From 5400096ec10da839cf2006a8a02e11ffeff521dd Mon Sep 17 00:00:00 2001
From: Cliff Click <cliffc@acm.org>
Date: Sun, 29 Dec 2024 16:06:07 -0800
Subject: More precise error reporting

Demand CFG cleanup, even if peeps are off; cleans up some tests & a lot of fuzzer bugs related to trying to find infinite loops in dead code.
Logical subclass for better error reporting on &,|,^,>>,<<,>>>
More precise error reporting on multi returns and trinaries
Fuzzer uses new Eval2
---
 .../main/java/com/seaofnodes/simple/CodeGen.java   |  5 ++-
 .../com/seaofnodes/simple/GlobalCodeMotion.java    |  7 ++--
 .../main/java/com/seaofnodes/simple/Parser.java    | 23 +++++++----
 .../java/com/seaofnodes/simple/node/AndNode.java   | 17 ++-------
 .../com/seaofnodes/simple/node/LogicalNode.java    | 25 ++++++++++++
 .../java/com/seaofnodes/simple/node/LoopNode.java  | 31 +++++++++------
 .../main/java/com/seaofnodes/simple/node/Node.java | 17 ++++++---
 .../java/com/seaofnodes/simple/node/OrNode.java    | 17 ++-------
 .../java/com/seaofnodes/simple/node/PhiNode.java   | 38 +++++++++++++++++--
 .../com/seaofnodes/simple/node/RegionNode.java     |  3 +-
 .../com/seaofnodes/simple/node/ReturnNode.java     | 44 +++++++++++++++++++++-
 .../java/com/seaofnodes/simple/node/SarNode.java   | 17 ++-------
 .../java/com/seaofnodes/simple/node/ShlNode.java   | 17 ++-------
 .../java/com/seaofnodes/simple/node/ShrNode.java   | 12 ++----
 .../java/com/seaofnodes/simple/node/XorNode.java   | 17 ++-------
 .../java/com/seaofnodes/simple/type/TypeMem.java   |  1 +
 16 files changed, 175 insertions(+), 116 deletions(-)
 create mode 100644 chapter18/src/main/java/com/seaofnodes/simple/node/LogicalNode.java

(limited to 'chapter18/src/main')

diff --git a/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java b/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
index 79d817f8..394b6d1c 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
@@ -57,7 +57,10 @@ public class CodeGen {
     }
 
     public CodeGen typeCheck() {
-        assert _phase == Phase.Opto || _phase == Phase.Parse;
+        // Demand phase Opto for cleaning up dead control flow at least,
+        // required for the following GCM.  Note that peeps can be disabled,
+        // but still the dead CFG will get cleaned.
+        assert _phase == Phase.Opto;
         _phase = Phase.TypeCheck;
 
         // Type check
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java b/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
index 6b0956ca..f1593a81 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
@@ -107,6 +107,8 @@ public abstract class GlobalCodeMotion {
                 if( n instanceof LoadNode ld )
                     for( Node memuse : ld.mem()._outputs )
                         if( late[memuse._nid]==null &&
+                            // New makes new memory, never crushes load memory
+                            !(memuse instanceof NewNode) &&
                             // Load-use directly defines memory
                             (memuse._type instanceof TypeMem ||
                              // Load-use indirectly defines memory
@@ -188,10 +190,6 @@ public abstract class GlobalCodeMotion {
                 assert late[st._nid]!=null;
                 lca = anti_dep(load,late[st._nid],st.cfg0(),lca,st);
                 break;
-            case NewNode st:
-                assert late[st._nid]!=null;
-                lca = anti_dep(load,late[st._nid],st.cfg0(),lca,st);
-                break;
             case PhiNode phi:
                 // Repeat anti-dep for matching Phi inputs.
                 // No anti-dep edges but may raise the LCA.
@@ -199,6 +197,7 @@ public abstract class GlobalCodeMotion {
                     if( phi.in(i)==load.mem() )
                         lca = anti_dep(load,phi.region().cfg(i),load.mem().cfg0(),lca,null);
                 break;
+            case NewNode st: break;
             case LoadNode ld: break; // Loads do not cause anti-deps on other loads
             case ReturnNode ret: break; // Load must already be ahead of Return
             case ScopeMinNode ret: break; // Mem uses now on ScopeMin
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/Parser.java b/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
index ac497fe8..235d2414 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
@@ -226,7 +226,7 @@ public class Parser {
         }
 
