new Encoding(code);
+                    int[] preserved={3,5,6,7,12,13,14,15};
+                    for( int r : preserved ) pushPop(body,r,false);
+                    X86.imm(body,0x81,5,4,16); // Reserve spill storage.
+                    X86.flags(body,TestC.OS.startsWith("Windows") ? 1 : 7,true);
+                    bytes(body,save);
+                    if( stack ) {
+                        for( int r=0; r<16; r++ ) if( r!=4 ) X86.constant(body,r,0);
+                    } else X86.reg(body,0x39,0,0); // CMP rax,rax clobbers only flags.
+                    bytes(body,restore);
+                    X86.flags(body,0,false); // Return the restored flags.
+                    X86.imm(body,0x81,0,4,16);
+                    for( int i=preserved.length-1; i>=0; i-- ) pushPop(body,preserved[i],true);
+                    X86.ret(body);
+                    String name=stack ? "flags_stack" : "flags_reg";
+                    asm.append(".globl ").append(name).append('\n').append(name).append(":\n.byte ");
+                    for( int i=0; i<body._bits.size(); i++ ) {
+                        if( i!=0 ) asm.append(',');
+                        asm.append(body._bits.buf()[i]&255);
+                    }
+                    asm.append('\n');
+                }
+            }
+        Path dir=Files.createTempDirectory(Path.of("build/objs"),"flag_spills_");
+        Files.writeString(dir.resolve("flags.s"),asm);
+        Files.writeString(dir.resolve("flags.c"),"""
+            typedef unsigned long long u64;
+            u64 __attribute__((CALL_CONV)) flags_reg(u64);
+            u64 __attribute__((CALL_CONV)) flags_stack(u64);
+            int main(void) {
+                const int positions[]={0,2,4,6,7,11};
+                for(int bits=0; bits<64; bits++) {
+                    u64 flags=0x202;
+                    for(int i=0;i<6;i++) if(bits&(1<<i)) flags|=1ULL<<positions[i];
+                    if((flags_reg(flags)&0x8d5)!=(flags&0x8d5)) return 1;
+                    if((flags_stack(flags)&0x8d5)!=(flags&0x8d5)) return 2;
+                }
+                return 0;
+            }
+            """);
+        String exe=dir.resolve(TestC.OS.startsWith("Windows") ? "flags.exe" : "flags").toString();
+        exec("gcc","-DCALL_CONV="+(TestC.OS.startsWith("Windows") ? "ms_abi" : "sysv_abi"),
+            "-o",exe,dir.resolve("flags.c").toString(),dir.resolve("flags.s").toString());
+        exec(exe);
+    }
+
+    private static void exec(String... command) throws Exception {
+        Process process = new ProcessBuilder(command).redirectErrorStream(true).start();
+        String output = new String(process.getInputStream().readAllBytes(),java.nio.charset.StandardCharsets.UTF_8);
+        assertEquals(String.join(" ",command)+"\n"+output,0,process.waitFor());
+    }
+
+    private static void bytes(Encoding dst,Encoding src) {
+        for( int i=0; i<src._bits.size(); i++ ) dst.add1(src._bits.buf()[i]&255);
+    }
+    private static void pushPop(Encoding enc,int reg,boolean pop) {
+        if( reg>=8 ) enc.add1(0x41);
+        enc.add1((pop ? 0x58 : 0x50)+(reg&7));
+    }
+}
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/codegen/RegAllocTestSupport.java b/chapter21/src/test/java/com/seaofnodes/simple/codegen/RegAllocTestSupport.java
index 739eba3b..06ed40d9 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/codegen/RegAllocTestSupport.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/codegen/RegAllocTestSupport.java
@@ -22,6 +22,11 @@ public class RegAllocTestSupport {
         @Override public Node copy() { return new Op(_def,_use,_kill,_clone,(Node)null); }
         @Override public void encoding(Encoding enc) { throw new AssertionError(); }
     }
+    private static SplitNode testSplit(CodeGen code, MachNode def) {
+        LRG lrg = new LRG((short)0);
+        lrg._machDef = def;
+        return code._mach.split("test",(byte)0,lrg);
+    }
     private static CodeGen graph() {
         CodeGen code = new CodeGen("");
         code._mach = new x86_64_v2(code);
@@ -168,11 +173,11 @@ public class RegAllocTestSupport {
             RegAlloc alloc = new RegAlloc(code);
             Op def = new Op(A,null,null,false,code._start);
             alloc.newLRG(def)._reg=0;
-            SplitNode hi = code._mach.split("test",(byte)0,null);
+            SplitNode hi = testSplit(code,def);
             hi.setDef(0,code._start); hi.setDef(1,def);
             alloc.newLRG(hi)._reg=1;
             new Op(null,null,killed ? A : null,false,code._start);
-            SplitNode lo = code._mach.split("test",(byte)0,null);
+            SplitNode lo = testSplit(code,hi);
             lo.setDef(0,code._start); lo.setDef(1,hi);
             alloc.newLRG(lo)._reg=0;
             Op use = new Op(null,A,null,false,code._start,lo);
@@ -186,7 +191,7 @@ public class RegAllocTestSupport {
         uncolored.newLRG(def0)._mask=A;
         Op fixed = new Op(A,null,null,false,pre._start);
         uncolored.newLRG(fixed)._mask=A;
-        SplitNode copy = pre._mach.split("test",(byte)0,null);
+        SplitNode copy = testSplit(pre,def0);
         copy.setDef(0,pre._start); copy.setDef(1,def0);
         assertFalse("A fixed definition clobbers even before coloring",uncolored.sameBlockNoClobber(copy));
     }
@@ -199,7 +204,7 @@ public class RegAllocTestSupport {
             RegAlloc alloc = new RegAlloc(code);
             Op def = new Op(A,null,null,false,code._start);
             alloc.newLRG(def)._reg=0;
-            SplitNode copy = code._mach.split("test",(byte)0,null);
+            SplitNode copy = testSplit(code,def);
             copy.setDef(0,code._start); copy.setDef(1,def);
             alloc.newLRG(copy)._reg=1;
             if( kind==3 ) new Op(null,null,A,false,code._start);
@@ -222,7 +227,7 @@ public class RegAllocTestSupport {
             CodeGen code = graph();
             RegAlloc alloc = new RegAlloc(code);
             Op def = new Op(new RegMask(3L),null,null,false,code._start);
-            SplitNode copy = code._mach.split("test",(byte)0,null);
+            SplitNode copy = testSplit(code,def);
             copy.setDef(0,code._start); copy.setDef(1,def);
             Op use = new Op(null,new RegMask(3L),null,false,code._start,copy);
             LRG from = alloc.newLRG(def), to = alloc.newLRG(copy);
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java b/chapter21/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java
index 586924b2..7e8bdbf1 100644
--- a/chapter21/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java
+++ b/chapter21/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java
@@ -133,7 +133,7 @@ public class Fuzzer {
             double parse_nop_ratio = (double)parse_nops/parse_peeps;
             double parse_peeps_per_node = (double)parse_peeps/nids;
 
-            code.opto();
+            code.iter();
             int iter_peeps= code._iter_cnt;
             int iter_nops = code._iter_nop_cnt;
             double iter_nop_ratio = (double)iter_nops/iter_peeps;