Index: llvm/lib/IR/ConstantFold.cpp
===================================================================
--- llvm/lib/IR/ConstantFold.cpp
+++ llvm/lib/IR/ConstantFold.cpp
@@ -1124,13 +1124,16 @@
       // [any flop] undef, undef -> undef
       if (isa<UndefValue>(C1) && isa<UndefValue>(C2))
         return C1;
-      // [any flop] C, undef -> NaN
-      // [any flop] undef, C -> NaN
-      // We could potentially specialize NaN/Inf constants vs. 'normal'
-      // constants (possibly differently depending on opcode and operand). This
-      // would allow returning undef sometimes. But it is always safe to fold to
-      // NaN because we can choose the undef operand as NaN, and any FP opcode
-      // with a NaN operand will propagate NaN.
+      // [any flop] with "regular" constant operand and undef --> undef
+      // TODO: We could try harder to fold zero operands based on opcode.
+      if (isa<UndefValue>(C1) && C2->isFiniteNonZeroFP())
+        return C1;
+      if (isa<UndefValue>(C2) && C1->isFiniteNonZeroFP())
+        return C2;
+      // [any flop] C, undef --> NaN
+      // [any flop] undef, C --> NaN
+      // It is always safe to fold to NaN because we can choose the undef
+      // operand as NaN. Any FP opcode with a NaN operand will propagate NaN.
       return ConstantFP::getNaN(C1->getType());
     case Instruction::BinaryOpsEnd:
       llvm_unreachable("Invalid BinaryOp");
Index: llvm/test/Analysis/ConstantFolding/vector-undef-elts.ll
===================================================================
--- llvm/test/Analysis/ConstantFolding/vector-undef-elts.ll
+++ llvm/test/Analysis/ConstantFolding/vector-undef-elts.ll
@@ -45,7 +45,7 @@
 
 define <3 x float> @fadd() {
 ; CHECK-LABEL: @fadd(
-; CHECK-NEXT:    ret <3 x float> <float undef, float 0x7FF8000000000000, float undef>
+; CHECK-NEXT:    ret <3 x float> undef
 ;
   %c = fadd <3 x float> <float undef, float 42.0, float undef>, undef
   ret <3 x float> %c
@@ -53,7 +53,7 @@
 
 define <3 x float> @fadd_commute() {
 ; CHECK-LABEL: @fadd_commute(
-; CHECK-NEXT:    ret <3 x float> <float 0x7FF8000000000000, float 0x7FF8000000000000, float undef>
+; CHECK-NEXT:    ret <3 x float> undef
 ;
   %c = fadd <3 x float> undef, <float -42.0, float 42.0, float undef>
   ret <3 x float> %c
Index: llvm/test/Transforms/InstSimplify/fp-undef.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/fp-undef.ll
+++ llvm/test/Transforms/InstSimplify/fp-undef.ll
@@ -209,7 +209,7 @@
 
 define float @fadd_undef_op0_nnan_constant(float %x) {
 ; CHECK-LABEL: @fadd_undef_op0_nnan_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fadd nnan float undef, 1.0
   ret float %r
@@ -217,7 +217,7 @@
 
 define float @fadd_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fadd_undef_op1_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fadd float 2.0, undef
   ret float %r
@@ -225,7 +225,7 @@
 
 define float @fsub_undef_op0_fast_constant(float %x) {
 ; CHECK-LABEL: @fsub_undef_op0_fast_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fsub fast float undef, 3.0
   ret float %r
@@ -233,7 +233,7 @@
 
 define float @fsub_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fsub_undef_op1_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fsub float 4.0, undef
   ret float %r
@@ -241,7 +241,7 @@
 
 define float @fmul_undef_op0_nnan_constant(float %x) {
 ; CHECK-LABEL: @fmul_undef_op0_nnan_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fmul nnan float undef, 5.0
   ret float %r
@@ -249,7 +249,7 @@
 
 define float @fmul_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fmul_undef_op1_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fmul float 6.0, undef
   ret float %r
@@ -257,7 +257,7 @@
 
 define float @fdiv_undef_op0_fast_constant(float %x) {
 ; CHECK-LABEL: @fdiv_undef_op0_fast_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fdiv fast float undef, 7.0
   ret float %r
@@ -265,7 +265,7 @@
 
 define float @fdiv_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fdiv_undef_op1_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fdiv float 8.0, undef
   ret float %r
@@ -273,7 +273,7 @@
 
 define float @frem_undef_op0_nnan_constant(float %x) {
 ; CHECK-LABEL: @frem_undef_op0_nnan_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = frem nnan float undef, 9.0
   ret float %r
@@ -281,7 +281,7 @@
 
 define float @frem_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @frem_undef_op1_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = frem float 10.0, undef
   ret float %r
@@ -453,7 +453,7 @@
 
 define <2 x double> @fadd_undef_op1_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @fadd_undef_op1_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double 0x7FF8000000000000, double undef>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = fadd <2 x double> <double 42.0, double undef>, undef
   ret <2 x double> %r
@@ -461,7 +461,7 @@
 
