Extend and backport SubNode peephole

2 files changed, 23 insertions, 0 deletions

diff --git a/chapter04/README.md b/chapter04/README.md
index 2c7f7e9c..07085e80 100644
--- a/chapter04/README.md
+++ b/chapter04/README.md
@@ -305,6 +305,7 @@ Here is a list of peepholes introduced in this Chapter, more will be introduced
 | Before                | After                 | Description                                    |
 |-----------------------|-----------------------|------------------------------------------------|
 | (arg + 0 )            |  arg                  | Add of zero identity                           |
+| (arg - 0 )            |  arg                  | Sub of zero identity                           |
 | (arg * 1 )            |  arg                  | Multiple of one identity                       |
 | (con + arg)           | (arg + con)           | Move constants to right, to encourage folding  |
 | (con * arg)           | (arg * con)           | Move constants to right, to encourage folding  |
@@ -312,6 +313,9 @@ Here is a list of peepholes introduced in this Chapter, more will be introduced
 | ((arg1 + con) + arg2) | ((arg1 + arg2) + con) | Move constants to right, to encourage folding  |
 | (arg + arg)           | (arg * 2)             | Sum-of-products form                           |
 | (arg - arg)           | 0                     | Sub of same is 0
+| (0 - arg)             | (-arg)                | Sub from 0 is negation
+| (arg1 - (-arg2))      | (arg1 + arg2)         | Sub of negation is add
+| ((-arg1) - arg2)      | -(arg1 + arg2)        | Sub from negation is negation of sum
 | (con / 1)             | con                   | Division by one
 | (arg == arg)          | 1                     | Compare of same
 | (arg != arg)          | 0                     | Compare of same
diff --git a/chapter04/src/main/java/com/seaofnodes/simple/node/SubNode.java b/chapter04/src/main/java/com/seaofnodes/simple/node/SubNode.java
index f5ad663d..304092f4 100644
--- a/chapter04/src/main/java/com/seaofnodes/simple/node/SubNode.java
+++ b/chapter04/src/main/java/com/seaofnodes/simple/node/SubNode.java
@@ -32,6 +32,25 @@ 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;
+
+        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);
+
+        // x - (-y) is x+y
+        if( rhs instanceof MinusNode minus )
+            return new AddNode(lhs,minus.in(1));
+
+        // (-x) - y is -(x+y)
+        if( lhs instanceof MinusNode minus )
+            return new MinusNode(new AddNode(minus.in(1),rhs).peephole());
+
         return null;
     }