New evaluator, more Compiler Driver cleanup

More compiler driver cleanup: Stop only has Return inputs, never an expr.  Drop extra lexical scope.  Linker table moves to CodeGen.
Replace evaluator with a simpler design; I was unable to get the current design to work with the other changes.
Updates to old eval; proper idoms; forced type checks; extensions for Fun, Return, Call, CallEnd.
Accessor find next control to execute.  Accessor for deciding when its ok to optimize a Function/Return, Call/CallEnd, Stop, ScopeMin (merge)
Add local scheduling pass.
IRPrinter uses SB instead of StringBuilder
Spelling SubF comment

23 files changed, 366 insertions, 165 deletions

diff --git a/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java b/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
index 184b6f9c..79d817f8 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
@@ -1,14 +1,13 @@
  package com.seaofnodes.simple;
 
-import com.seaofnodes.simple.Parser;
 import com.seaofnodes.simple.node.*;
 import com.seaofnodes.simple.type.*;
-import java.util.*;
+import java.util.HashMap;
 
 public class CodeGen {
 
     public enum Phase {
-        Parse, Opto, TypeCheck, Schedule, RegAlloc;
+        Parse, Opto, TypeCheck, Schedule, LocalSched, RegAlloc;
     }
     public Phase _phase;
 
@@ -20,6 +19,11 @@ public class CodeGen {
     public StartNode _start;
     public  StopNode _stop ;
 
+
+    // "Linker" mapping from constant TypeFunPtrs to heads of function
+    public final HashMap<TypeFunPtr,FunNode> _linker = new HashMap<>();
+
+
     // Last created CodeGen as a global, for easier debugging prints
     public static CodeGen CODE;
 
@@ -30,6 +34,8 @@ public class CodeGen {
     public CodeGen parse(boolean disable) {
         assert _phase == null;
         _phase = Phase.Parse;
+        _linker.clear();
+
         P = new Parser(_src,_arg);
         Node._disablePeephole = disable;
         _stop = P.parse();
@@ -51,7 +57,7 @@ public class CodeGen {
     }
 
     public CodeGen typeCheck() {
-        assert _phase == Phase.Opto;
+        assert _phase == Phase.Opto || _phase == Phase.Parse;
         _phase = Phase.TypeCheck;
 
         // Type check
@@ -62,8 +68,8 @@ public class CodeGen {
     }
 
 
-    public CodeGen codegen() { return codegen(false); }
-    public CodeGen codegen(boolean show) {
+    public CodeGen GCM() { return GCM(false); }
+    public CodeGen GCM( boolean show) {
         assert _phase == Phase.TypeCheck;
         _phase = Phase.Schedule;
 
@@ -74,14 +80,23 @@ public class CodeGen {
 
         // TODO:
         // loop unroll, peel, RCE, etc
-
         GlobalCodeMotion.buildCFG(_start,_stop);
-
-        _phase = Phase.RegAlloc;
-
         return this;
     }
 
+     public CodeGen localSched() {
+        assert _phase == Phase.Schedule;
+        _phase = Phase.LocalSched;
+        ListScheduler.sched(this);
+        return this;
+     }
+
+
+    // Testing shortcuts
+    Node ctrl() { return _stop.ret().ctrl(); }
+    Node expr() { return _stop.ret().expr(); }
+    String print() { return _stop.print(); }
+
     // Debugging helper
     @Override public String toString() {
         return _stop.p(9999);
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java b/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
index 7ce12091..6b0956ca 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
@@ -54,7 +54,7 @@ public abstract class GlobalCodeMotion {
         visit.set(n._nid);
         // Schedule not-pinned not-CFG inputs before self.  Since skipping
         // Pinned, this never walks the backedge of Phis (and thus spins around
-        // a data-only loop, eventually attempting relying on some pre-visited-
+        // a data-only loop), eventually attempting relying on some pre-visited-
         // not-post-visited data op with no scheduled control.
         for( Node def : n._inputs )
             if( def!=null && !(def instanceof PhiNode) )
@@ -65,7 +65,7 @@ public abstract class GlobalCodeMotion {
             CFGNode early = Parser.START; // Maximally early, lowest idepth
             if( n.in(0) instanceof CFGNode cfg ) early = cfg;
             for( int i=1; i<n.nIns(); i++ )
-                if( n.in(i).cfg0().idepth() > early.idepth() )
+                if( n.in(i)!=null && n.in(i).cfg0().idepth() > early.idepth() )
                     early = n.in(i).cfg0(); // Latest/deepest input
             n.setDef(0,early);              // First place this can go
         }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter18/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 4f422a25..adf67d8b 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -1,7 +1,6 @@
 package com.seaofnodes.simple;
 
 import com.seaofnodes.simple.node.*;
-import java.lang.StringBuilder;
 import java.util.ArrayList;
 import java.util.BitSet;
 import java.util.HashMap;
@@ -11,33 +10,31 @@ public class IRPrinter {
     // Print a node on 1 line, columnar aligned, as:
     // NNID NNAME DDEF DDEF  [[  UUSE UUSE  ]]  TYPE
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
-    public static void printLine( Node n, StringBuilder sb ) {
-        sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
-            sb.append("DEAD\n");
-            return;
-        }
+    public static SB printLine( Node n, SB sb ) {
+        sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
+        if( n._inputs==null )
+            return sb.p("DEAD\n");
         for( Node def : n._inputs )
-            sb.append(def==null ? "____" : "%4d".formatted(def._nid))
+            sb.p(def==null ? "____" : "%4d".formatted(def._nid))
                 // Lazy Phi indicator
-                .append(n instanceof ScopeMinNode && def instanceof ScopeMinNode ? "^" : " ");
+                .p(n instanceof ScopeMinNode && def instanceof ScopeMinNode ? "^" : " ");
         for( int i = n._inputs.size(); i<4; i++ )
-            sb.append("     ");
-        sb.append(" [[  ");
+            sb.p("     ");
+        sb.p(" [[  ");
         for( Node use : n._outputs )
-            sb.append(use==null ? "____ " : "%4d ".formatted(use._nid));
+            sb.p(use==null ? "____ " : "%4d ".formatted(use._nid));
         int lim = 6 - Math.max(n._inputs.size(),4);
         for( int i = n._outputs.size(); i<lim; i++ )
-            sb.append("     ");
-        sb.append(" ]]  ");
-        if( n._type!= null ) sb.append(n._type.str());
-        sb.append("\n");
+            sb.p("     ");
+        sb.p(" ]]  ");
+        if( n._type!= null ) sb.p(n._type.str());
+        return sb.p("\n");
     }
 