         // Parse the body
-        Node last=null;
+        Node last=ZERO;
         while (!peek('}') && !_lexer.isEOF())
             last = parseStatement();
 
@@ -562,6 +562,9 @@ public class Parser {
 
         RegionNode r = ctrl(tScope.mergeScopes(fScope));
         Node ret = peep(new PhiNode("",lhs._type.meet(rhs._type),r,lhs.unkeep(),rhs.unkeep()));
+        // Immediately fail e.g. `arg ? 7 : ptr`
+        if( !stmt && ret.err() !=null )
+            throw error(ret.err());
         r.peephole();
         return ret;
     }
@@ -938,6 +941,8 @@ public class Parser {
             else if( match(">>") ) lhs = new SarNode(lhs,null);
             else break;
             lhs.setDef(2,parseAddition());
+            if( lhs.err() != null )
+                throw error(lhs.err());
             lhs = peep(lhs.widen());
         }
         return lhs;
@@ -973,7 +978,7 @@ public class Parser {
      * @return a multiply expression {@link Node}, never {@code null}
      */
     private Node parseMultiplication() {
-        var lhs = parseUnary();  boolean mul;
+        var lhs = parseUnary();
         while( true ) {
             if( false ) ;
             else if( match("*") ) lhs = new MulNode(lhs,null);
@@ -1004,7 +1009,9 @@ public class Parser {
                 if( n != null ) {
                     if( n._final )
                         throw error("Cannot reassign final '"+n._name+"'");
-                    Node expr = zsMask(peep(new AddNode(_scope.in(n),con(delta))),n.type());
+                    Node expr = n.type() instanceof TypeFloat
+                        ?        peep(new AddFNode(_scope.in(n),con(TypeFloat.constant(-1))))
+                        : zsMask(peep(new  AddNode(_scope.in(n),con(delta))),n.type());
                     _scope.update(n,expr);
                     return expr;
                 }
@@ -1054,7 +1061,7 @@ public class Parser {
         // 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()) )
+        if( !(rhs._type instanceof TypeInteger) && !rhs._type.isa(n.type()) )
             throw error("Type " + rhs._type.str() + " is not of declared type " + n.type().str());
 
         Node op = switch(ch) {
@@ -1124,7 +1131,7 @@ public class Parser {
         }
         // Check that all fields are initialized
         for( int i=idx; i<init.size(); i++ )
-            if( /*init.at(i)._type == Type.TOP ||*/ init.at(i)._type == Type.BOTTOM )
+            if( init.at(i)._type == Type.TOP || init.at(i)._type == Type.BOTTOM )
                 throw error("'"+tmp._obj._name+"' is not fully initialized, field '" + fs[i-idx]._fname + "' needs to be set in a constructor");
         Node ptr = newStruct(tmp._obj, con(tmp._obj.offset(fs.length)), idx, init );
         if( hasConstructor )
@@ -1196,9 +1203,11 @@ public class Parser {
         // Happens when parsing known dead code, which often has other typing
         // issues.  Since the code is dead, possibly due to inlining, lets not
         // spoil the user experience with error messages.
-        if( ctrl()._type==Type.XCONTROL )
+        if( ctrl()._type==Type.XCONTROL ) {
+            if( expr.isUnused() ) expr.kill(); // Losing last use of expr
             // Exit out via parsing the trailing expression
-            return matchOpx('=','=') ? parseAsgn() : parsePostfix(con(Type.TOP));
+            return matchOpx( '=', '=' ) ? parseAsgn() : parsePostfix( con( Type.TOP ) );
+        }
 
