Index: lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- lib/Transforms/Vectorize/LoopVectorize.cpp
+++ lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -7179,8 +7179,21 @@
 
   // Check the loop for a trip count threshold:
   // do not vectorize loops with a tiny trip count.
-  const unsigned TC = SE->getSmallConstantTripCount(L);
-  if (TC > 0u && TC < TinyTripCountVectorThreshold) {
+  bool KnownTC = false;
+  unsigned TC = SE->getSmallConstantTripCount(L);
+  if (TC) {
+    KnownTC = true;
+  } else if (F->getEntryCount()) {
+    // If the tripcount is unknown, but profile information is available,
+    // use a profile-based estimate.
+    auto EstimatedTC = getLoopEstimatedTripCount(L);
+    if (EstimatedTC) {
+      TC = *EstimatedTC;
+      KnownTC = true;
+    }
+  }
+
+  if (KnownTC && TC < TinyTripCountVectorThreshold) {
     DEBUG(dbgs() << "LV: Found a loop with a very small trip count. "
                  << "This loop is not worth vectorizing.");
     if (Hints.getForce() == LoopVectorizeHints::FK_Enabled)
Index: test/Transforms/LoopVectorize/X86/runtime-trip-count.ll
===================================================================
--- test/Transforms/LoopVectorize/X86/runtime-trip-count.ll
+++ test/Transforms/LoopVectorize/X86/runtime-trip-count.ll
@@ -0,0 +1,57 @@
+; RUN: opt < %s  -loop-vectorize -force-vector-interleave=1 -force-vector-width=4 -S -debug-only=loop-vectorize 2>&1 | FileCheck %s
+; REQUIRES: asserts
+
+; CHECK: Checking a loop in "high_dynamic"
+; CHECK: We can vectorize this loop
+; CHECK: Checking a loop in "low_dynamic"
+; CHECK: Found a loop with a very small trip count.
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: @high_dynamic
+; CHECK: fadd <4 x float>
+define void @high_dynamic(float* nocapture %a, i32 %k) !prof !0 {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds float, float* %a, i64 %indvars.iv
+  %0 = load float, float* %arrayidx, align 4
+  %add = fadd float %0, 1.000000e+00
+  store float %add, float* %arrayidx, align 4
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %k
+  br i1 %exitcond, label %for.end, label %for.body, !prof !1
+
+for.end:                                          ; preds = %for.body
+  ret void
+}
+
+; CHECK-LABEL: @low_dynamic
+; CHECK-NOT: <4 x float>
+define void @low_dynamic(float* nocapture %a, i32 %k) !prof !0 {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds float, float* %a, i64 %indvars.iv
+  %0 = load float, float* %arrayidx, align 4
+  %add = fadd float %0, 1.000000e+00
+  store float %add, float* %arrayidx, align 4
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %k
+  br i1 %exitcond, label %for.end, label %for.body, !prof !2
+
+for.end:                                          ; preds = %for.body
+  ret void
+}
+
+
+!0 = !{!"function_entry_count", i64 1}
+!1 = !{!"branch_weights", i32 1001, i32 400001}
+!2 = !{!"branch_weights", i32 1001, i32 4001}