-    private static StringBuilder nodeId(StringBuilder sb, Node n) {
-        sb.append("%%%d".formatted(n._nid));
+    private static SB nodeId(SB sb, Node n) {
+        sb.p("%%%d".formatted(n._nid));
         if (n instanceof ProjNode proj) {
-            sb.append(".").append(proj._idx);
+            sb.p(".").p(proj._idx);
         }
         return sb;
     }
@@ -58,19 +55,19 @@ public class IRPrinter {
         for( int i=bfs._lim; i< bfs._bfs.size(); i++ )
             postOrd( bfs._bfs.get(i), null, rpos, visit, bfs._bs);
         // Reverse the post-order walk
-        StringBuilder sb = new StringBuilder();
+        SB sb = new SB();
         boolean gap=false;
         for( int i=rpos.size()-1; i>=0; i-- ) {
             Node n = rpos.get(i);
             if( n instanceof CFGNode || n.isMultiHead() ) {
-                if( !gap ) sb.append("\n"); // Blank before multihead
+                if( !gap ) sb.p("\n"); // Blank before multihead
                 printLine( n, sb );         // Print head
                 while( --i >= 0 ) {
                     Node t = rpos.get(i);
                     if( !t.isMultiTail() ) { i++; break; }
                     printLine( t, sb );
                 }
-                sb.append("\n"); // Blank after multitail
+                sb.p("\n"); // Blank after multitail
                 gap = true;
             } else {
                 printLine( n, sb );
@@ -180,7 +177,7 @@ public class IRPrinter {
             }
         }
         // Print by block with least idepth
-        StringBuilder sb = new StringBuilder();
+        SB sb = new SB();
         Ary<Node> bns = new Ary<>(Node.class);
         while( !ds.isEmpty() ) {
             CFGNode blk = null;
@@ -193,16 +190,15 @@ public class IRPrinter {
             ns.remove(blk);
 
             // Print block header
-            sb.append("%-13.13s".formatted(label(blk)+":"));
-            sb.append( "     ".repeat(4) ).append(" [[  ");
+            sb.p("%-13.13s".formatted(label(blk)+":"));
+            sb.p( "     ".repeat(4) ).p(" [[  ");
             if( blk instanceof StartNode ) ;
             else if( blk instanceof RegionNode || blk instanceof StopNode )
                 for( int i=(blk instanceof StopNode ? 3 : 1); i<blk.nIns(); i++ )
                     label(sb,blk.cfg(i));
             else
                 label(sb,blk.cfg(0));
-            sb.append(" ]]  \n");
-            sb.append(d);
+            sb.p(" ]]  \n");
             printLine(blk,sb);
 
             // Collect block contents that are in the depth limit
@@ -234,20 +230,21 @@ public class IRPrinter {
                         }
                     }
                 }
-            sb.append("\n");
+            sb.p("\n");
         }
         return sb.toString();
     }
 
     private static void _walk( HashMap<Integer,Integer> ds, ArrayList<Node> ns, Node node, int d ) {
         Integer nd = ds.get(node._nid);
-        if( nd!=null && d >= nd ) return; // Been there, done that
+        if( nd!=null && d <= nd ) return; // Been there, done that
         Integer old = ds.put(node._nid,d) ;
         if( old == null )
           ns.add(node);
         if( d == 0 ) return;    // Depth cutoff
         for( Node def : node._inputs )
             if( def != null &&
+                !(node instanceof LoopNode loop && loop.back()==def) &&
                 // Don't walk into or out of functions
                 !(node instanceof CallEndNode && def instanceof ReturnNode) &&
                 !(node instanceof FunNode && def instanceof CallNode) )
@@ -258,12 +255,11 @@ public class IRPrinter {
         if( blk instanceof StartNode ) return "START";
         return (blk instanceof LoopNode ? "LOOP" : "L")+blk._nid;
     }
-    static void label( StringBuilder sb, CFGNode blk ) {
+    static void label( SB sb, CFGNode blk ) {
         if( !blk.blockHead() ) blk = blk.cfg(0);
-        sb.append( "%-9.9s ".formatted( label( blk ) ) );
+        sb.p( "%-9.9s ".formatted( label( blk ) ) );
     }
-    static void printLine( Node n, StringBuilder sb, Ary<Node> bns, int i, HashMap<Integer,Integer> ds, ArrayList<Node> ns ) {
-        sb.append(ds.get(n._nid));
+    static void printLine( Node n, SB sb, Ary<Node> bns, int i, HashMap<Integer,Integer> ds, ArrayList<Node> ns ) {
         printLine( n, sb );
         if( i != -1 ) bns.del(i);
         ds.remove(n._nid);
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/IterPeeps.java b/chapter18/src/main/java/com/seaofnodes/simple/IterPeeps.java
index a321b13e..998bd4f0 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/IterPeeps.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/IterPeeps.java
@@ -182,6 +182,7 @@ public abstract class IterPeeps {
          * True if Node is on the WorkList
          */
         boolean on( E x ) { return _on.get(x._nid); }
+        boolean isEmpty() { return _len==0; }
 
