From 6e59fc1b71f2098c8baf90a6c54e5a33b7291e94 Mon Sep 17 00:00:00 2001
From: Cliff Click <cliffc@acm.org>
Date: Sat, 19 Sep 2026 22:31:42 -0700
Subject: Unify mixed-return checking and results

Unify dominator caching.
---
 .../main/java/com/seaofnodes/simple/CodeGen.java   | 10 +++++++++
 .../com/seaofnodes/simple/GlobalCodeMotion.java    |  6 +++---
 .../java/com/seaofnodes/simple/node/CFGNode.java   | 24 ++++++++++++++++------
 .../com/seaofnodes/simple/node/CallEndNode.java    |  2 ++
 .../java/com/seaofnodes/simple/node/FunNode.java   |  4 ++--
 .../java/com/seaofnodes/simple/node/LoopNode.java  |  2 +-
 .../com/seaofnodes/simple/node/RegionNode.java     | 13 ++++++------
 .../com/seaofnodes/simple/node/ReturnNode.java     |  6 ++----
 .../java/com/seaofnodes/simple/node/StopNode.java  |  4 ++--
 .../java/com/seaofnodes/simple/Chapter10Test.java  | 13 ++++++++++++
 .../java/com/seaofnodes/simple/Chapter12Test.java  | 14 +++++++++++++
 .../java/com/seaofnodes/simple/Chapter18Test.java  | 16 +++++++++++++++
 .../seaofnodes/simple/node/PrintTestSupport.java   | 11 ++++++++--
 13 files changed, 98 insertions(+), 27 deletions(-)

(limited to 'chapter18/src')

diff --git a/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java b/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
index 47616b51..1586b3c5 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/CodeGen.java
@@ -6,6 +6,16 @@ import java.util.HashMap;
 
 public class CodeGen {
 
+    // Inlining can insert CFG edges and invalidate cached depth ordering.
+    private char _iDepthVersion;
+    public char iDepthVersion() { return _iDepthVersion; }
+    public void invalidateIDepthCaches() {
+        assert _iDepthVersion < Character.MAX_VALUE : "Dominator cache version exceeds 65535";
+        ++_iDepthVersion;
+    }
+
+
+
     public enum Phase {
         Parse, Opto, TypeCheck, Schedule, LocalSched, RegAlloc;
     }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java b/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
index d3ee62c3..f68043c4 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
@@ -140,7 +140,7 @@ public abstract class GlobalCodeMotion {
         CFGNode lca = null;
         for( Node use : n._outputs )
             if( use != null )
-              lca = use_block(n,use, late)._idom(lca,null);
+              lca = use_block(n,use, late).domLCA(lca,null);
 
         // Loads may need anti-dependencies, raising their LCA
         if( n instanceof LoadNode load )
@@ -167,7 +167,7 @@ public abstract class GlobalCodeMotion {
         CFGNode found=null;
         for( int i=1; i<phi.nIns(); i++ )
             if( phi.in(i)==n )
-                found = phi.region().cfg(i)._idom(found,null);
+                found = phi.region().cfg(i).domLCA(found,null);
 
         assert found!=null;
         return found;
@@ -220,7 +220,7 @@ public abstract class GlobalCodeMotion {
         for( ; stblk != defblk.idom(); stblk = stblk.idom() ) {
             // Store and Load overlap, need anti-dependence
             if( stblk._anti==load._nid ) {
-                lca = stblk._idom(lca,null); // Raise Loads LCA
+                lca = stblk.domLCA(lca,null); // Raise Loads LCA
                 if( lca == stblk && st != null && st._inputs.find(load) == -1 ) // And if something moved,
                     st.addDef(load);   // Add anti-dep as well
                 return lca;            // Cap this stores' anti-dep to here
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 b051b3ce..dc486549 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/CFGNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/CFGNode.java
@@ -1,5 +1,6 @@
 package com.seaofnodes.simple.node;
 
