Nick pointed out on IRC that GVN's propagateEquality wasn't propagating

equalities into phi node operands for which the equality is known to
hold in the incoming basic block.  That's because replaceAllDominatedUsesWith
wasn't handling phi nodes correctly in general (that this didn't give wrong
results was just luck: the specific way GVN uses replaceAllDominatedUsesWith
precluded wrong changes to phi nodes).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152006 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 11 insertions, 1 deletions

diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index 37e970b..dc13edf 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -1953,7 +1953,17 @@ unsigned GVN::replaceAllDominatedUsesWith(Value *From, Value *To,
   for (Value::use_iterator UI = From->use_begin(), UE = From->use_end();
        UI != UE; ) {
     Use &U = (UI++).getUse();
-    if (DT->dominates(Root, cast<Instruction>(U.getUser())->getParent())) {
+
+    // If From occurs as a phi node operand then the use implicitly lives in the
+    // corresponding incoming block.  Otherwise it is the block containing the
+    // user that must be dominated by Root.
+    BasicBlock *UsingBlock;
+    if (PHINode *PN = dyn_cast<PHINode>(U.getUser()))
+      UsingBlock = PN->getIncomingBlock(U);
+    else
+      UsingBlock = cast<Instruction>(U.getUser())->getParent();
+
+    if (DT->dominates(Root, UsingBlock)) {
       U.set(To);
       ++Count;
     }