         //
         if( expr._type==Type.NIL )
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/AndNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/AndNode.java
index 68a8079d..7f080ff2 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/AndNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/AndNode.java
@@ -5,20 +5,14 @@ import com.seaofnodes.simple.type.TypeInteger;
 
 import java.util.BitSet;
 
-public class AndNode extends Node {
-    public AndNode(Node lhs, Node rhs) { super(null, lhs, rhs); }
+public class AndNode extends LogicalNode {
+    public AndNode(Node lhs, Node rhs) { super(lhs, rhs); }
 
     @Override public String label() { return "And"; }
+    @Override public String op() { return "&"; }
 
     @Override public String glabel() { return "&"; }
 
-    @Override
-    StringBuilder _print1(StringBuilder sb, BitSet visited) {
-        in(1)._print0(sb.append("("), visited);
-        in(2)._print0(sb.append("&"), visited);
-        return sb.append(")");
-    }
-
     @Override
     public Type compute() {
         Type t1 = in(1)._type, t2 = in(2)._type;
@@ -60,9 +54,4 @@ public class AndNode extends Node {
         return null;
     }
     @Override Node copy(Node lhs, Node rhs) { return new AndNode(lhs,rhs); }
-    @Override public String err() {
-        if( !(in(1)._type instanceof TypeInteger) ) return "Cannot '&' " + in(1)._type;
-        if( !(in(2)._type instanceof TypeInteger) ) return "Cannot '&' " + in(2)._type;
-        return null;
-    }
 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/LogicalNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/LogicalNode.java
new file mode 100644
index 00000000..7f959ff3
--- /dev/null
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/LogicalNode.java
@@ -0,0 +1,25 @@
+package com.seaofnodes.simple.node;
+
+import com.seaofnodes.simple.type.Type;
+import com.seaofnodes.simple.type.TypeInteger;
+import java.util.BitSet;
+
+public abstract class LogicalNode extends Node {
+    public LogicalNode(Node lhs, Node rhs) { super(null, lhs, rhs); }
+    abstract String op();
+
+    @Override
+    StringBuilder _print1(StringBuilder sb, BitSet visited) {
+        in(1)._print0(sb.append("("), visited);
+        in(2)._print0(sb.append(op()), visited);
+        return sb.append(")");
+    }
+
+
+    @Override public String err() {
+        if( in(1)._type.isHigh() || in(2)._type.isHigh() ) return null;
+        if( !(in(1)._type instanceof TypeInteger) ) return "Cannot '"+op()+"' " + in(1)._type;
+        if( !(in(2)._type instanceof TypeInteger) ) return "Cannot '"+op()+"' " + in(2)._type;
+        return null;
+    }
+}
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 3f2ae149..001ab8f2 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/LoopNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/LoopNode.java
@@ -43,7 +43,7 @@ public class LoopNode extends RegionNode {
         while( x != this ) {
             if( x instanceof CProjNode exit && exit.in(0) instanceof IfNode iff ) {
                 CFGNode other = iff.cproj(1-exit._idx);
-                if( other.loopDepth() < loopDepth() )
+                if( other!=null && other.loopDepth() < loopDepth() )
                     return stop; // Found an exit, not an infinite loop
             }
             x = x.idom();
@@ -57,18 +57,25 @@ public class LoopNode extends RegionNode {
         CProjNode f = new CProjNode(iff,1,"False");
         setDef(2,f);
 
