[MemorySSA] Small fix for the clobber limit.

Summary:
After introducing the limit for clobber walking, `walkToPhiOrClobber` would assert that the limit is at least 1 on entry.
The test included triggered that assert.

The callsite in `tryOptimizePhi` making the calls to `walkToPhiOrClobber` is structured like this:
```
while (true) {
   if (getBlockingAccess()) { // calls walkToPhiOrClobber
   }
   for (...) {
     walkToPhiOrClobber();
   }
}
```

The cleanest fix is to check if the limit was reached inside `walkToPhiOrClobber`, and give an allowence of 1.
This approach not make any alias() calls (no calls to instructionClobbersQuery), so the performance condition is enforced.
The limit is set back to 0 if not used, as this provides info on the fact that we stopped before reaching a true clobber.

Reviewers: george.burgess.iv

Subscribers: jlebar, Prazek, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60479

llvm-svn: 358303

1 files changed, 12 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/MemorySSA.cpp b/llvm/lib/Analysis/MemorySSA.cpp
index a8100312f21..cd6235de685 100644
--- a/llvm/lib/Analysis/MemorySSA.cpp
+++ b/llvm/lib/Analysis/MemorySSA.cpp
@@ -544,10 +544,15 @@ template <class AliasAnalysisType> class ClobberWalker {
                      const MemoryAccess *SkipStopAt = nullptr) const {
     assert(!isa<MemoryUse>(Desc.Last) && "Uses don't exist in my world");
     assert(UpwardWalkLimit && "Need a valid walk limit");
-    // This will not do any alias() calls. It returns in the first iteration in
-    // the loop below.
-    if (*UpwardWalkLimit == 0)
-      (*UpwardWalkLimit)++;
+    bool LimitAlreadyReached = false;
+    // (*UpwardWalkLimit) may be 0 here, due to the loop in tryOptimizePhi. Set
+    // it to 1. This will not do any alias() calls. It either returns in the
+    // first iteration in the loop below, or is set back to 0 if all def chains
+    // are free of MemoryDefs.
+    if (!*UpwardWalkLimit) {
+      *UpwardWalkLimit = 1;
+      LimitAlreadyReached = true;
+    }
 
     for (MemoryAccess *Current : def_chain(Desc.Last)) {
       Desc.Last = Current;
@@ -568,6 +573,9 @@ template <class AliasAnalysisType> class ClobberWalker {
       }
     }
 
+    if (LimitAlreadyReached)
+      *UpwardWalkLimit = 0;
+
     assert(isa<MemoryPhi>(Desc.Last) &&
            "Ended at a non-clobber that's not a phi?");
     return {Desc.Last, false, MayAlias};