From 64ebe7529f9798ce036ab86ad4e09071152fe64d Mon Sep 17 00:00:00 2001
From: Cliff Click <cliffc@acm.org>
Date: Thu, 9 Jan 2025 12:08:51 -0800
Subject: Final vars and forced constants do not lazy-phi

Better comments on why a field is final
---
 chapter18/src/main/java/com/seaofnodes/simple/Parser.java    | 12 +++++++++---
 .../src/main/java/com/seaofnodes/simple/node/ScopeNode.java  |  5 ++++-
 2 files changed, 13 insertions(+), 4 deletions(-)

(limited to 'chapter18/src/main')

diff --git a/chapter18/src/main/java/com/seaofnodes/simple/Parser.java b/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
index 3f4484f5..a6185aa0 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/Parser.java
@@ -694,10 +694,16 @@ public class Parser {
         Node expr;
         if( match("=") ) {
             expr = parseAsgn();
-            // `val` is always final
+            // TOP means val and val is always final
             xfinal = (t==Type.TOP) ||
-                // var is always not-final, final if no Bang AND TMP since primitives are not-final by default
-                (t!=Type.BOTTOM && !hasBang && (t instanceof TypeMemPtr));
+                // BOTTOM is var and var is always not-final
+                (t!=Type.BOTTOM &&
+                 // no Bang AND
+                 !hasBang &&
+                 // not-null (expecting null to be set to not-null)
+                 expr._type != Type.NIL &&
+                 // Pointers are final by default; int/flt are not-final by default.
+                 (t instanceof TypeNil));
             // var/val, then type comes from expression
             if( inferType ) {
                 if( expr._type==Type.NIL )
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 5620a2d2..d0d86b79 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
@@ -209,6 +209,7 @@ public class ScopeNode extends MemMergeNode {
                 : loop.setDef(v._idx,new PhiNode(v._name, v.lazyGLB(), loop.ctrl(), loop.in(loop.update(v,null)._idx),null).peephole());
             setDef(v._idx,old);
         }
+        assert !v._final || st==null;
         if( st!=null ) setDef(v._idx,st); // Set new value
         return v;
     }
@@ -258,7 +259,7 @@ public class ScopeNode extends MemMergeNode {
         for( int i=2; i<nIns(); i++ )
             // For lazy phis on loops we use a sentinel
             // that will trigger phi creation on update
-            dup.addDef(loop ? this : in(i));
+            dup.addDef(loop && !_vars.at(i)._final ? this : in(i));
         return dup;
     }
 
@@ -327,6 +328,8 @@ public class ScopeNode extends MemMergeNode {
     // Fill in the backedge of any inserted Phis
     void _endLoop( ScopeNode scope, Node back, Node exit ) {
         for( int i=2; i<nIns(); i++ ) {
+            if( _vars.at(i)._final ) continue; // Final vars did not get modified in the loop
+            if( _vars.at(i).type().isHighOrConst() ) continue; // Cannot lift higher than a constant, so no Phi
             if( back.in(i) != scope ) {
                 PhiNode phi = (PhiNode)in(i);
                 assert phi.region()==scope.ctrl() && phi.in(2)==null;
-- 
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