[mssa] Fix case when there is no definition in a block prior to an inserted use.

Summary:
Check that the first access before one being tested is valid.
Before this patch, if there was no definition prior to the Use being tested,
the first time Iter was deferenced, it hit the sentinel.

Reviewers: dberlin, gbiv

Subscribers: sanjoy, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D33950

llvm-svn: 304926

1 files changed, 6 insertions, 11 deletions

diff --git a/llvm/lib/Analysis/MemorySSAUpdater.cpp b/llvm/lib/Analysis/MemorySSAUpdater.cpp
index 86a4eb66fc9..1ff84471c09 100644
--- a/llvm/lib/Analysis/MemorySSAUpdater.cpp
+++ b/llvm/lib/Analysis/MemorySSAUpdater.cpp
@@ -124,17 +124,12 @@ MemoryAccess *MemorySSAUpdater::getPreviousDefInBlock(MemoryAccess *MA) {
         return &*Iter;
     } else {
       // Otherwise, have to walk the all access iterator.
-      auto Iter = MA->getReverseIterator();
-      ++Iter;
-      while (&*Iter != &*Defs->begin()) {
-        if (!isa<MemoryUse>(*Iter))
-          return &*Iter;
-        --Iter;
-      }
-      // At this point it must be pointing at firstdef
-      assert(&*Iter == &*Defs->begin() &&
-             "Should have hit first def walking backwards");
-      return &*Iter;
+      auto End = MSSA->getWritableBlockAccesses(MA->getBlock())->rend();
+      for (auto &U : make_range(++MA->getReverseIterator(), End))
+        if (!isa<MemoryUse>(U))
+          return cast<MemoryAccess>(&U);
+      // Note that if MA comes before Defs->begin(), we won't hit a def.
+      return nullptr;
     }
   }
   return nullptr;