Restrict subtraction zero peepholes to integer operandspr254-fixes

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.

3 files changed, 21 insertions, 5 deletions

diff --git a/chapter23/src/main/java/com/seaofnodes/simple/node/SubNode.java b/chapter23/src/main/java/com/seaofnodes/simple/node/SubNode.java
index 6dbd55cb..2c4fed39 100644
--- a/chapter23/src/main/java/com/seaofnodes/simple/node/SubNode.java
+++ b/chapter23/src/main/java/com/seaofnodes/simple/node/SubNode.java
@@ -32,11 +32,11 @@ public class SubNode extends ArithNode {
         Type t1 = lhs._type;
         Type t2 = rhs._type;
 
-        if( t2.isConstant() && t2 instanceof TypeInteger i && i.value()==0 )
-            return lhs;
-
-        if( t1.isConstant() && t1 instanceof TypeInteger i && i.value()==0 )
-            return new MinusNode(rhs);
+        // 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( rhs instanceof MinusNode minus )
diff --git a/chapter23/src/test/java/com/seaofnodes/simple/Chapter12Test.java b/chapter23/src/test/java/com/seaofnodes/simple/Chapter12Test.java
index 21759b78..c50be559 100644
--- a/chapter23/src/test/java/com/seaofnodes/simple/Chapter12Test.java
+++ b/chapter23/src/test/java/com/seaofnodes/simple/Chapter12Test.java
@@ -7,6 +7,13 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 
 public class Chapter12Test {
+    @Test public void testSubZeroFloat() {
+        var code = new CodeGen("flt x = arg; return 0-x;").parse().opto().typeCheck();
+        // Compare strings so +0.0 and -0.0 remain distinct.
+        assertEquals("0.0", Eval2.eval(code,0));
+        assertEquals("-1.0", Eval2.eval(code,1));
+    }
+
     @Test public void testDeadNumericReturns() {
         for( String[] test : new String[][] {
             {"if(1) return 7; else return 2.5;", "7"},
diff --git a/chapter23/src/test/java/com/seaofnodes/simple/Chapter22Test.java b/chapter23/src/test/java/com/seaofnodes/simple/Chapter22Test.java
index a3e9a016..bca06f0c 100644
--- a/chapter23/src/test/java/com/seaofnodes/simple/Chapter22Test.java
+++ b/chapter23/src/test/java/com/seaofnodes/simple/Chapter22Test.java
@@ -13,6 +13,15 @@ import org.junit.Test;
 import static org.junit.Assert.*;
 
 public class Chapter22Test {
+    @Test public void testSubZeroTypeError() {
+        try {
+            new CodeGen("return null-0;").parse().opto().typeCheck();
+            fail("Subtraction must reject null even when the other operand is zero");
+        } catch( Parser.ParseException e ) {
+            assertEquals("Cannot '-' null",e.getMessage());
+        }
+    }
+
     @Test public void testInfiniteReturn() {
         String src = "struct S { int i; }; S !s = new S; while(1) s.i++; return s.i;";
         testCPU(src,"x86_64_v2","SystemV",0,"return Top;");