diff --git a/lib/Transforms/Vectorize/LoopVectorize.cpp b/lib/Transforms/Vectorize/LoopVectorize.cpp
index 78674952847..c72c2dc41fc 100644
--- a/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -172,6 +172,12 @@ static cl::opt<unsigned> SmallLoopCost(
     "small-loop-cost", cl::init(20), cl::Hidden,
     cl::desc("The cost of a loop that is considered 'small' by the unroller."));
 
+static cl::opt<bool> LoopVectorizeWithBlockFrequency(
+    "loop-vectorize-with-block-frequency", cl::init(false), cl::Hidden,
+    cl::desc("Enable the use of the block frequency analysis to access PGO "
+             "heuristics minimizing code growth in cold regions and being more "
+             "aggressive in hot regions."));
+
 // Runtime unroll loops for load/store throughput.
 static cl::opt<bool> EnableLoadStoreRuntimeUnroll(
     "enable-loadstore-runtime-unroll", cl::init(false), cl::Hidden,
@@ -1099,9 +1105,13 @@ struct LoopVectorize : public FunctionPass {
     // Compute the weighted frequency of this loop being executed and see if it
     // is less than 20% of the function entry baseline frequency. Note that we
     // always have a canonical loop here because we think we *can* vectoriez.
-    BlockFrequency LoopEntryFreq = BFI->getBlockFreq(L->getLoopPreheader());
-    if (Hints.Force != 1 && LoopEntryFreq < ColdEntryFreq)
-      OptForSize = true;
+    // FIXME: This is hidden behind a flag due to pervasive problems with
+    // exactly what block frequency models.
+    if (LoopVectorizeWithBlockFrequency) {
+      BlockFrequency LoopEntryFreq = BFI->getBlockFreq(L->getLoopPreheader());
+      if (Hints.Force != 1 && LoopEntryFreq < ColdEntryFreq)
+        OptForSize = true;
+    }
 
     // Check the function attributes to see if implicit floats are allowed.a
     // FIXME: This check doesn't seem possibly correct -- what if the loop is
diff --git a/test/Transforms/LoopVectorize/X86/small-size.ll b/test/Transforms/LoopVectorize/X86/small-size.ll
index 1d46366369c..dfa4faaf09e 100644
--- a/test/Transforms/LoopVectorize/X86/small-size.ll
+++ b/test/Transforms/LoopVectorize/X86/small-size.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s  -loop-vectorize -force-vector-unroll=1 -force-vector-width=4 -dce -instcombine -S | FileCheck %s
+; RUN: opt < %s  -loop-vectorize -force-vector-unroll=1 -force-vector-width=4 -loop-vectorize-with-block-frequency -dce -instcombine -S | FileCheck %s
 
 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-apple-macosx10.8.0"