Index: llvm/trunk/lib/IR/ConstantFold.cpp
===================================================================
--- llvm/trunk/lib/IR/ConstantFold.cpp
+++ llvm/trunk/lib/IR/ConstantFold.cpp
@@ -1009,7 +1009,10 @@
     case Instruction::FMul:
     case Instruction::FDiv:
     case Instruction::FRem:
-      // TODO: UNDEF handling for binary float instructions.
+      // [any flop] undef, undef -> undef
+      if (isa<UndefValue>(C1) && isa<UndefValue>(C2))
+        return C1;
+      // TODO: Handle one undef operand and some other constant.
       return nullptr;
     case Instruction::BinaryOpsEnd:
       llvm_unreachable("Invalid BinaryOp");
Index: llvm/trunk/test/Transforms/InstCombine/X86/X86FsubCmpCombine.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/X86/X86FsubCmpCombine.ll
+++ llvm/trunk/test/Transforms/InstCombine/X86/X86FsubCmpCombine.ll
@@ -155,7 +155,7 @@
 define i8 @sub_compare_folding_swapPD256_undef(<4 x double> %a, <4 x double> %b) {
 ; CHECK-LABEL: @sub_compare_folding_swapPD256_undef(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TMP:%.*]] = call <4 x i1> @llvm.x86.avx512.mask.cmp.pd.256(<4 x double> fsub (<4 x double> undef, <4 x double> undef), <4 x double> zeroinitializer, i32 5)
+; CHECK-NEXT:    [[TMP:%.*]] = call <4 x i1> @llvm.x86.avx512.mask.cmp.pd.256(<4 x double> undef, <4 x double> zeroinitializer, i32 5)
 ; CHECK-NEXT:    [[TMP0:%.*]] = shufflevector <4 x i1> [[TMP]], <4 x i1> zeroinitializer, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
 ; CHECK-NEXT:    [[TMP1:%.*]] = bitcast <8 x i1> [[TMP0]] to i8
 ; CHECK-NEXT:    ret i8 [[TMP1]]
Index: llvm/trunk/test/Transforms/InstSimplify/fp-undef.ll
===================================================================
--- llvm/trunk/test/Transforms/InstSimplify/fp-undef.ll
+++ llvm/trunk/test/Transforms/InstSimplify/fp-undef.ll
@@ -179,7 +179,7 @@
 
 define double @fadd_undef_undef(double %x) {
 ; CHECK-LABEL: @fadd_undef_undef(
-; CHECK-NEXT:    ret double fadd (double undef, double undef)
+; CHECK-NEXT:    ret double undef
 ;
   %r = fadd double undef, undef
   ret double %r
@@ -187,7 +187,7 @@
 
 define double @fsub_undef_undef(double %x) {
 ; CHECK-LABEL: @fsub_undef_undef(
-; CHECK-NEXT:    ret double fsub (double undef, double undef)
+; CHECK-NEXT:    ret double undef
 ;
   %r = fsub double undef, undef
   ret double %r
@@ -195,7 +195,7 @@
 
 define double @fmul_undef_undef(double %x) {
 ; CHECK-LABEL: @fmul_undef_undef(
-; CHECK-NEXT:    ret double fmul (double undef, double undef)
+; CHECK-NEXT:    ret double undef
 ;
   %r = fmul double undef, undef
   ret double %r
@@ -203,7 +203,7 @@
 
 define double @fdiv_undef_undef(double %x) {
 ; CHECK-LABEL: @fdiv_undef_undef(
-; CHECK-NEXT:    ret double fdiv (double undef, double undef)
+; CHECK-NEXT:    ret double undef
 ;
   %r = fdiv double undef, undef
   ret double %r
@@ -211,7 +211,7 @@
 
 define double @frem_undef_undef(double %x) {
 ; CHECK-LABEL: @frem_undef_undef(
-; CHECK-NEXT:    ret double frem (double undef, double undef)
+; CHECK-NEXT:    ret double undef
 ;
   %r = frem double undef, undef
   ret double %r
Index: llvm/trunk/test/Transforms/Reassociate/factorize-again.ll
===================================================================
--- llvm/trunk/test/Transforms/Reassociate/factorize-again.ll
+++ llvm/trunk/test/Transforms/Reassociate/factorize-again.ll
@@ -1,15 +1,16 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -reassociate < %s | FileCheck %s
 
