[NewGVN] Make sure we do not add a user to itself.

If we simplify an instruction to itself, we do not need to add a user to
itself. For congruence classes with a defining expression, we already
use a similar logic.

Fixes PR38259.

Reviewers: davide, efriedma, mcrosier

Reviewed By: davide

Differential Revision: https://reviews.llvm.org/D51168

llvm-svn: 346335

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Scalar/NewGVN.cpp b/llvm/lib/Transforms/Scalar/NewGVN.cpp
index cd57ebd0c6f..9803bcb485d 100644
--- a/llvm/lib/Transforms/Scalar/NewGVN.cpp
+++ b/llvm/lib/Transforms/Scalar/NewGVN.cpp
@@ -1086,9 +1086,13 @@ const Expression *NewGVN::checkSimplificationResults(Expression *E,
   CongruenceClass *CC = ValueToClass.lookup(V);
   if (CC) {
     if (CC->getLeader() && CC->getLeader() != I) {
-      // Don't add temporary instructions to the user lists.
-      if (!AllTempInstructions.count(I))
-        addAdditionalUsers(V, I);
+      // If we simplified to something else, we need to communicate
+      // that we're users of the value we simplified to.
+      if (I != V) {
+        // Don't add temporary instructions to the user lists.
+        if (!AllTempInstructions.count(I))
+          addAdditionalUsers(V, I);
+      }
       return createVariableOrConstant(CC->getLeader());
     }
     if (CC->getDefiningExpr()) {