         /**
          * Removes a random Node from the WorkList; null if WorkList is empty
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/ListScheduler.java b/chapter18/src/main/java/com/seaofnodes/simple/ListScheduler.java
new file mode 100644
index 00000000..252a8f8e
--- /dev/null
+++ b/chapter18/src/main/java/com/seaofnodes/simple/ListScheduler.java
@@ -0,0 +1,110 @@
+ package com.seaofnodes.simple;
+
+import com.seaofnodes.simple.IterPeeps.WorkList;
+import com.seaofnodes.simple.node.*;
+import com.seaofnodes.simple.type.*;
+import java.util.*;
+
+public abstract class ListScheduler {
+
+    private static final BitSet VISIT = new BitSet();
+    private static int[] BCNTS;
+    public static void sched( CodeGen code ) {
+        assert VISIT.isEmpty();
+        BCNTS = new int[Node.UID()];
+        walk(code._start,VISIT);
+        VISIT.clear();
+    }
+
+    // Visit all CFG nodes
+    static void walk( CFGNode cfg, BitSet visit ) {
+        if( visit.get(cfg._nid) ) return;
+        visit.set(cfg._nid);
+        local(cfg);
+        for( Node n : cfg._outputs )
+            if( n instanceof CFGNode c )
+                walk(c,visit);
+    }
+
+    // Local schedule this one CFG
+    static void local( CFGNode bb ) {
+        if( !bb.blockHead() || bb instanceof StopNode ) return;
+        Node[] uses = bb._outputs._es;
+        int ulen = bb._outputs._len;
+
+        // Count block-locals
+        for( int i=0; i<ulen; i++ ) {
+            Node use = uses[i];
+            if( use.in(0)==bb && !(use instanceof PhiNode) ) {
+                // Normal basic block member: in(0)==bb and not a Phi
+                for( Node def : use._inputs )
+                    // Has defs from same block?  Then block-local inputs
+                    if( def != null && def.cfg0()==bb ) // Same block?
+                        BCNTS[use._nid]++;     // Raise block-local input count
+            }
+        }
+
+        // Nodes are ready if they are not used by other nodes in this block.
+        // Move ready up front. We will never look at remaining nodes, later
+        // nodes are found by looking at users of ready nodes.
+        int ready = 0;
+        for( int i=0; i<ulen; i++ )
+            if( BCNTS[uses[i]._nid]==0 )   // No block-local inputs?
+                uses[ready++] = uses[i];   // Move into ready set
+        assert ready > 0;
+
+        // Classic list scheduler.  Behind sched is scheduled.  Between sched
+        // and ready have zero counts and are ready to schedule.  Ahead of
+        // ready is undefined.
+        //
+        // As nodes are scheduled, the nodes they use decrement their _bcnt. Once
+        // that gets to 0, all the nodes using it are scheduled, so it's ready.
+        int sched = 0;
+        while( sched < ulen ) {
+            assert sched < ready : "Scheduling nodes that are not ready. Probably due to duplicate nodes in the user list.";
+            int pick = best(uses, sched,ready); // Pick best
+            Node best = uses[pick];       // Swap best to head of ready
+            uses[pick] = uses[sched];
+            uses[sched++] = best; // And move it into the scheduled set
+            // Lower ready count of users
+            for( Node use : best._outputs )
+                if( use instanceof ProjNode ) {
+                    for( Node useuse : use._outputs )
+                        ready = ready(bb,useuse,ready);
+                } else
+                    ready = ready(bb, use,ready);
+        }
+    }
+
+    private static int ready(CFGNode bb, Node use, int ready) {
+        if( use != null && use.in(0)==bb && !(use instanceof PhiNode) ) {
+            assert BCNTS[use._nid] >= 1;
+            if( --BCNTS[use._nid] == 0 )
+                bb._outputs._es[ready++] = use; // Became ready, move into ready set
+        }
+        return ready;
+    }
+
+
+    // Pick best between sched and ready.
+    private static int best( Node[] uses, int sched, int ready ) {
+        int pick = sched;
+        int score = score(uses[pick]);
+        for( int i=sched+1; i<ready; i++ ) {
+            int nscore = score(uses[i]);
+            if( nscore > score )
+                { score = nscore; pick = i; }
+        }
+        return pick;
+    }
+
+    // Highest score wins.  Max at 1000, min at 10, except specials.
+    private static int score( Node n ) {
+        if( n.isMultiTail()  ) return 1001; // Pinned just behind the multi-head
+        if( n instanceof CFGNode ) return n instanceof ReturnNode ? 2 : 1; // Pinned at block exit
+
+        // Nothing special
+        return 500;
+    }
+
+}
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/Parser.java b/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
index f6bf71a9..ac497fe8 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
@@ -98,16 +98,12 @@ public class Parser {
     // Mapping from a type name to the constructor for a Type.
     public final HashMap<String, StructNode> INITS;
 
-    // "Linker" mapping from constant TypeFunPtrs to heads of function
-    public static final HashMap<TypeFunPtr,FunNode> LINKER = new HashMap<>();
-
 
