diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 544b71c..0f5103b 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -4884,8 +4884,15 @@
 }
 
 SDValue X86TargetLowering::LowerUINT_TO_FP(SDValue Op, SelectionDAG &DAG) {
-  MVT SrcVT = Op.getOperand(0).getValueType();
+  SDValue N0 = Op.getOperand(0);
 
+  // Now not UINT_TO_FP is legal (it's marked custom), dag combiner won't
+  // optimize it to a SINT_TO_FP when the sign bit is known zero. Perform
+  // the optimization here.
+  if (DAG.SignBitIsZero(N0))
+    return DAG.getNode(ISD::SINT_TO_FP, Op.getValueType(), N0);
+
+  MVT SrcVT = N0.getValueType();
   if (SrcVT == MVT::i64) {
     // We only handle SSE2 f64 target here; caller can handle the rest.
     if (Op.getValueType() != MVT::f64 || !X86ScalarSSEf64)
diff --git a/test/CodeGen/X86/uint_to_fp.ll b/test/CodeGen/X86/uint_to_fp.ll
index 3ea7a94..148437f 100644
--- a/test/CodeGen/X86/uint_to_fp.ll
+++ b/test/CodeGen/X86/uint_to_fp.ll
@@ -1,4 +1,5 @@
 ; RUN: llvm-as < %s | llc -march=x86 -mcpu=yonah | not grep {sub.*esp}
+; RUN: llvm-as < %s | llc -march=x86 -mcpu=yonah | grep cvtsi2ss
 ; rdar://6034396
 
 target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"