+import com.seaofnodes.simple.CodeGen;
 import com.seaofnodes.simple.Parser;
 import com.seaofnodes.simple.type.*;
 import com.seaofnodes.simple.Utils;
@@ -47,21 +48,32 @@ public abstract class CFGNode extends Node {
      * <p>
      * See {@link <a href="https://en.wikipedia.org/wiki/Dominator_(graph_theory)">...</a>}
      */
-    public int _idepth;
-    public int idepth() { return _idepth==0 ? (_idepth=idom().idepth()+1) : _idepth; }
+    public char _idepth;
+    public char _idepthVersion;
+    public int idepth() { return validIDepth() ? _idepth : cacheIDepth(idom().idepth()+1); }
+    // Zero depth means uncached. Check before narrowing so overflow cannot wrap.
+    final int cacheIDepth(int depth) {
+        assert 0 <= depth && depth <= Character.MAX_VALUE : "Dominator depth exceeds 65535";
+        _idepthVersion = CodeGen.CODE.iDepthVersion();
+        return _idepth = (char)depth;
+    }
+
+    final boolean validIDepth() { return _idepth!=0 && _idepthVersion==CodeGen.CODE.iDepthVersion(); }
+
 
     // Return the immediate dominator of this Node and compute dom tree depth.
     public CFGNode idom(Node dep) { return cfg(0); }
     public final CFGNode idom() { return idom(null); }
 
     // Return the LCA of two idoms
-    public CFGNode _idom(CFGNode rhs, Node dep) {
+    public CFGNode domLCA(CFGNode rhs, Node dep) {
         if( rhs==null ) return this;
         CFGNode lhs = this;
         while( lhs != rhs ) {
-            var comp = lhs.idepth() - rhs.idepth();
-            if( comp >= 0 ) lhs = ((CFGNode)lhs.addDep(dep)).idom();
-            if( comp <= 0 ) rhs = ((CFGNode)rhs.addDep(dep)).idom();
+            if( lhs==null || rhs==null ) return null;
+            int comp = lhs.idepth() - rhs.idepth();
+            if( comp >= 0 ) lhs = (dep==null ? lhs : (CFGNode)lhs.addDep(dep)).idom();
+            if( comp <= 0 ) rhs = (dep==null ? rhs : (CFGNode)rhs.addDep(dep)).idom();
         }
         return lhs;
     }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/CallEndNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/CallEndNode.java
index 07a2b5a3..6d7aa323 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/CallEndNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/CallEndNode.java
@@ -1,5 +1,6 @@
 package com.seaofnodes.simple.node;
 
+import com.seaofnodes.simple.CodeGen;
 import com.seaofnodes.simple.IterPeeps;
 import com.seaofnodes.simple.Parser;
 import com.seaofnodes.simple.Utils;
@@ -73,6 +74,7 @@ public class CallEndNode extends CFGNode implements MultiNode {
                         fun.setDef(2,call.ctrl());  // Bypass the Call;
                         fun.ret().setDef(3,null);   // Return is folding also
                         IterPeeps.addAll(fun._outputs);
+                        CodeGen.CODE.invalidateIDepthCaches();
                         return this;
                     }
                 } else {
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 1f0c76eb..5873f6b6 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/FunNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/FunNode.java
@@ -78,8 +78,8 @@ public class FunNode extends RegionNode {
         return progress;
     }
 
-    // Bypass Region idom, always assume depth == 1, one more than Start
-    @Override public int idepth() { return (_idepth=1); }
+    // Unfolded functions are roots; inlining reconnects their depth to the caller.
+    @Override public int idepth() { return _folding ? super.idepth() : 1; }
     // Bypass Region idom, always assume idom is Start
     @Override public CFGNode idom(Node dep) { return cfg(1); }
 
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 f70a1012..097ddea7 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/LoopNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/LoopNode.java
@@ -24,7 +24,7 @@ public class LoopNode extends RegionNode {
     }
 
     // Bypass Region idom, same as the default idom() using use in(1) instead of in(0)
-    @Override public int idepth() { return _idepth==0 ? (_idepth=idom().idepth()+1) : _idepth; }
+    @Override public int idepth() { return validIDepth() ? _idepth : cacheIDepth(entry().idepth()+1); }
     // Bypass Region idom, same as the default idom() using use in(1) instead of in(0)
     @Override public CFGNode idom(Node dep) { return entry(); }
 