-        // Now fold control into the exit
-        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);
+        // Now fold control into the exit.  Might have 1 valid exit, or an
+        // XCtrl or a bunch of prior NeverNode exits.
         Node top = new ConstantNode(Type.TOP).peephole();
-        ctrl.addDef(t  );
-        mem .addDef(top);
-        expr.addDef(top);
+        ReturnNode ret = fun.ret();
+        if( ret.ctrl()._type == Type.XCONTROL ) {
+            ret.setDef(0,t);
+            ret.setDef(1,top);
+            ret.setDef(2,top);
+        } else {
+            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).peepholeOpt();
+            ctrl.addDef(t  );
+            mem .addDef(top);
+            expr.addDef(top);
+        }
         return stop;
     }
 }
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 cc7a1d13..9aa123a5 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -302,10 +302,6 @@ public abstract class Node {
      * Always returns some not-null Node (often this).
      */
     public final Node peephole( ) {
-        if (_disablePeephole) {
-            _type = compute();
-            return this;        // Peephole optimizations turned off
-        }
         if( _type==null )       // Brand-new node, never peeped before
             JSViewer.show();
         Node n = peepholeOpt();
@@ -337,8 +333,17 @@ public abstract class Node {
         // Compute initial or improved Type
         Type old = setType(compute());
 
-        // Replace constant computations from non-constants with a constant node
-        if( _type.isHighOrConst() && !isConst() )
+        // Peepholes can be turned off - except for cleaning up dead CFG paths.
+        // These need to clean up so the following code motion algorithms don't
+        // get confused by dead or infinite paths.
+        if( _disablePeephole && !(this instanceof RegionNode) && !(this instanceof PhiNode) )
+            return old==_type ? null : this;   // Peephole optimizations turned off
+
+        // Replace constant computations from non-constants with a constant
+        // node.  If peeps are disabled, still allow high Phis to collapse;
+        // they typically come from dead Regions and we want the Region to
+        // collapse, which requires the Phis to die first.
+        if( _type.isHighOrConst() && !isConst() && (!_disablePeephole || _type.isHigh()) )
             return ConstantNode.make(_type).peepholeOpt();
 
         // Global Value Numbering
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/OrNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/OrNode.java
index bd771198..42bedc46 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/OrNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/OrNode.java
@@ -5,20 +5,14 @@ import com.seaofnodes.simple.type.TypeInteger;
 
 import java.util.BitSet;
 
-public class OrNode extends Node {
-    public OrNode(Node lhs, Node rhs) { super(null, lhs, rhs); }
+public class OrNode extends LogicalNode {
+    public OrNode(Node lhs, Node rhs) { super(lhs, rhs); }
 
     @Override public String label() { return "Or"; }
+    @Override public String op() { return "|"; }
 
     @Override public String glabel() { return "|"; }
 
-    @Override
-    StringBuilder _print1(StringBuilder sb, BitSet visited) {
-        in(1)._print0(sb.append("("), visited);
-        in(2)._print0(sb.append("|"), visited);
-        return sb.append(")");
-    }
-
     @Override
     public Type compute() {
         Type t1 = in(1)._type, t2 = in(2)._type;
@@ -57,9 +51,4 @@ public class OrNode extends Node {
         return null;
     }
     @Override Node copy(Node lhs, Node rhs) { return new OrNode(lhs,rhs); }
-    @Override public  String err() {
-        if( !(in(1)._type instanceof TypeInteger) ) return "Cannot '|' " + in(1)._type;
-        if( !(in(2)._type instanceof TypeInteger) ) return "Cannot '|' " + in(2)._type;
-        return null;
-    }
 }
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 91334b6c..ccbdd460 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/PhiNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/PhiNode.java
@@ -1,9 +1,9 @@
 package com.seaofnodes.simple.node;
 
+import com.seaofnodes.simple.Ary;
+import com.seaofnodes.simple.SB;
 import com.seaofnodes.simple.Utils;
-import com.seaofnodes.simple.type.Type;
-import com.seaofnodes.simple.type.TypeMem;
-import com.seaofnodes.simple.type.TypeNil;
+import com.seaofnodes.simple.type.*;
 import java.util.BitSet;
 
 public class PhiNode extends Node {
@@ -75,13 +75,21 @@ public class PhiNode extends Node {
         Node live = singleUniqueInput();
         if (live != null)
             return live;
+        // No more fancy peeps
+        if( _disablePeephole )
+            return null;
+
+        // No bother if region is going to fold dead paths soon
+        for( int i=1; i<nIns(); i++ )
+            if( r.in(i)._type == Type.XCONTROL )
+                return null;
 
