Turn off Store GVN with explaination

1 files changed, 9 insertions, 0 deletions

diff --git a/chapter18/src/main/java/com/seaofnodes/simple/node/StoreNode.java b/chapter18/src/main/java/com/seaofnodes/simple/node/StoreNode.java
index de17bbad..93e71554 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/node/StoreNode.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/node/StoreNode.java
@@ -167,4 +167,13 @@ public class StoreNode extends MemOpNode {
         Type t = val()._type;
         return _init || t.isa(_declaredType) ? null : Parser.error("Cannot store "+t+" into field "+_declaredType+" "+_name,_loc);
     }
+
+    // Do not GVN Stores. Two equal Stores on two of three paths are the easy
+    // failure: sharing them makes GCM place the Store on all three paths.
+    // Sharing is safe only if every exit path goes through an original Store,
+    // or GCM splits the shared Store to keep it on those paths. Stores must
+    // sink, not run on extra paths. GVN does not check this; GCM does not split.
+    // Identity also disables generic Phi factoring of Stores.
+    // The closed-loop Store sink is a separate peep and still applies.
+    @Override boolean eq(Node n) { return this == n; }
 }