Extend and backport SubNode peephole (#254)

* Extend and backport SubNode peephole

* Add tests

* Restrict subtraction zero peepholes to integer operands

Require resolved integer mode in chapter 25 so zero subtraction preserves floating-point signed zero. Keep nonnumeric operands in the graph for subtraction diagnostics.

Add signed-zero coverage from chapter 12 and null-subtraction diagnostics from chapter 22. Confirmed the regressions fail on the original PR. Full Make suites pass in chapters 4-25 (5462 tests), including fresh chapter 25 sys.o compilation.

---------

Co-authored-by: Cliff Click <cliffc@acm.org>

1 files changed, 15 insertions, 4 deletions

diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/SubNode.java b/chapter21/src/main/java/com/seaofnodes/simple/node/SubNode.java
index 51164dcc..3b3a6321 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/node/SubNode.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/SubNode.java
@@ -41,13 +41,24 @@ public class SubNode extends Node {
 
     @Override
     public Node idealize() {
+        Node lhs = in(1);
+        Node rhs = in(2);
+        Type t1 = lhs._type;
+        Type t2 = rhs._type;
+
+        // Keep invalid operands for type checking.
+        if( t1 instanceof TypeInteger x && t2 instanceof TypeInteger y ) {
+            if( y.isConstant() && y.value()==0 ) return lhs;
+            if( x.isConstant() && x.value()==0 ) return new MinusNode(rhs);
+        }
+
         // x - (-y) is x+y
-        if( in(2) instanceof MinusNode minus )
-            return new AddNode(in(1),minus.in(1));
+        if( rhs instanceof MinusNode minus )
+            return new AddNode(lhs,minus.in(1));
 
         // (-x) - y is -(x+y)
-        if( in(1) instanceof MinusNode minus )
-            return new MinusNode(new AddNode(minus.in(1),in(2)).peephole());
+        if( lhs instanceof MinusNode minus )
+            return new MinusNode(new AddNode(minus.in(1),rhs).peephole());
 
         return null;
     }