2 files changed, 49 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 05f34e9eaa0..e946acf84f5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -794,6 +794,11 @@ unsigned FAddCombine::calcInstrNumber(const AddendVect &Opnds) {
     if (Opnd->isConstant())
       continue;
 
+    // The constant check above is really for a few special constant
+    // coefficients.
+    if (isa<UndefValue>(Opnd->getSymVal()))
+      continue;
+
     const FAddendCoef &CE = Opnd->getCoef();
     if (CE.isMinusOne() || CE.isMinusTwo())
       NegOpndNum++;
diff --git a/llvm/test/Transforms/InstCombine/fsub.ll b/llvm/test/Transforms/InstCombine/fsub.ll
index af2fadd2867..6b62f5dd7e3 100644
--- a/llvm/test/Transforms/InstCombine/fsub.ll
+++ b/llvm/test/Transforms/InstCombine/fsub.ll
@@ -21,3 +21,47 @@ define double @test2(double %x, double %y) nounwind {
 
   ret double %t2
 }
+
+; CHECK-LABEL: @fsub_undef(
+; CHECK: %sub = fsub float %val, undef
+define float @fsub_undef(float %val) {
+bb:
+  %sub = fsub float %val, undef
+  ret float %sub
+}
+
+; XXX - Why doesn't this fold to undef?
+; CHECK-LABEL: @fsub_fast_undef(
+; CHCK: %sub = fsub fast float %val, undef
+define float @fsub_fast_undef(float %val) {
+bb:
+  %sub = fsub fast float %val, undef
+  ret float %sub
+}
+
+; CHECK-LABEL: @fneg_undef(
+; CHECK: ret float fsub (float -0.000000e+00, float undef)
+define float @fneg_undef(float %val) {
+bb:
+  %sub = fsub float -0.0, undef
+  ret float %sub
+}
+
+; CHECK-LABEL: @fneg_fast_undef(
+; CHECK: ret float fsub (float -0.000000e+00, float undef)
+define float @fneg_fast_undef(float %val) {
+bb:
+  %sub = fsub fast float -0.0, undef
+  ret float %sub
+}
+
+; This folds to a constant expression, which produced 0 instructions
+; contrary to the expected one for negation.
+; CHECK-LABEL: @inconsistent_numbers_fsub_undef(
+; CHECK: ret float fsub (float -0.000000e+00, float undef)
+define float @inconsistent_numbers_fsub_undef(float %val) {
+bb:
+  %sub0 = fsub fast float %val, undef
+  %sub1 = fsub fast float %sub0, %val
+  ret float %sub1
+}