     public Parser(String source) { this(source, TypeInteger.BOT); }
 
     public Parser(String source, TypeInteger arg) {
         Node.reset();
         IterPeeps.reset();
-        LINKER.clear();
         _lexer = new Lexer(source);
         _scope = new ScopeNode();
         _continueScope = _breakScope = null;
@@ -154,38 +150,38 @@ public class Parser {
     public StopNode parse() { return parse(false); }
     public StopNode parse(boolean show) {
         _xScopes.push(_scope);
-        // Enter a new scope for the initial control and arguments
-        _scope.push();
-        Node ctrl = new CProjNode(START, 0, ScopeNode.CTRL).peephole();
-        Node pmem = new  ProjNode(START, 1, ScopeNode.MEM0).peephole();
-        Node arg  = new  ProjNode(START, 2, ScopeNode.ARG0).peephole();
-        ScopeMinNode mem = new ScopeMinNode();
-        mem.addDef(null);
-        mem.addDef(pmem);
-        _scope.define(ScopeNode.CTRL, Type.CONTROL   , false, ctrl);
-        _scope.define(ScopeNode.MEM0, TypeMem.BOT    , false, mem );
-        _scope.define(ScopeNode.ARG0, TypeInteger.BOT, false, arg );
-
-        // Call main
-        CallNode main = (CallNode)new CallNode(ctrl,pmem,arg,con(TypeFunPtr.MAIN)).peephole();
-        // Call End
-        CallEndNode cend = (CallEndNode)new CallEndNode(main).peephole();
-        STOP.addDef(new CProjNode(cend,0,ScopeNode.CTRL).peephole());
-        STOP.addDef(new  ProjNode(cend,1,ScopeNode.MEM0).peephole());
-        STOP.addDef(new  ProjNode(cend,2,ScopeNode.ARG0).peephole());
+        //// Enter a new scope for the initial control and arguments
+        //_scope.push();
+        //Node ctrl = new CProjNode(START, 0, ScopeNode.CTRL).peephole();
+        //Node pmem = new  ProjNode(START, 1, ScopeNode.MEM0).peephole();
+        //Node arg  = new  ProjNode(START, 2, ScopeNode.ARG0).peephole();
+        //ScopeMinNode mem = new ScopeMinNode();
+        //mem.addDef(null);
+        //mem.addDef(pmem);
+        _scope.define(ScopeNode.CTRL, Type.CONTROL   , false, null);
+        _scope.define(ScopeNode.MEM0, TypeMem.BOT    , false, null);
+        _scope.define(ScopeNode.ARG0, TypeInteger.BOT, false, null);
+        //
+        //// Call main
+        //CallNode main = (CallNode)new CallNode(ctrl,pmem,arg,con(TypeFunPtr.MAIN)).peephole();
+        //// Call End
+        //CallEndNode cend = (CallEndNode)new CallEndNode(main).peephole();
+        //STOP.addDef(new CProjNode(cend,0,ScopeNode.CTRL).peephole());
+        //STOP.addDef(new  ProjNode(cend,1,ScopeNode.MEM0).peephole());
+        //STOP.addDef(new  ProjNode(cend,2,ScopeNode.ARG0).peephole());
 
         // Parse whole program, as-if function header "{ int arg -> body }"
         ReturnNode ret = parseFunctionBody("main",TypeFunPtr.MAIN,"arg");
-        LINKER.put(TypeFunPtr.MAIN,ret.fun());
-        main.peephole();
-        if( cend.peephole() != null )
-            IterPeeps.addAll(cend._outputs);
+        CodeGen.CODE._linker.put(TypeFunPtr.MAIN,ret.fun());
+        //main.peephole();
+        //if( cend.peephole() != null )
+        //    IterPeeps.addAll(cend._outputs);
 
         if( !_lexer.isEOF() ) throw _errorSyntax("unexpected");
 
         // Clean up and reset
-        _scope.kill();
         _xScopes.pop();
+        _scope.kill();
         for( StructNode init : INITS.values() )
             init.unkeep().kill();
         INITS.clear();
@@ -208,6 +204,7 @@ public class Parser {
         PhiNode rmem = new PhiNode(ScopeNode.MEM0,TypeMem.BOT,r,null);
         PhiNode rrez = new PhiNode(ScopeNode.ARG0,Type.BOTTOM,r,null);
         ReturnNode ret = new ReturnNode(r, rmem, rrez, fun);
+        fun.setRet(ret);
         assert ret.inProgress();
         STOP.addDef(ret);
 
@@ -217,10 +214,9 @@ public class Parser {
         _scope.push();
         ctrl(fun);              // Scope control from function
         // Private mem alias tracking per function
-        ScopeMinNode startMem = _scope.mem();
-        ScopeMinNode mem = new ScopeMinNode();
+        ScopeMinNode mem = new ScopeMinNode(true);
         mem.addDef(null);       // Alias#0
-        mem.addDef(new ParmNode(ScopeNode.MEM0,1,TypeMem.BOT,fun,startMem).peephole()); // All aliases
+        mem.addDef(new ParmNode(ScopeNode.MEM0,1,TypeMem.BOT,fun,con(TypeMem.BOT)).peephole()); // All aliases
         _scope.mem(mem);
         // All args, "as-if" called externally
         for( int i=0; i<ids.length; i++ ) {
@@ -236,7 +232,7 @@ public class Parser {
 
         // Last expression is the return
         if( ctrl()._type==Type.CONTROL )
-            STOP.addReturn(ctrl(), _scope.mem(), last, fun);
+            STOP.addReturn(ctrl(), _scope.mem().merge(), last, fun);
 
         // Function scope ends
         _scope.pop();
@@ -581,7 +577,9 @@ public class Parser {
      */
     private Node parseReturn() {
         var expr = require(parseAsgn(), ";");
-        STOP.addReturn(ctrl(), _scope.mem(), expr, _fun);
+        // Need default memory, since it can be lazy, need to force
+        // a non-lazy Phi
+        STOP.addReturn(ctrl(), _scope.mem().merge(), expr, _fun);
         ctrl(XCTRL);            // Kill control
         return expr;
     }
@@ -1043,26 +1041,36 @@ public class Parser {
         // Check for assign-update, x += e0;
         char ch = _lexer.matchOperAssign();
         if( ch==0  ) return rvalue;
+
+        // the rvalue is an l-value now; it will be updated
+        Node lhs = rvalue.keep();
         if( n._final )
             throw error("Cannot reassign final '"+n._name+"'");
+        // RHS of the update
+        Node rhs =
+            (byte)ch ==  1 ? con( 1) : // var++
+            (byte)ch == -1 ? con(-1) : // var--
+            parseAsgn();         // var op= rhs
+        // Widen RHS int to a RHS float
+        rhs = widenInt(rhs,n.type());
+        // Complain a RHS float into a LHS int
+        if( !rhs._type.isa(n.type()) )
+            throw error("Type " + rhs._type.str() + " is not of declared type " + n.type().str());
+
         Node op = switch(ch) {
-        case '+'      -> new AddNode(rvalue,null);
-        case '-'      -> new SubNode(rvalue,null);
-        case '*'      -> new MulNode(rvalue,null);
-        case '/'      -> new DivNode(rvalue,null);
-        case        1 -> new AddNode(rvalue,con( 1));
-        case (char)-1 -> new AddNode(rvalue,con(-1));
-        default -> throw Utils.TODO();
+        case 1, (char)-1,
+             '+' -> new AddNode(lhs,rhs);
+        case '-' -> new SubNode(lhs,rhs);
+        case '*' -> new MulNode(lhs,rhs);
+        case '/' -> new DivNode(lhs,rhs);
+        default  -> throw Utils.TODO();
         };
-        // Return pre-value (x+=1) or post-value (x++)
-        boolean pre = op.in(2)==null;
-        // Parse RHS argument as needed
-        if( pre )
-            { op.keep().setDef(2,parseAsgn());  op.unkeep(); }
-        else rvalue.keep();     // Keep post-value across peeps
-        op = zsMask(peep(op),n.type());
+
+        op = zsMask(peep(op.widen()),n.type());
         _scope.update(n,op);
-        return pre ? op : rvalue.unkeep();
+        // Return pre-value (x+=1) or post-value (x++)
+        lhs.unkeep();
+        return (byte)ch== 1 || (byte)ch== -1 ? lhs : op;
     }
 