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 ea053c02..9a076dea 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/RegionNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/RegionNode.java
@@ -100,20 +100,19 @@ public class RegionNode extends CFGNode {
 
     // Immediate dominator of Region is a little more complicated.
     @Override public int idepth() {
-        if( _idepth!=0 ) return _idepth;
-        int d=0;
+        if( validIDepth() ) return _idepth;
+        int depth=0;
         for( Node n : _inputs )
             if( n!=null )
-                d = Math.max(d,((CFGNode)n).idepth()+1);
-        return _idepth=d;
+                depth = Math.max(depth,((CFGNode)n).idepth()+1);
+        return cacheIDepth(depth);
     }
 
     @Override public CFGNode idom(Node dep) {
         CFGNode lca = null;
-        // Walk the LHS & RHS idom trees in parallel until they match, or either fails.
-        // Because this does not cache, it can be linear in the size of the program.
+        // Recompute from predecessors: CFG edits can change the dominator.
         for( int i=1; i<nIns(); i++ )
-            lca = cfg(i)._idom(lca,dep);
+            lca = cfg(i).domLCA(lca,dep);
         return lca;
     }
 
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 e543f9ea..e516859b 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
@@ -77,8 +77,7 @@ public class ReturnNode extends CFGNode {
         return ctrl().getClass() == RegionNode.class && ((RegionNode)ctrl()).inProgress();
     }
 
-    // Gather parse-time return types for error reporting
-    private Type mt = Type.TOP;
+    // Remember parsed type kinds for diagnostics; the optimized expression decides validity.
     private boolean ti=false, tf=false, tp=false, tn=false;
 
     // Add a return exit to the current parsing function
@@ -87,7 +86,6 @@ public class ReturnNode extends CFGNode {
 
         // 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;
@@ -113,7 +111,7 @@ public class ReturnNode extends CFGNode {
     }
 
     @Override public Parser.ParseException err() {
-        return mt==Type.BOTTOM ? mixerr(ti,tf,tp,tn,_fun._loc) : null;
+        return expr()._type==Type.BOTTOM ? mixerr(ti,tf,tp,tn,_fun._loc) : null;
     }
 
     static Parser.ParseException mixerr( boolean ti, boolean tf, boolean tp, boolean tn, Parser.Lexer loc ) {
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 feb8347f..cabb7393 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/StopNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/StopNode.java
@@ -54,10 +54,10 @@ public class StopNode extends CFGNode {
     }
 
     @Override public int idepth() {
-        if( _idepth!=0 ) return _idepth;
+        if( validIDepth() ) return _idepth;
         int d=0;
         for( Node ret : _inputs )
             d = Math.max(d,((ReturnNode)ret).idepth()+1);
-        return _idepth=d;
+        return cacheIDepth(d);
     }
 }
diff --git a/chapter18/src/test/java/com/seaofnodes/simple/Chapter10Test.java b/chapter18/src/test/java/com/seaofnodes/simple/Chapter10Test.java
index 42684416..378e4f40 100644
--- a/chapter18/src/test/java/com/seaofnodes/simple/Chapter10Test.java
+++ b/chapter18/src/test/java/com/seaofnodes/simple/Chapter10Test.java
@@ -5,6 +5,19 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 
 public class Chapter10Test {
+    @Test public void testDeadReferenceReturn() {
+        for( String body : new String[] {
+            "return new S; return 0;",
+            "if(1) return new S; else return 0;"
+        } ) {
+            String src = "struct S { int x; }; " + body;
+            var code = new CodeGen(src).parse().opto().typeCheck();
+            org.junit.Assert.assertTrue(body,
+                code.expr()._type instanceof com.seaofnodes.simple.type.TypeMemPtr);
+        }
+    }
+
+
 
