[LAA] Avoid generating RT checks for known deps preventing vectorization.

If we found unsafe dependences other than 'unknown', we already know at
compile time that they are unsafe and the runtime checks should always
fail. So we can avoid generating them in those cases.

This should have no negative impact on performance as the runtime checks
that would be created previously should always fail. As a sanity check,
I measured the test-suite, spec2k and spec2k6 and there were no regressions.

Reviewers: Ayal, anemet, hsaito

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D55798

llvm-svn: 349794

1 files changed, 6 insertions, 5 deletions

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 245f318e308..7f3480f512a 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -342,7 +342,7 @@ void RuntimePointerChecking::groupChecks(
   //
   // The above case requires that we have an UnknownDependence between
   // accesses to the same underlying object. This cannot happen unless
-  // ShouldRetryWithRuntimeCheck is set, and therefore UseDependencies
+  // FoundNonConstantDistanceDependence is set, and therefore UseDependencies
   // is also false. In this case we will use the fallback path and create
   // separate checking groups for all pointers.
 
@@ -556,7 +556,7 @@ public:
   /// perform dependency checking.
   ///
   /// Note that this can later be cleared if we retry memcheck analysis without
-  /// dependency checking (i.e. ShouldRetryWithRuntimeCheck).
+  /// dependency checking (i.e. FoundNonConstantDistanceDependence).
   bool isDependencyCheckNeeded() { return !CheckDeps.empty(); }
 
   /// We decided that no dependence analysis would be used.  Reset the state.
@@ -604,8 +604,8 @@ private:
   ///
   /// Note that, this is different from isDependencyCheckNeeded.  When we retry
   /// memcheck analysis without dependency checking
-  /// (i.e. ShouldRetryWithRuntimeCheck), isDependencyCheckNeeded is cleared
-  /// while this remains set if we have potentially dependent accesses.
+  /// (i.e. FoundNonConstantDistanceDependence), isDependencyCheckNeeded is
+  /// cleared while this remains set if we have potentially dependent accesses.
   bool IsRTCheckAnalysisNeeded;
 
   /// The SCEV predicate containing all the SCEV-related assumptions.
@@ -1230,6 +1230,7 @@ MemoryDepChecker::Dependence::isSafeForVectorization(DepType Type) {
     return VectorizationSafetyStatus::Safe;
 
   case Unknown:
+    return VectorizationSafetyStatus::PossiblySafeWithRtChecks;
   case ForwardButPreventsForwarding:
   case Backward:
   case BackwardVectorizableButPreventsForwarding:
@@ -1491,7 +1492,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
       return Dependence::NoDep;
 
     LLVM_DEBUG(dbgs() << "LAA: Dependence because of non-constant distance\n");
-    ShouldRetryWithRuntimeCheck = true;
+    FoundNonConstantDistanceDependence = true;
     return Dependence::Unknown;
   }