         // Pull "down" a common data op.  One less op in the world.  One more
         // Phi, but Phis do not make code.
         //   Phi(op(A,B),op(Q,R),op(X,Y)) becomes
         //     op(Phi(A,Q,X), Phi(B,R,Y)).
         Node op = in(1);
-        if( op.nIns()==3 && op.in(0)==null && same_op() ) {
+        if( !isMem() && op.nIns()==3 && op.in(0)==null && same_op() ) {
             assert !(op instanceof CFGNode);
             Node[] lhss = new Node[nIns()];
             Node[] rhss = new Node[nIns()];
@@ -171,4 +179,26 @@ public class PhiNode extends Node {
     @Override boolean eq( Node n ) {
         return !inProgress();
     }
+
+    @Override
+    public String err() {
+        if( _type != Type.BOTTOM ) return null;
+
+        // BOTTOM means we mixed e.g. int and ptr
+        for( int i=1; i<nIns(); i++ )
+            // Already an error, but better error messages come from elsewhere
+            if( in(i)._type == Type.BOTTOM )
+                return null;
+
+        // Gather a minimal set of types that "cover" all the rest
+        boolean ti=false, tf=false, tp=false, tn=false;
+        for( int i=1; i<nIns(); i++ ) {
+            Type t = in(i)._type;
+            ti |= t instanceof TypeInteger x;
+            tf |= t instanceof TypeFloat   x;
+            tp |= t instanceof TypeMemPtr  x;
+            tn |= t==Type.NIL;
+        }
+        return ReturnNode.mixerr(ti,tf,tp,tn);
+    }
 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/RegionNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/RegionNode.java
index 0f9fce7a..5b3d90fd 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/RegionNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/RegionNode.java
@@ -65,7 +65,8 @@ public class RegionNode extends CFGNode {
             return in(1);       // Collapse if no Phis; 1-input Phis will collapse on their own
 
         // If a CFG diamond with no merging, delete: "if( pred ) {} else {};"
-        if( !hasPhi() &&       // No Phi users, just a control user
+        if( !_disablePeephole &&
+            !hasPhi() &&       // No Phi users, just a control user
             in(1) instanceof CProjNode p1 &&
             in(2) instanceof CProjNode p2 &&
             p1.in(0).addDep(this)==p2.in(0).addDep(this) &&
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 35563d6e..5232cfbb 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
@@ -1,6 +1,7 @@
 package com.seaofnodes.simple.node;
 
-import com.seaofnodes.simple.CodeGen;
+import com.seaofnodes.simple.SB;
+import com.seaofnodes.simple.Utils;
 import com.seaofnodes.simple.type.*;
 import java.util.BitSet;
 
@@ -16,6 +17,7 @@ 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 = fun;
@@ -42,15 +44,40 @@ public class ReturnNode extends CFGNode {
         return TypeTuple.make(ctrl()._type,mem()._type,expr()._type);
     }
 
-    @Override public Node idealize() { return null; }
+    @Override public Node idealize() {
+        if( inProgress() ) return null;
+        // If dead (cant be reached; infinite loop), kill the exit values
+        if( ctrl()._type==Type.XCONTROL &&
+            !(mem() instanceof ConstantNode && expr() instanceof ConstantNode) ) {
+            Node top = new ConstantNode(Type.TOP).peephole();
+            setDef(1,top);
+            setDef(2,top);
+            return this;
+        }
+        return null;
+    }
 
     public boolean inProgress() {
         return ctrl().getClass() == RegionNode.class && ((RegionNode)ctrl()).inProgress();
     }
 
