[LV] Avoid vectorizing unsafe dependencies in uniform address

Summary:
Currently, when vectorizing stores to uniform addresses, the only
instance we prevent vectorization is if there are multiple stores to the
same uniform address causing an unsafe dependency.
This patch teaches LAA to avoid vectorizing loops that have an unsafe
cross-iteration dependency between a load and a store to the same uniform address.

Fixes PR39653.

Reviewers: Ayal, efriedma

Subscribers: rkruppe, llvm-commits

Differential Revision: https://reviews.llvm.org/D54538

llvm-svn: 347220

1 files changed, 12 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 3c6c2ab9c99..bc01f04243f 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -1870,7 +1870,7 @@ void LoopAccessInfo::analyzeLoop(AliasAnalysis *AA, LoopInfo *LI,
     Value *Ptr = ST->getPointerOperand();
 
     if (isUniform(Ptr))
-      HasMultipleStoresToLoopInvariantAddress |=
+      HasDependenceInvolvingLoopInvariantAddress |=
           !UniformStores.insert(Ptr).second;
 
     // If we did *not* see this pointer before, insert it to  the read-write
@@ -1914,6 +1914,14 @@ void LoopAccessInfo::analyzeLoop(AliasAnalysis *AA, LoopInfo *LI,
       IsReadOnlyPtr = true;
     }
 
+    // See if there is an unsafe dependency between a load to a uniform address and
+    // store to the same uniform address.
+    if (UniformStores.count(Ptr)) {
+      LLVM_DEBUG(dbgs() << "LAA: Found an unsafe dependency between a uniform "
+                           "load and uniform store to the same address!\n");
+      HasDependenceInvolvingLoopInvariantAddress = true;
+    }
+
     MemoryLocation Loc = MemoryLocation::get(LD);
     // The TBAA metadata could have a control dependency on the predication
     // condition, so we cannot rely on it when determining whether or not we
@@ -2272,7 +2280,7 @@ LoopAccessInfo::LoopAccessInfo(Loop *L, ScalarEvolution *SE,
       PtrRtChecking(llvm::make_unique<RuntimePointerChecking>(SE)),
       DepChecker(llvm::make_unique<MemoryDepChecker>(*PSE, L)), TheLoop(L),
       NumLoads(0), NumStores(0), MaxSafeDepDistBytes(-1), CanVecMem(false),
-      HasMultipleStoresToLoopInvariantAddress(false) {
+      HasDependenceInvolvingLoopInvariantAddress(false) {
   if (canAnalyzeLoop())
     analyzeLoop(AA, LI, TLI, DT);
 }
@@ -2304,8 +2312,8 @@ void LoopAccessInfo::print(raw_ostream &OS, unsigned Depth) const {
   PtrRtChecking->print(OS, Depth);
   OS << "\n";
 
-  OS.indent(Depth) << "Multiple stores to invariant address were "
-                   << (HasMultipleStoresToLoopInvariantAddress ? "" : "not ")
+  OS.indent(Depth) << "Non vectorizable stores to invariant address were "
+                   << (HasDependenceInvolvingLoopInvariantAddress ? "" : "not ")
                    << "found in loop.\n";
 
   OS.indent(Depth) << "SCEV assumptions:\n";