Index: llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h
===================================================================
--- llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h
+++ llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h
@@ -104,7 +104,9 @@
     // Can vectorize safely without RT checks. All dependences are known to be
     // safe.
     Safe,
-    // Cannot vectorize due to unsafe or unknown dependencies.
+    // Can possibly vectorize with RT checks to overcome unknown dependencies.
+    PossiblySafeWithRtChecks,
+    // Cannot vectorize due to known unsafe dependencies.
     Unsafe,
   };
 
@@ -175,7 +177,7 @@
 
   MemoryDepChecker(PredicatedScalarEvolution &PSE, const Loop *L)
       : PSE(PSE), InnermostLoop(L), AccessIdx(0), MaxSafeRegisterWidth(-1U),
-        ShouldRetryWithRuntimeCheck(false),
+        FoundNonConstantDistanceDependence(false),
         Status(VectorizationSafetyStatus::Safe), RecordDependences(true) {}
 
   /// Register the location (instructions are given increasing numbers)
@@ -218,7 +220,10 @@
 
   /// In same cases when the dependency check fails we can still
   /// vectorize the loop with a dynamic array access check.
-  bool shouldRetryWithRuntimeCheck() { return ShouldRetryWithRuntimeCheck; }
+  bool shouldRetryWithRuntimeCheck() const {
+    return FoundNonConstantDistanceDependence &&
+           Status == VectorizationSafetyStatus::PossiblySafeWithRtChecks;
+  }
 
   /// Returns the memory dependences.  If null is returned we exceeded
   /// the MaxDependences threshold and this information is not
@@ -280,10 +285,11 @@
 
   /// If we see a non-constant dependence distance we can still try to
   /// vectorize this loop with runtime checks.
-  bool ShouldRetryWithRuntimeCheck;
+  bool FoundNonConstantDistanceDependence;
 
   /// Result of the dependence checks, indicating whether the checked
-  /// dependences are safe for vectorization or not.
+  /// dependences are safe for vectorization, require RT checks or are known to
+  /// be unsafe.
   VectorizationSafetyStatus Status;
 
   //// True if Dependences reflects the dependences in the
@@ -319,7 +325,8 @@
   bool couldPreventStoreLoadForward(uint64_t Distance, uint64_t TypeByteSize);
 
   /// Updates the current safety status with \p S. We can go from Safe to
-  /// to Unsafe.
+  /// either PossiblySafeWithRtChecks or Unsafe and from
+  /// PossiblySafeWithRtChecks to Unsafe.
   void mergeInStatus(VectorizationSafetyStatus S);
 };
 
Index: llvm/trunk/lib/Analysis/LoopAccessAnalysis.cpp
===================================================================
--- llvm/trunk/lib/Analysis/LoopAccessAnalysis.cpp
+++ llvm/trunk/lib/Analysis/LoopAccessAnalysis.cpp
@@ -342,7 +342,7 @@
   //
   // 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 @@
   /// 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 @@
   ///
   /// 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 @@
     return VectorizationSafetyStatus::Safe;
 
   case Unknown:
+    return VectorizationSafetyStatus::PossiblySafeWithRtChecks;
   case ForwardButPreventsForwarding:
   case Backward:
   case BackwardVectorizableButPreventsForwarding:
@@ -1491,7 +1492,7 @@
       return Dependence::NoDep;
 
     LLVM_DEBUG(dbgs() << "LAA: Dependence because of non-constant distance\n");
-    ShouldRetryWithRuntimeCheck = true;
+    FoundNonConstantDistanceDependence = true;
     return Dependence::Unknown;
   }
 
Index: llvm/trunk/test/Transforms/LoopVectorize/runtime-check.ll
===================================================================
--- llvm/trunk/test/Transforms/LoopVectorize/runtime-check.ll
+++ llvm/trunk/test/Transforms/LoopVectorize/runtime-check.ll
@@ -117,7 +117,9 @@
   ret void
 }
 
-; Check we do generate unnecessary runtime checks. They will always fail.
+; Check we do not generate runtime checks if we found a known dependence preventing
+; vectorization. In this case, it is a read of c[i-1] followed by a write of c[i].
+; The runtime checks would always fail.
 
 ; void test_runtime_check2(float *a, float b, unsigned offset, unsigned offset2, unsigned n, float *c) {
 ;   for (unsigned i = 1; i < n; i++) {
@@ -127,7 +129,7 @@
 ; }
 ;
 ; CHECK-LABEL: test_runtime_check2
-; CHECK:      <4 x float>
+; CHECK-NOT:      <4 x float>
 define void @test_runtime_check2(float* %a, float %b, i64 %offset, i64 %offset2, i64 %n, float* %c) {
 entry:
   br label %for.body