+    // Gather parse-time return types for error reporting
+    private Type mt = Type.TOP;
+    private boolean ti=false, tf=false, tp=false, tn=false;
+
     // Add a return exit to the current parsing function
     void addReturn(Node ctrl, Node rmem, Node expr) {
         assert inProgress();
+
+        // Gather parse-time return types for error reporting
+        Type t = expr._type;
+        mt = mt.meet(t);
+        ti |= t instanceof TypeInteger x;
+        tf |= t instanceof TypeFloat   x;
+        tp |= t instanceof TypeMemPtr  x;
+        tn |= t==Type.NIL;
+
+        // Merge path into the One True Return
         RegionNode r = (RegionNode)ctrl();
         // Assert that the Phis are in particular outputs; not reordered or shuffled
         PhiNode mem = (PhiNode)r.out(0); assert mem._declaredType == TypeMem.BOT;
@@ -68,4 +95,17 @@ public class ReturnNode extends CFGNode {
         mem.addDef(null);
         rez.addDef(null);
     }
+
+    @Override public String err() {
+        return mt==Type.BOTTOM ? mixerr(ti,tf,tp,tn) : null;
+    }
+
+    static String mixerr( boolean ti, boolean tf, boolean tp, boolean tn ) {
+        SB sb = new SB().p("No common type amongst ");
+        if( ti ) sb.p("int and ");
+        if( tf ) sb.p("f64 and ");
+        if( tp ) sb.p("reference and ");
+        if( !tp && tn ) sb.p("nil and ");
+        return sb.unchar(5).toString();
+    }
 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/SarNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/SarNode.java
index a3d1045f..a3e2a562 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/SarNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/SarNode.java
@@ -4,20 +4,14 @@ import com.seaofnodes.simple.type.Type;
 import com.seaofnodes.simple.type.TypeInteger;
 import java.util.BitSet;
 
-public class SarNode extends Node {
-    public SarNode(Node lhs, Node rhs) { super(null, lhs, rhs); }
+public class SarNode extends LogicalNode {
+    public SarNode(Node lhs, Node rhs) { super(lhs, rhs); }
 
     @Override public String label() { return "Sar"; }
+    @Override public String op() { return ">>"; }
 
     @Override public String glabel() { return "&gt;&gt;"; }
 
-    @Override
-    StringBuilder _print1(StringBuilder sb, BitSet visited) {
-        in(1)._print0(sb.append("("), visited);
-        in(2)._print0(sb.append(">>"), visited);
-        return sb.append(")");
-    }
-
     @Override
     public Type compute() {
         Type t1 = in(1)._type, t2 = in(2)._type;
@@ -52,9 +46,4 @@ public class SarNode extends Node {
         return null;
     }
     @Override Node copy(Node lhs, Node rhs) { return new SarNode(lhs,rhs); }
-    @Override public String err() {
-        if( !(in(1)._type instanceof TypeInteger) ) return "Cannot '>>' " + in(1)._type;
-        if( !(in(2)._type instanceof TypeInteger) ) return "Cannot '>>' " + in(2)._type;
-        return null;
-    }
 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/ShlNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/ShlNode.java
index e454362a..24dff5e2 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ShlNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ShlNode.java
@@ -5,20 +5,14 @@ import com.seaofnodes.simple.type.Type;
 import com.seaofnodes.simple.type.TypeInteger;
 import java.util.BitSet;
 
-public class ShlNode extends Node {
-    public ShlNode(Node lhs, Node rhs) { super(null, lhs, rhs); }
+public class ShlNode extends LogicalNode {
+    public ShlNode(Node lhs, Node rhs) { super(lhs, rhs); }
 
     @Override public String label() { return "Shl"; }
+    @Override public String op() { return "<<"; }
 