 define <2 x double> @fadd_undef_op0_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @fadd_undef_op0_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double undef, double 0x7FF8000000000000>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = fadd <2 x double> undef, <double undef, double 42.0>
   ret <2 x double> %r
@@ -469,7 +469,7 @@
 
 define <2 x double> @fsub_undef_op1_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @fsub_undef_op1_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double undef, double 0x7FF8000000000000>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = fsub <2 x double> <double undef, double 42.0>, undef
   ret <2 x double> %r
@@ -477,7 +477,7 @@
 
 define <2 x double> @fsub_undef_op0_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @fsub_undef_op0_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double 0x7FF8000000000000, double undef>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = fsub <2 x double> undef, <double 42.0, double undef>
   ret <2 x double> %r
@@ -485,7 +485,7 @@
 
 define <2 x double> @fmul_undef_op1_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @fmul_undef_op1_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double 0x7FF8000000000000, double undef>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = fmul <2 x double> <double 42.0, double undef>, undef
   ret <2 x double> %r
@@ -493,7 +493,7 @@
 
 define <2 x double> @fmul_undef_op0_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @fmul_undef_op0_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double undef, double 0x7FF8000000000000>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = fmul <2 x double> undef, <double undef, double 42.0>
   ret <2 x double> %r
@@ -501,7 +501,7 @@
 
 define <2 x double> @fdiv_undef_op1_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @fdiv_undef_op1_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double 0x7FF8000000000000, double undef>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = fdiv <2 x double> <double 42.0, double undef>, undef
   ret <2 x double> %r
@@ -509,7 +509,7 @@
 
 define <2 x double> @fdiv_undef_op0_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @fdiv_undef_op0_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double undef, double 0x7FF8000000000000>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = fdiv <2 x double> undef, <double undef, double 42.0>
   ret <2 x double> %r
@@ -517,7 +517,7 @@
 
 define <2 x double> @frem_undef_op1_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @frem_undef_op1_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double undef, double 0x7FF8000000000000>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = frem <2 x double> <double undef, double 42.0>, undef
   ret <2 x double> %r
@@ -525,7 +525,7 @@
 
 define <2 x double> @frem_undef_op0_constant_vec(<2 x double> %x) {
 ; CHECK-LABEL: @frem_undef_op0_constant_vec(
-; CHECK-NEXT:    ret <2 x double> <double 0x7FF8000000000000, double undef>
+; CHECK-NEXT:    ret <2 x double> undef
 ;
   %r = frem <2 x double> undef, <double 42.0, double undef>
   ret <2 x double> %r
Index: llvm/test/Transforms/NewGVN/pr32607.ll
===================================================================
--- llvm/test/Transforms/NewGVN/pr32607.ll
+++ llvm/test/Transforms/NewGVN/pr32607.ll
@@ -1,12 +1,11 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -newgvn %s -S -o - | FileCheck %s
+
 define hidden void @foo() {
 ; CHECK-LABEL: @foo(
 ; CHECK-NEXT:  top:
 ; CHECK-NEXT:    br label [[IF:%.*]]
 ; CHECK:       if:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi double [ [[TMP1:%.*]], [[IF]] ], [ undef, [[TOP:%.*]] ]
-; CHECK-NEXT:    [[TMP1]] = fadd double [[TMP0]], 1.000000e+00
 ; CHECK-NEXT:    br i1 false, label [[L50:%.*]], label [[IF]]
 ; CHECK:       L50:
 ; CHECK-NEXT:    store i8 undef, i8* null
@@ -31,3 +30,33 @@
   ret void
 }
 
+define hidden void @foo_less_undef() {
+; CHECK-LABEL: @foo_less_undef(
+; CHECK-NEXT:  top:
+; CHECK-NEXT:    br label [[IF:%.*]]
+; CHECK:       if:
+; CHECK-NEXT:    [[TMP0:%.*]] = phi double [ [[TMP1:%.*]], [[IF]] ], [ undef, [[TOP:%.*]] ]
+; CHECK-NEXT:    [[TMP1]] = fadd double [[TMP0]], 0x7FF0000000000000
+; CHECK-NEXT:    br i1 false, label [[L50:%.*]], label [[IF]]
+; CHECK:       L50:
+; CHECK-NEXT:    store i8 undef, i8* null
+; CHECK-NEXT:    ret void
+;
+top:
+  %.promoted = load double, double* undef, align 8
+  br label %if
+
+;; This is really a multi-valued phi, because the phi is defined by an expression of the phi.
+;; This means that we can't propagate the value over the backedge, because we'll just cycle
+;; through every value.
+
+if:                                               ; preds = %if, %top
+  %0 = phi double [ %1, %if ], [ %.promoted, %top ]
+  %1 = fadd double %0, 0x7ff0000000000000 ; undef + infinity --> NaN (not undef)
+  br i1 false, label %L50, label %if
+
+L50:                                              ; preds = %if
+  %.lcssa = phi double [ %1, %if ]
+  store double %.lcssa, double* undef, align 8
+  ret void
+}