 define void @main(float, float) {
 ; CHECK-LABEL: @main(
 ; CHECK-NEXT:  wrapper_entry:
-; CHECK-NEXT:    [[TMP2:%.*]] = fsub float undef, %0
-; CHECK-NEXT:    [[TMP3:%.*]] = fsub float undef, %1
+; CHECK-NEXT:    [[TMP2:%.*]] = fsub float undef, [[TMP0:%.*]]
+; CHECK-NEXT:    [[TMP3:%.*]] = fsub float undef, [[TMP1:%.*]]
 ; CHECK-NEXT:    [[TMP4:%.*]] = call float @llvm.rsqrt.f32(float undef)
 ; CHECK-NEXT:    [[REASS_ADD2:%.*]] = fadd fast float [[TMP3]], [[TMP2]]
-; CHECK-NEXT:    [[REASS_MUL3:%.*]] = fmul fast float [[REASS_ADD2]], undef
-; CHECK-NEXT:    [[REASS_ADD1:%.*]] = fadd fast float [[REASS_MUL3]], fmul (float undef, float undef)
-; CHECK-NEXT:    [[REASS_MUL:%.*]] = fmul fast float [[REASS_ADD1]], [[TMP4]]
+; CHECK-NEXT:    [[REASS_MUL3:%.*]] = fmul fast float [[TMP4]], [[REASS_ADD2]]
+; CHECK-NEXT:    [[REASS_ADD1:%.*]] = fadd fast float [[REASS_MUL3]], [[TMP4]]
+; CHECK-NEXT:    [[REASS_MUL:%.*]] = fmul fast float [[REASS_ADD1]], undef
 ; CHECK-NEXT:    [[TMP5:%.*]] = call float @foo2(float [[REASS_MUL]], float 0.000000e+00)
 ; CHECK-NEXT:    [[MUL36:%.*]] = fmul fast float [[TMP5]], 1.500000e+00
 ; CHECK-NEXT:    call void @foo1(i32 4, float [[MUL36]])
Index: llvm/trunk/test/Transforms/Reassociate/fp-expr.ll
===================================================================
--- llvm/trunk/test/Transforms/Reassociate/fp-expr.ll
+++ llvm/trunk/test/Transforms/Reassociate/fp-expr.ll
@@ -1,10 +1,11 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -reassociate < %s | FileCheck %s
 
 define void @test1() {
 ; CHECK-LABEL: @test1(
 ; CHECK-NEXT:    [[T1:%.*]] = tail call <4 x float> @blam()
 ; CHECK-NEXT:    [[T1_NEG:%.*]] = fsub fast <4 x float> <float -0.000000e+00, float -0.000000e+00, float -0.000000e+00, float -0.000000e+00>, [[T1]]
-; CHECK-NEXT:    [[T24:%.*]] = fadd fast <4 x float> [[T1_NEG]], fadd (<4 x float> undef, <4 x float> undef)
+; CHECK-NEXT:    [[T24:%.*]] = fadd fast <4 x float> [[T1_NEG]], undef
 ; CHECK-NEXT:    tail call void @wombat(<4 x float> [[T24]])
 ; CHECK-NEXT:    ret void
 ;
@@ -19,7 +20,7 @@
 ; CHECK-LABEL: @test2(
 ; CHECK-NEXT:    [[T15:%.*]] = fsub fast half undef, undef
 ; CHECK-NEXT:    [[T15_NEG:%.*]] = fsub fast half 0xH8000, [[T15]]
-; CHECK-NEXT:    [[T18:%.*]] = fadd fast half [[T15_NEG]], fadd (half undef, half undef)
+; CHECK-NEXT:    [[T18:%.*]] = fadd fast half [[T15_NEG]], undef
 ; CHECK-NEXT:    ret half [[T18]]
 ;
   %t15 = fsub fast half undef, undef