     // Issue #246: null-check guards start in Chapter 10; arrays arrive in Chapter 15.
     private static final String NULLABLE_POINT_SOURCE = """
diff --git a/chapter18/src/test/java/com/seaofnodes/simple/Chapter12Test.java b/chapter18/src/test/java/com/seaofnodes/simple/Chapter12Test.java
index 21df8205..43345809 100644
--- a/chapter18/src/test/java/com/seaofnodes/simple/Chapter12Test.java
+++ b/chapter18/src/test/java/com/seaofnodes/simple/Chapter12Test.java
@@ -8,6 +8,20 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 
 public class Chapter12Test {
+    @Test public void testDeadNumericReturns() {
+        for( String[] test : new String[][] {
+            {"if(1) return 7; else return 2.5;", "7"},
+            {"if(0) return 7; else return 2.5;", "2.5"},
+            {"return 7; return 2.5;", "7"},
+            {"return 2.5; return 7;", "2.5"}
+        } ) {
+            String src = test[0];
+            var code = new CodeGen(src).parse().opto().typeCheck();
+            assertEquals(src,test[1],Eval2.eval(code,0));
+        }
+    }
+
+
     @Test
     public void testJig() {
         CodeGen code = new CodeGen(
diff --git a/chapter18/src/test/java/com/seaofnodes/simple/Chapter18Test.java b/chapter18/src/test/java/com/seaofnodes/simple/Chapter18Test.java
index f9be0e93..e4a9e409 100644
--- a/chapter18/src/test/java/com/seaofnodes/simple/Chapter18Test.java
+++ b/chapter18/src/test/java/com/seaofnodes/simple/Chapter18Test.java
@@ -9,6 +9,22 @@ import static org.junit.Assert.fail;
 import org.junit.Ignore;
 
 public class Chapter18Test {
+    @Test public void testReachableMixedReturns() {
+        for( String[] test : new String[][] {
+            {"if(arg) return 7; else return 2.5;", "No common type amongst int and f64"},
+            {"struct S { int x; }; if(arg) return new S; else return 0;",
+             "No common type amongst int and reference"}
+        } ) {
+            try {
+                new CodeGen(test[0]).parse().opto().typeCheck();
+                fail("Expected incompatible return types: " + test[0]);
+            } catch( Parser.ParseException e ) {
+                assertEquals(test[1],e.getMessage());
+            }
+        }
+    }
+
+
     @Test public void testPrintingForwardReferenceScope() throws Exception {
         new CodeGen("return 0;").parse();
         var declared = TypeMemPtr.make(TypeStruct.makeFRef("PrintForward"));
diff --git a/chapter18/src/test/java/com/seaofnodes/simple/node/PrintTestSupport.java b/chapter18/src/test/java/com/seaofnodes/simple/node/PrintTestSupport.java
index 01d471bf..ceea1de5 100644
--- a/chapter18/src/test/java/com/seaofnodes/simple/node/PrintTestSupport.java
+++ b/chapter18/src/test/java/com/seaofnodes/simple/node/PrintTestSupport.java
@@ -21,17 +21,24 @@ public class PrintTestSupport {
         var ins = new IdentityHashMap<Node,Node[]>();
         var outs = new IdentityHashMap<Node,Node[]>();
         var hashes = new IdentityHashMap<Node,Integer>();
+        var versions = new IdentityHashMap<Node,Integer>();
         var depths = new IdentityHashMap<Node,Integer>();
         for( Node n : nodes ) {
             ins.put(n,n._inputs.toArray(new Node[0]));
             outs.put(n,n._outputs.toArray(new Node[0]));
             hashes.put(n,hash.getInt(n));
-            if( n instanceof CFGNode cfg ) depths.put(n,cfg._idepth);
+            if( n instanceof CFGNode cfg ) {
+                depths.put(n,(int)cfg._idepth);
+                versions.put(n,(int)cfg._idepthVersion);
+            }
         }
         print.run();
         for( Node n : nodes ) {
             assertEquals("hash of node "+n._nid,(int)hashes.get(n),hash.getInt(n));
-            if( n instanceof CFGNode cfg ) assertEquals("depth of node "+n._nid,(int)depths.get(n),cfg._idepth);
+            if( n instanceof CFGNode cfg ) {
+                assertEquals("depth of node "+n._nid,(int)depths.get(n),cfg._idepth);
+                assertEquals("depth version of node "+n._nid,(int)versions.get(n),cfg._idepthVersion);
+            }
             assertEquals(ins.get(n).length,n.nIns());
             assertEquals(outs.get(n).length,n.nOuts());
             for( int i=0; i<n.nIns(); i++ ) assertSame(ins.get(n)[i],n.in(i));
-- 
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