     ScopeMinNode.Var requireLookupId(String msg) {
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/CFGNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/CFGNode.java
index bed4eb17..bacc1935 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/CFGNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/CFGNode.java
@@ -28,6 +28,17 @@ public abstract class CFGNode extends Node {
     // Block head is Start, Region, CProj, but not e.g. If, Return, Stop
     public boolean blockHead() { return false; }
 
+    // Get the one control following; error to call with more than one e.g. an
+    // IfNode or other multi-way branch.
+    public CFGNode uctrl() {
+        CFGNode c = null;
+        for( Node n : _outputs )
+            if( n instanceof CFGNode cfg )
+                {  assert c==null;  c = cfg; }
+        return c;
+    }
+
+
     // ------------------------------------------------------------------------
     /**
      * Immediate dominator tree depth, used to approximate a real IDOM during
@@ -90,13 +101,14 @@ public abstract class CFGNode extends Node {
         _pre = pre++;
         // Pre-walk
         for( Node use : _outputs )
-            if( use instanceof CFGNode usecfg )
+            if( use instanceof CFGNode usecfg && !(usecfg instanceof XCtrlNode) )
                 pre = usecfg._bltWalk(pre,use instanceof FunNode fuse ? fuse : fun,stop,post);
 
         // Post-order work: find innermost loop
         LoopTree inner = null, ltree;
         for( Node use : _outputs ) {
             if( !(use instanceof CFGNode usecfg) ) continue;
+            if( usecfg instanceof XCtrlNode ) continue;
             // Child visited but not post-visited?
             if( !post.get(usecfg._nid) ) {
                 // Must be a backedge to a LoopNode then
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/CallNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/CallNode.java
index 0839ad2c..3ec1cd0c 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/CallNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/CallNode.java
@@ -1,6 +1,6 @@
 package com.seaofnodes.simple.node;
 
-import com.seaofnodes.simple.Parser;
+import com.seaofnodes.simple.CodeGen;
 import com.seaofnodes.simple.Utils;
 import com.seaofnodes.simple.type.Type;
 import com.seaofnodes.simple.type.TypeFunPtr;
@@ -21,7 +21,7 @@ public class CallNode extends CFGNode {
         String fname = null;
         Node fptr = fptr();
         if( fptr._type instanceof TypeFunPtr tfp && tfp.isConstant() )
-            fname = Parser.LINKER.get(tfp)._name;
+            fname = CodeGen.CODE._linker.get(tfp)._name;
         if( fname==null ) fptr._print0(sb,visited);
         else sb.append(fname);
         sb.append("( ");
@@ -62,7 +62,7 @@ public class CallNode extends CFGNode {
                 for( ; fidxs!=0; fidxs = TypeFunPtr.nextFIDX(fidxs) ) {
                     int fidx = Long.numberOfTrailingZeros(fidxs);
                     TypeFunPtr tfp0 = tfp.makeFrom(fidx);
-                    FunNode fun = Parser.LINKER.get(tfp0);
+                    FunNode fun = CodeGen.CODE._linker.get(tfp0);
                     if( fun!=null && !fun._folding && !linked(fun) )
                         link(fun);
                 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/ConstantNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/ConstantNode.java
index 406b20ca..e54797a5 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ConstantNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ConstantNode.java
@@ -19,12 +19,18 @@ import java.util.BitSet;
  * The Constant's value is the value stored in it.
  */
 public class ConstantNode extends Node {
-    Type _con;
+    public final Type _con;
     public ConstantNode( Type type ) {
         super(Parser.START);
         _con = type;
     }
 
+    public static Node make( Type type ) {
+        if( type==Type. CONTROL ) return new CtrlNode();
+        if( type==Type.XCONTROL ) return new XCtrlNode();
+        return new ConstantNode(type);
+    }
+
     @Override
     public String label() { return "#"+_con; }
 
@@ -37,6 +43,7 @@ public class ConstantNode extends Node {
     }
 
     @Override public boolean isMultiTail() { return true; }
+    @Override public boolean isConst() { return true; }
 
     @Override
     public Type compute() { return _con; }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/CtrlNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/CtrlNode.java
new file mode 100644
index 00000000..162417bb
--- /dev/null
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/CtrlNode.java
@@ -0,0 +1,14 @@
+package com.seaofnodes.simple.node;
+
+import com.seaofnodes.simple.Parser;
+import com.seaofnodes.simple.type.Type;
+import java.util.BitSet;
+
+public class CtrlNode extends CFGNode {
+    public CtrlNode() { super(Parser.START); }
+    @Override public String label() { return "Ctrl"; }
+    @Override StringBuilder _print1(StringBuilder sb, BitSet visited) { return sb.append("Cctrl"); }
+    @Override public boolean isConst() { return true; }
+    @Override  public Type compute() { return Type.CONTROL; }
+    @Override public Node idealize() { return null; }
+}
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/FunNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/FunNode.java
index 9087926e..94d7eff7 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/FunNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/FunNode.java
@@ -9,24 +9,27 @@ public class FunNode extends RegionNode {
     // When set true, this Call/CallEnd/Fun/Return is being trivially inlined
     boolean _folding;
 
