Index: include/llvm/Analysis/LoopAccessAnalysis.h
===================================================================
--- include/llvm/Analysis/LoopAccessAnalysis.h
+++ include/llvm/Analysis/LoopAccessAnalysis.h
@@ -102,6 +102,8 @@
     // Can vectorize safely without RT checks. All dependences are known to be
     // safe.
     Safe,
+    // Can vectorize with RT checks to overcome unknown dependencies.
+    SafeWithRtChecks,
     // Cannot vectorize due to known unsafe dependencies.
     Unsafe,
   };
@@ -218,7 +220,7 @@
   /// vectorize the loop with a dynamic array access check.
   bool shouldRetryWithRuntimeCheck() const {
     return ShouldRetryWithRuntimeCheck &&
-           Status == VectorizationSafetyStatus::Unsafe;
+           Status == VectorizationSafetyStatus::SafeWithRtChecks;
   }
 
   /// Returns the memory dependences.  If null is returned we exceeded
@@ -284,7 +286,8 @@
   bool ShouldRetryWithRuntimeCheck;
 
   /// 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
@@ -320,7 +323,7 @@
   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 SafeWithRtChecks or Unsafe and from SafeWithRtChecks to Unsafe.
   void mergeInStatus(VectorizationSafetyStatus S);
 };
 
Index: lib/Analysis/LoopAccessAnalysis.cpp
===================================================================
--- lib/Analysis/LoopAccessAnalysis.cpp
+++ lib/Analysis/LoopAccessAnalysis.cpp
@@ -1230,6 +1230,7 @@
     return VectorizationSafetyStatus::Safe;
 
   case Unknown:
+    return VectorizationSafetyStatus::SafeWithRtChecks;
   case ForwardButPreventsForwarding:
   case Backward:
   case BackwardVectorizableButPreventsForwarding:
Index: test/Transforms/LoopVectorize/runtime-check.ll
===================================================================
--- test/Transforms/LoopVectorize/runtime-check.ll
+++ 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