Index: lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -794,6 +794,11 @@
     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++;
Index: test/Transforms/InstCombine/fsub.ll
===================================================================
--- test/Transforms/InstCombine/fsub.ll
+++ test/Transforms/InstCombine/fsub.ll
@@ -21,3 +21,47 @@
 
   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
+}