Handle double-negation preserving not-null knowledge

1 files changed, 7 insertions, 0 deletions

diff --git a/chapter15/src/main/java/com/seaofnodes/simple/node/ScopeNode.java b/chapter15/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
index 877ae4a4..9c4c4403 100644
--- a/chapter15/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
+++ b/chapter15/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
@@ -252,6 +252,13 @@ public class ScopeNode extends Node {
     // combinations, and replaces the variable with the upcast variant.
     public Node upcast( Node ctrl, Node pred, boolean invert ) {
         if( ctrl._type==Type.XCONTROL ) return null;
+        // A single negation is handled below. For !!p (including p != null),
+        // also refine the underlying p without changing the Boolean result.
+        if( pred instanceof NotNode not && not.in(1) instanceof NotNode ) {
+            pred.keep();        // Recursive peepholes may rediscover this Not.
+            upcast(ctrl,not.in(1),!invert);
+            pred.unkeep();
+        }
         // Invert the If conditional
         if( invert )
             pred = pred instanceof NotNode not ? not.in(1) : IterPeeps.add(new NotNode(pred).peephole());