Index: llvm/trunk/lib/Analysis/ConstantFolding.cpp
===================================================================
--- llvm/trunk/lib/Analysis/ConstantFolding.cpp
+++ llvm/trunk/lib/Analysis/ConstantFolding.cpp
@@ -1443,6 +1443,11 @@
                                  ArrayRef<Constant *> Operands,
                                  const TargetLibraryInfo *TLI) {
   if (Operands.size() == 1) {
+    if (isa<UndefValue>(Operands[0])) {
+      // cosine(arg) is between -1 and 1. cosine(invalid arg) is NaN
+      if (IntrinsicID == Intrinsic::cos)
+        return Constant::getNullValue(Ty);
+    }
     if (ConstantFP *Op = dyn_cast<ConstantFP>(Operands[0])) {
       if (IntrinsicID == Intrinsic::convert_to_fp16) {
         APFloat Val(Op->getValueAPF());
Index: llvm/trunk/test/Transforms/InstCombine/cos-intrinsic.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/cos-intrinsic.ll
+++ llvm/trunk/test/Transforms/InstCombine/cos-intrinsic.ll
@@ -0,0 +1,26 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+; This test makes sure that the undef is propagated for the cos instrinsic
+
+declare double    @llvm.cos.f64(double %Val)
+declare float     @llvm.cos.f32(float %Val)
+
+; Function Attrs: nounwind readnone
+define double @test1() {
+; CHECK-LABEL: define double @test1(
+; CHECK-NEXT: ret double 0.000000e+00
+  %1 = call double @llvm.cos.f64(double undef)
+  ret double %1
+}
+
+
+; Function Attrs: nounwind readnone
+define float @test2(float %d) {
+; CHECK-LABEL: define float @test2(
+; CHECK-NEXT: %cosval = call float @llvm.cos.f32(float %d)
+   %cosval   = call float @llvm.cos.f32(float %d)
+   %cosval2  = call float @llvm.cos.f32(float undef)
+   %fsum   = fadd float %cosval2, %cosval
+   ret float %fsum
+; CHECK-NEXT: %fsum
+; CHECK: ret float %fsum
+}