-    TypeFunPtr _sig;            // Initial signature may be missing return
-    String _name;               // Debug label, optional
+    public final TypeFunPtr _sig; // Initial signature
+    public String _name;        // Debug label, optional
+    private ReturnNode _ret;     // Return pointer
 
     public FunNode( StartNode start, TypeFunPtr sig, String name ) { super(null,start); _sig=sig; _name=name; }
 
     @Override
     public String label() { return _name; }
 
-    ReturnNode ret() {
-        ReturnNode ret=null;
+    // Find the one CFG user from Fun.  It's not always the Return, but always
+    // the Return *is* a CFG user of Fun.
+    @Override public CFGNode uctrl() {
         for( Node n : _outputs )
-            if( n instanceof ReturnNode ret0 ) {
-                assert ret==null || ret==ret0; // Found more than one return
-                ret=ret0;
-            }
-        return ret;
+            if( n instanceof CFGNode cfg &&
+                (cfg instanceof RegionNode || cfg.cfg0()==this) )
+                return cfg;
+        return null;
     }
 
+    public void setRet(ReturnNode ret) { _ret=ret; }
+    ReturnNode ret() { assert _ret!=null; return _ret; }
 
     @Override
     public Type compute() {
@@ -36,12 +39,23 @@ public class FunNode extends RegionNode {
 
     @Override
     public Node idealize() {
+        // When can we assume no callers?  Or no other callers (except main)?
+        // In a partial compilation, we assume Start gets access to any/all
+        // top-level public structures and recursively what they point to.
+        // This in turn is valid arguments to every callable function.
+        //
+        // In a total compilation, we can start from Start and keep things
+        // more contained.
+
+
         // If no default/unknown caller, use the normal RegionNode ideal rules
         // to collapse
-        if( inProgress() ) return null;
+        if( unknownCallers() ) return null;
         return super.idealize();
     }
 
     // Always in-progress until we run out of unknown callers
-    @Override public boolean inProgress() { return in(1) instanceof StartNode; }
+    public boolean unknownCallers() { return in(1) instanceof StartNode; }
+
+    @Override public boolean inProgress() { return unknownCallers(); }
 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/IfNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/IfNode.java
index dcd2d4c7..1e6b2878 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/IfNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/IfNode.java
@@ -26,6 +26,9 @@ public class IfNode extends CFGNode implements MultiNode {
     public Node ctrl() { return in(0); }
     public Node pred() { return in(1); }
 
+    // No one unique control follows
+    @Override public CFGNode uctrl() { return null; }
+
     @Override
     public Type compute() {
         // If the If node is not reachable then neither is any following Proj
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/LoopNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/LoopNode.java
index 33420eb9..3f2ae149 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/LoopNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/LoopNode.java
@@ -58,17 +58,13 @@ public class LoopNode extends RegionNode {
         setDef(2,f);
 
         // Now fold control into the exit
-        if( fun!=null )
-            throw Utils.TODO();
-
-        // If not in a function, look for a last-level merge point;
-        // make if not there.
-        RegionNode ctrl = stop.ctrl() instanceof RegionNode r ? r : new RegionNode(null,stop.ctrl());
-        PhiNode    mem  = stop.mem () instanceof PhiNode phi && phi.in(0)==ctrl ? phi : new PhiNode("",stop.mem ()._type,ctrl,stop.mem ());
-        PhiNode    expr = stop.expr() instanceof PhiNode phi && phi.in(0)==ctrl ? phi : new PhiNode("",stop.expr()._type,ctrl,stop.expr());
-        if( stop.ctrl()!=ctrl ) stop.setDef(0,ctrl);
-        if( stop.mem ()!=mem  ) stop.setDef(1,mem );
-        if( stop.expr()!=expr ) stop.setDef(2,expr);
+        ReturnNode ret = fun.ret();
+        RegionNode ctrl = ret.ctrl() instanceof RegionNode r ? r : new RegionNode(null,ret.ctrl());
+        PhiNode    mem  = ret.mem () instanceof PhiNode phi && phi.in(0)==ctrl ? phi : new PhiNode(ctrl,ret.mem ());
+        PhiNode    expr = ret.expr() instanceof PhiNode phi && phi.in(0)==ctrl ? phi : new PhiNode(ctrl,ret.expr());
+        if( ret.ctrl()!=ctrl ) ret.setDef(0,ctrl);
+        if( ret.mem ()!=mem  ) ret.setDef(1,mem );
+        if( ret.expr()!=expr ) ret.setDef(2,expr);
         Node top = new ConstantNode(Type.TOP).peephole();
         ctrl.addDef(t  );
         mem .addDef(top);
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter18/src/main/java/com/seaofnodes/simple/node/Node.java
index 6369fb01..cc7a1d13 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -114,6 +114,7 @@ public abstract class Node {
 
     public boolean isMultiHead() { return false; }
     public boolean isMultiTail() { return false; }
+    public boolean isConst    () { return false; }
 
     // ------------------------------------------------------------------------
     // Graph Node & Edge manipulation
@@ -337,8 +338,8 @@ public abstract class Node {
         Type old = setType(compute());
 
         // Replace constant computations from non-constants with a constant node
-        if( !(this instanceof ConstantNode) && !(this instanceof XCtrlNode) && _type.isHighOrConst() )
-            return (_type==Type.XCONTROL ? new XCtrlNode() : new ConstantNode(_type)).peepholeOpt();
+        if( _type.isHighOrConst() && !isConst() )
+            return ConstantNode.make(_type).peepholeOpt();
 
         // Global Value Numbering
         if( _hash==0 ) {
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/ParmNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/ParmNode.java
index 136494b5..f139e4ca 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ParmNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ParmNode.java
@@ -7,7 +7,7 @@ import java.util.BitSet;
 public class ParmNode extends PhiNode {
 