     @Override public String glabel() { return "&lt;&lt;"; }
 
-    @Override
-    StringBuilder _print1(StringBuilder sb, BitSet visited) {
-        in(1)._print0(sb.append("("), visited);
-        in(2)._print0(sb.append("<<"), visited);
-        return sb.append(")");
-    }
-
     @Override
     public Type compute() {
         Type t1 = in(1)._type, t2 = in(2)._type;
@@ -56,9 +50,4 @@ public class ShlNode extends Node {
         return null;
     }
     @Override Node copy(Node lhs, Node rhs) { return new ShlNode(lhs,rhs); }
-    @Override public String err() {
-        if( !(in(1)._type instanceof TypeInteger) ) return "Cannot '<<' " + in(1)._type;
-        if( !(in(2)._type instanceof TypeInteger) ) return "Cannot '<<' " + in(2)._type;
-        return null;
-    }
 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/ShrNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/ShrNode.java
index 6f4a2c6c..52d2971d 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ShrNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ShrNode.java
@@ -4,20 +4,14 @@ import com.seaofnodes.simple.type.Type;
 import com.seaofnodes.simple.type.TypeInteger;
 import java.util.BitSet;
 
-public class ShrNode extends Node {
-    public ShrNode(Node lhs, Node rhs) { super(null, lhs, rhs); }
+public class ShrNode extends LogicalNode {
+    public ShrNode(Node lhs, Node rhs) { super(lhs, rhs); }
 
     @Override public String label() { return "Shr"; }
+    @Override public String op() { return ">>>"; }
 
     @Override public String glabel() { return "&gt;&gt;&gt;"; }
 
-    @Override
-    StringBuilder _print1(StringBuilder sb, BitSet visited) {
-        in(1)._print0(sb.append("("), visited);
-        in(2)._print0(sb.append(">>>"), visited);
-        return sb.append(")");
-    }
-
     @Override
     public Type compute() {
         Type t1 = in(1)._type, t2 = in(2)._type;
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/XorNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/XorNode.java
index c140ba83..a23e1304 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/XorNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/XorNode.java
@@ -5,20 +5,14 @@ import com.seaofnodes.simple.type.TypeInteger;
 
 import java.util.BitSet;
 
-public class XorNode extends Node {
-    public XorNode(Node lhs, Node rhs) { super(null, lhs, rhs); }
+public class XorNode extends LogicalNode {
+    public XorNode(Node lhs, Node rhs) { super(lhs, rhs); }
 
     @Override public String label() { return "Xor"; }
+    @Override public String op() { return "^"; }
 
     @Override public String glabel() { return "^"; }
 
-    @Override
-    StringBuilder _print1(StringBuilder sb, BitSet visited) {
-        in(1)._print0(sb.append("("), visited);
-        in(2)._print0(sb.append("^"), visited);
-        return sb.append(")");
-    }
-
     @Override
     public Type compute() {
         Type t1 = in(1)._type, t2 = in(2)._type;
@@ -57,9 +51,4 @@ public class XorNode extends Node {
         return null;
     }
     @Override Node copy(Node lhs, Node rhs) { return new XorNode(lhs,rhs); }
-    @Override public String err() {
-        if( !(in(1)._type instanceof TypeInteger) ) return "Cannot '^' " + in(1)._type;
-        if( !(in(2)._type instanceof TypeInteger) ) return "Cannot '^' " + in(2)._type;
-        return null;
-    }
 }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/type/TypeMem.java b/chapter18/src/main/java/com/seaofnodes/simple/type/TypeMem.java
index 60b3612f..9438abe2 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/type/TypeMem.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/type/TypeMem.java
@@ -44,6 +44,7 @@ public class TypeMem extends Type {
         return make(_alias,_t.dual());
     }
 
+    @Override public boolean isHigh() { return _t.isHigh(); }
     @Override public Type glb() { return make(_alias,_t.glb()); }
 
     @Override int hash() { return 9876543 + _alias + _t.hashCode(); }
-- 
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