[LV] Fix maximum legal VF calculation

This patch fixes pr34283, which exposed that the computation of
maximum legal width for vectorization was wrong, because it relied
on MaxInterleaveFactor to obtain the maximum stride used in the loop,
however not all strided accesses in the loop have an interleave-group
associated with them.
Instead of recording the maximum stride in the loop, which can be over
conservative (e.g. if the access with the maximum stride is not involved
in the dependence limitation), this patch tracks the actual maximum legal
width imposed by accesses that are involved in dependencies.

Differential Revision: https://reviews.llvm.org/D37507

llvm-svn: 313237

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 9cda14db294..eb633196d33 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -1540,10 +1540,11 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
       couldPreventStoreLoadForward(Distance, TypeByteSize))
     return Dependence::BackwardVectorizableButPreventsForwarding;
 
+  uint64_t MaxVF = MaxSafeDepDistBytes / (TypeByteSize * Stride);
   DEBUG(dbgs() << "LAA: Positive distance " << Val.getSExtValue()
-               << " with max VF = "
-               << MaxSafeDepDistBytes / (TypeByteSize * Stride) << '\n');
-
+               << " with max VF = " << MaxVF << '\n');
+  uint64_t MaxVFInBits = MaxVF * TypeByteSize * 8;
+  MaxSafeRegisterWidth = std::min(MaxSafeRegisterWidth, MaxVFInBits);
   return Dependence::BackwardVectorizable;
 }