[MemorySSA] Don't optimize incomplete phis.

Summary:
MemoryPhis cannot be optimized out until they are complete.
Resolves PR41254.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59966

llvm-svn: 357315

1 files changed, 9 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/MemorySSAUpdater.cpp b/llvm/lib/Analysis/MemorySSAUpdater.cpp
index f3ea8827370..fe84ca1207e 100644
--- a/llvm/lib/Analysis/MemorySSAUpdater.cpp
+++ b/llvm/lib/Analysis/MemorySSAUpdater.cpp
@@ -309,8 +309,15 @@ void MemorySSAUpdater::insertDef(MemoryDef *MD, bool RenameUses) {
       IDFs.calculate(IDFBlocks);
       SmallVector<AssertingVH<MemoryPhi>, 4> NewInsertedPHIs;
       for (auto *BBIDF : IDFBlocks)
-        if (!MSSA->getMemoryAccess(BBIDF))
-          NewInsertedPHIs.push_back(MSSA->createMemoryPhi(BBIDF));
+        if (!MSSA->getMemoryAccess(BBIDF)) {
+          auto *MPhi = MSSA->createMemoryPhi(BBIDF);
+          NewInsertedPHIs.push_back(MPhi);
+          // Add the phis created into the IDF blocks to NonOptPhis, so they are
+          // not optimized out as trivial by the call to getPreviousDefFromEnd
+          // below. Once they are complete, all these Phis are added to the
+          // FixupList, and removed from NonOptPhis inside fixupDefs().
+          NonOptPhis.insert(MPhi);
+        }
 
       for (auto &MPhi : NewInsertedPHIs) {
         auto *BBIDF = MPhi->getBlock();