From dd907c2d86eaaad0910e7bfe3f0d25e5caf8bdf7 Mon Sep 17 00:00:00 2001
From: Hsiang-Ying Fu <66062782+fuhsnn@users.noreply.github.com>
Date: Sat, 26 Sep 2026 06:35:22 +0800
Subject: 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>
---
 .../java/com/seaofnodes/simple/node/SubNode.java   | 19 +++++++++++----
 .../java/com/seaofnodes/simple/Chapter04Test.java  | 28 ++++++++++++++++++++++
 .../java/com/seaofnodes/simple/Chapter12Test.java  |  7 ++++++
 3 files changed, 50 insertions(+), 4 deletions(-)

(limited to 'chapter15/src')

diff --git a/chapter15/src/main/java/com/seaofnodes/simple/node/SubNode.java b/chapter15/src/main/java/com/seaofnodes/simple/node/SubNode.java
index 9eb30089..237e40ed 100644
--- a/chapter15/src/main/java/com/seaofnodes/simple/node/SubNode.java
+++ b/chapter15/src/main/java/com/seaofnodes/simple/node/SubNode.java
@@ -37,13 +37,24 @@ 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;
     }
diff --git a/chapter15/src/test/java/com/seaofnodes/simple/Chapter04Test.java b/chapter15/src/test/java/com/seaofnodes/simple/Chapter04Test.java
index 80cdb3f2..a3109ff7 100644
--- a/chapter15/src/test/java/com/seaofnodes/simple/Chapter04Test.java
+++ b/chapter15/src/test/java/com/seaofnodes/simple/Chapter04Test.java
@@ -50,6 +50,34 @@ public class Chapter04Test {
         assertEquals("return arg;", ret.print());
     }
 
+    @Test
+    public void testSub0() {
+        Parser parser = new Parser("return arg-0;");
+        StopNode ret = parser.parse();
+        assertEquals("return arg;", ret.print());
+    }
+
+    @Test
+    public void test0Sub() {
+        Parser parser = new Parser("return 0-arg;");
+        StopNode ret = parser.parse();
+        assertEquals("return (-arg);", ret.print());
+    }
+
+    @Test
+    public void testSubNeg() {
+        Parser parser = new Parser("return 3--arg;");
+        StopNode ret = parser.parse();
+        assertEquals("return (arg+3);", ret.print());
+    }
+
+    @Test
+    public void testNegSub() {
+        Parser parser = new Parser("return -arg-3;");
+        StopNode ret = parser.parse();
+        assertEquals("return (-(arg+3));", ret.print());
+    }
+
     @Test
     public void testVarArg() {
         Parser parser = new Parser("return arg; ");
diff --git a/chapter15/src/test/java/com/seaofnodes/simple/Chapter12Test.java b/chapter15/src/test/java/com/seaofnodes/simple/Chapter12Test.java
index c8802f4c..4eaf81df 100644
--- a/chapter15/src/test/java/com/seaofnodes/simple/Chapter12Test.java
+++ b/chapter15/src/test/java/com/seaofnodes/simple/Chapter12Test.java
@@ -8,6 +8,13 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 
 public class Chapter12Test {
+    @Test public void testSubZeroFloat() {
+        var stop = new Parser("flt x = arg; return 0-x;").parse().iterate();
+        // Compare strings so +0.0 and -0.0 remain distinct.
+        assertEquals("0.0", Evaluator.evaluate(stop,0).toString());
+        assertEquals("-1.0", Evaluator.evaluate(stop,1).toString());
+    }
+
     @Test public void testDeadNumericReturns() {
         for( String[] test : new String[][] {
             {"if(1) return 7; else return 2.5;", "7"},
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