     // Argument indices are mapped one-to-one on CallNode inputs
-    final int _idx;             // Argument index
+    public final int _idx;             // Argument index
 
     public ParmNode(String label, int idx, Type declaredType, Node... inputs) {
         super(label,declaredType,inputs);
@@ -18,7 +18,7 @@ public class ParmNode extends PhiNode {
 
     @Override public String glabel() { return _label; }
 
-    FunNode fun() { return (FunNode)in(0); }
+    public FunNode fun() { return (FunNode)in(0); }
 
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/PhiNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/PhiNode.java
index 6dd4f57d..91334b6c 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/PhiNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/PhiNode.java
@@ -8,7 +8,7 @@ import java.util.BitSet;
 
 public class PhiNode extends Node {
 
-    final String _label;
+    public final String _label;
 
     // The Phi type we compute must stay within the domain of the Phi.
     // Example Int stays Int, Ptr stays Ptr, Control stays Control, Mem stays Mem.
@@ -16,6 +16,14 @@ public class PhiNode extends Node {
 
     public PhiNode(String label, Type declaredType, Node... inputs) { super(inputs); _label = label;  assert declaredType!=null; _declaredType = declaredType; }
 
+    public PhiNode(RegionNode r, Node sample) {
+        super(r);
+        _label = "";
+        _declaredType = sample._type;
+        while( nIns() < r.nIns() )
+            addDef(sample);
+    }
+
     @Override public String label() { return "Phi_"+MemOpNode.mlabel(_label); }
 
     @Override public String glabel() { return "&phi;_"+_label; }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/ReturnNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
index b305a96e..35563d6e 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
@@ -1,8 +1,7 @@
 package com.seaofnodes.simple.node;
 
-import com.seaofnodes.simple.Parser;
+import com.seaofnodes.simple.CodeGen;
 import com.seaofnodes.simple.type.*;
-
 import java.util.BitSet;
 
 /**
@@ -16,14 +15,16 @@ import java.util.BitSet;
  */
 public class ReturnNode extends CFGNode {
 
+    public final FunNode _fun;
     public ReturnNode(Node ctrl, Node mem, Node data, FunNode fun) {
-        super(ctrl, mem, data, fun);
+        super(ctrl, mem, data);
+        _fun = fun;
     }
 
     public Node ctrl() { return in(0); }
     public Node mem () { return in(1); }
     public Node expr() { return in(2); }
-    public FunNode fun() { return (FunNode)in(3); }
+    public FunNode fun() { return _fun; }
 
     @Override
     public String label() { return "Return"; }
@@ -31,7 +32,6 @@ public class ReturnNode extends CFGNode {
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         sb.append("return ");
-        if( expr()==null ) return sb.append("INPROGRESS");
         expr()._print0(sb, visited);
         return sb.append(";");
     }
@@ -45,7 +45,7 @@ public class ReturnNode extends CFGNode {
     @Override public Node idealize() { return null; }
 
     public boolean inProgress() {
-        return ctrl() instanceof RegionNode r && r.inProgress();
+        return ctrl().getClass() == RegionNode.class && ((RegionNode)ctrl()).inProgress();
     }
 
     // Add a return exit to the current parsing function
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
index f5a33890..f29ed20b 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
@@ -11,6 +11,8 @@ import static com.seaofnodes.simple.Utils.TODO;
 /* */
 public class ScopeMinNode extends Node {
 
+    public final boolean _inProgress;
+
     /** The tracked fields are now complex enough to deserve a array-of-structs layout
      */
     public static class Var {
@@ -39,8 +41,12 @@ public class ScopeMinNode extends Node {
         }
     }
 
-    public ScopeMinNode() { _type = TypeMem.BOT; }
+    public ScopeMinNode(boolean inProgress) { _type = TypeMem.BOT; _inProgress = inProgress; }
+
 
+    // If being used by a Scope, this is "in progress" from the Parser.
+    // Otherwise its a memory merge
+    boolean inProgress() { return _inProgress; }
 
     @Override public String label() { return "ALLMEM"; }
     @Override public boolean isMem() { return true; }
@@ -63,6 +69,17 @@ public class ScopeMinNode extends Node {
         return sb.append("]");
     }
 
+    // Make a memory merge: no longer a Scope really, tracking memory state but
+    // not related to the parser in any way.
+    public Node merge() {
+        // Force default memory to not be lazy
+        ScopeMinNode merge = new ScopeMinNode(false);
+        for( Node n : _inputs )
+            merge.addDef(n);
+        merge._mem(1,null);
+        return merge.peephole();
+    }
+
 
     @Override public Type compute() { return TypeMem.BOT; }
 
@@ -127,13 +144,13 @@ public class ScopeMinNode extends Node {
 
     // Fill in the backedge of any inserted Phis
     void _endLoopMem( ScopeNode scope, ScopeMinNode back, ScopeMinNode exit ) {
-        for( int i=2; i<back.nIns(); i++ ) {
-            if( back.in(i) != scope ) {
-                PhiNode phi = (PhiNode)in(i);
-                assert phi.region()==scope.ctrl() && phi.in(2)==null;
-                phi.setDef(2,back.in(i)); // Fill backedge
+        Node exit_def = exit.alias(1);
+        for( int i=1; i<back.nIns(); i++ ) {
+            if( in(i) instanceof PhiNode phi && phi.region()==scope.ctrl() ) {
+                assert phi.in(2)==null;
+                phi.setDef(2,back.in(i)==scope ? phi : back.in(i)); // Fill backedge
             }
-            if( exit.alias(i) == scope ) // Replace a lazy-phi on the exit path also
+            if( exit_def == scope ) // Replace a lazy-phi on the exit path also
                 exit.alias(i,in(i));
         }
     }
@@ -155,16 +172,7 @@ public class ScopeMinNode extends Node {
         }
     }
 
-    // If being used by a Scope, this is "in progress" from the Parser.
-    // Once no more scope users, this is just a memory-merge.
-    boolean inProgress() {
-        for( Node n : _outputs )
-            if( n instanceof ScopeNode scope )
-                return true;
-        return false;
-    }
-
     @Override boolean eq( Node n ) {
-        return this==n || !inProgress();
+        return this==n || !_inProgress;
     }
 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
index 8685e66b..e4a4ddc0 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
@@ -3,7 +3,6 @@ package com.seaofnodes.simple.node;
 import com.seaofnodes.simple.*;
 import com.seaofnodes.simple.type.*;
 import java.util.*;
-import static com.seaofnodes.simple.Utils.TODO;
 
 /**
  * The Scope node is purely a parser helper - it tracks names to nodes with a
@@ -33,6 +32,7 @@ public class ScopeNode extends ScopeMinNode {
 
     // A new ScopeNode
     public ScopeNode() {
+        super(true);
         _vars   = new Ary<>(Var    .class);
         _lexSize= new Ary<>(Integer.class);
         _inCons = new Ary<>(Boolean.class);
@@ -113,8 +113,8 @@ public class ScopeNode extends ScopeMinNode {
      * Create a new variable name in the current scope
      */
     public boolean define( String name, Type declaredType, boolean xfinal, Node init ) {
-        assert name.charAt(0)!='$' || _lexSize.size()==1; // Later scopes do not define memory
-        if( _lexSize._len > 1 )
+        assert _lexSize.isEmpty() || name.charAt(0)!='$' ; // Later scopes do not define memory
+        if( _lexSize._len > 0 )
             for( int i=_vars.size()-1; i>=_lexSize.last(); i-- )
                 if( _vars.get(i)._name.equals(name) )
                     return false;   // Double define
@@ -203,7 +203,7 @@ public class ScopeNode extends ScopeMinNode {
         dup.addDef(ctrl());     // Control input is just copied
 
         // Memory input is a shallow copy
-        ScopeMinNode memdup = new ScopeMinNode(), mem = mem();
+        ScopeMinNode memdup = new ScopeMinNode(true), mem = mem();
         memdup.addDef(null);
         memdup.addDef(loop ? this : mem.in(1));
         for( int i=2; i<mem.nIns(); i++ )
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/StartNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/StartNode.java
index ab6e4931..008a910f 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/StartNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/StartNode.java
@@ -29,7 +29,19 @@ public class StartNode extends LoopNode implements MultiNode {
 
     @Override public boolean isMultiHead() { return true; }
     @Override public boolean blockHead() { return true; }
-    @Override public CFGNode cfg0() { return this; }
+    @Override public CFGNode cfg0() { return null; }
+
+    // Get the one control following; error to call with more than one e.g. an
+    // IfNode or other multi-way branch.  For Start, its "main"
+    @Override public CFGNode uctrl() {
+        // Find "main", its the start.
+        CFGNode C = null;
+        for( Node use : _outputs )
+            if( use instanceof FunNode fun && fun._sig==TypeFunPtr.MAIN )
+                { assert C==null; C = fun; }
+        return C;
+    }
+
 
     @Override public TypeTuple compute() {
         return TypeTuple.make(Type.CONTROL,TypeMem.TOP,_arg);
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/StopNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/StopNode.java
index 4b91f1eb..fb78f720 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/StopNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/StopNode.java
@@ -22,7 +22,6 @@ public class StopNode extends CFGNode {
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         // For the sake of many old tests, and single value prints as "return val"
-        if( nIns() == 3 ) return expr()._print0(sb.append("return "),visited).append(";");
         if( ret()!=null ) return ret()._print0(sb,visited);
         sb.append("Stop[ ");
         for( Node ret : _inputs )
@@ -33,14 +32,10 @@ public class StopNode extends CFGNode {
     @Override public boolean blockHead() { return true; }
 
 
-    public CFGNode ctrl() { return (CFGNode)in(0); }
-    public Node mem () { return in(1); }
-    public Node expr() { return in(2); }
-
     // If a single Return, return it.
     // Otherwise, null because ambiguous.
     public ReturnNode ret() {
-        return nIns()==4 && in(3) instanceof ReturnNode ret ? ret : null;
+        return nIns()==1 && in(0) instanceof ReturnNode ret ? ret : null;
     }
 
     @Override
@@ -51,7 +46,7 @@ public class StopNode extends CFGNode {
     @Override
     public Node idealize() {
         int len = nIns();
-        for( int i=3; i<nIns(); i++ )
+        for( int i=0; i<nIns(); i++ )
             if( ((ReturnNode)in(i)).fun()==null )
                 delDef(i--);
         if( len != nIns() ) return this;
@@ -61,9 +56,8 @@ public class StopNode extends CFGNode {
     @Override public int idepth() {
         if( _idepth!=0 ) return _idepth;
         int d=0;
-        for( Node n : _inputs )
-            if( n instanceof CFGNode cfg )
-                d = Math.max(d,cfg.idepth()+1);
+        for( Node ret : _inputs )
+            d = Math.max(d,((ReturnNode)ret).idepth()+1);
         return _idepth=d;
     }
 
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/SubFNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/SubFNode.java
index 682930e0..b591718d 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/SubFNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/SubFNode.java
@@ -34,7 +34,7 @@ public class SubFNode extends Node {
         Node lhs = in(1);
         Type t2 = in(2)._type;
 
-        // Sub of 1.
+        // Sub of 0.
         if ( t2.isConstant() && t2 instanceof TypeFloat i && i.value()==0 )
             return lhs;
 
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/XCtrlNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/XCtrlNode.java
index 83779dd2..3c6a5df3 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/XCtrlNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/XCtrlNode.java
@@ -5,9 +5,11 @@ import com.seaofnodes.simple.type.Type;
 import java.util.BitSet;
 
 public class XCtrlNode extends CFGNode {
-    public XCtrlNode( ) { super(Parser.START); }
+    public XCtrlNode() { super(Parser.START); }
     @Override public String label() { return "Xctrl"; }
     @Override StringBuilder _print1(StringBuilder sb, BitSet visited) { return sb.append("Xctrl"); }
+    @Override public boolean isConst() { return true; }
+    @Override public boolean isMultiTail() { return true; }
     @Override  public Type compute() { return Type.XCONTROL; }
     @Override public Node idealize() { return null; }
 }