[ConstantFold] fp_binop AnyConstant, undef --> NaN

[spatel]

Summary

With the first update to the LangRef ( D44216 / rL327138 ) in place, we can proceed with more constant folding.

I'm intentionally taking the conservative path here: no matter what the constant or the FMF, we can always fold to NaN. This is because the undef operand can be chosen as NaN, and in our simplified default FP env, nothing else happens - NaN just propagates to the result. If we find some way/need to propagate undef instead, that can be added subsequently.

The tests show that we always choose the same quiet NaN constant (0x7FF8000000000000 in IR text). There were suggestions to improve that with a 'NaN' string token or not always print a 64-bit hex value, but those are independent changes. We might also consider setting/propagating the payload of NaN constants as an enhancement.

[arsenm]

test/CodeGen/AMDGPU/skip-if-dead.ll

279–281 ↗

(On Diff #137752)

This is potentially a bad test change because this is now a divergent branch. Adding inreg to the parameter should fix it

[spatel]

test/CodeGen/AMDGPU/skip-if-dead.ll

279–281 ↗

(On Diff #137752)

Thanks - I didn't understand what was happening there. I'll fix it and remove this test from the patch.

[spatel]

Patch updated:

1. Fixed the AMDGPU test to not be affected ( rL327147 ).
2. Fixed the 'Inf' constants in some tests to actually be 'Inf' ( rL327144 ).

[majnemer]

LGTM

lib/IR/ConstantFold.cpp

if (isa<UndefValue>(C1) || isa<UndefValue>(C2)) {
     case Instruction::FAdd:
     case Instruction::FSub:
     case Instruction::FMul:
     case Instruction::FDiv:
     case Instruction::FRem:
       // [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;
+      // [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.
+      return ConstantFP::getNaN(C1->getType());
     case Instruction::BinaryOpsEnd:
       llvm_unreachable("Invalid BinaryOp");
     }
   }
 
   // At this point neither constant should be an UndefValue.
   assert(!isa<UndefValue>(C1) && !isa<UndefValue>(C2) &&
          "Unexpected UndefValue");

test/Transforms/InstCombine/fsub.ll

 ; CHECK-LABEL: @fsub_fast_undef(
 ; CHECK-NEXT:    [[SUB:%.*]] = fsub fast float [[VAL:%.*]], undef
 ; CHECK-NEXT:    ret float [[SUB]]
 ;
   %sub = fsub fast float %val, undef
   ret float %sub
 }
 
-define float @fneg_undef(float %val) {
-; CHECK-LABEL: @fneg_undef(
-; CHECK-NEXT:    ret float fsub (float -0.000000e+00, float undef)
-;
-  %sub = fsub float -0.0, undef
-  ret float %sub
-}
-
-define float @fneg_fast_undef(float %val) {
-; CHECK-LABEL: @fneg_fast_undef(
-; CHECK-NEXT:    ret float fsub (float -0.000000e+00, float undef)
-;
-  %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.
-
-define float @inconsistent_numbers_fsub_undef(float %val) {
-; CHECK-LABEL: @inconsistent_numbers_fsub_undef(
-; CHECK-NEXT:    ret float fsub (float -0.000000e+00, float undef)
-;
-  %sub0 = fsub fast float %val, undef
-  %sub1 = fsub fast float %sub0, %val
-  ret float %sub1
-}
-

test/Transforms/InstSimplify/fp-undef.ll

;
   %r = frem double undef, undef
   ret double %r
 }
 
 ; Constant folding.
 
 define float @fadd_undef_op0_nnan_constant(float %x) {
 ; CHECK-LABEL: @fadd_undef_op0_nnan_constant(
-; CHECK-NEXT:    ret float fadd (float undef, float 1.000000e+00)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = fadd nnan float undef, 1.0
   ret float %r
 }
 
 define float @fadd_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fadd_undef_op1_constant(
-; CHECK-NEXT:    ret float fadd (float 2.000000e+00, float undef)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = fadd float 2.0, undef
   ret float %r
 }
 
 define float @fsub_undef_op0_fast_constant(float %x) {
 ; CHECK-LABEL: @fsub_undef_op0_fast_constant(
-; CHECK-NEXT:    ret float fsub (float undef, float 3.000000e+00)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = fsub fast float undef, 3.0
   ret float %r
 }
 
 define float @fsub_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fsub_undef_op1_constant(
-; CHECK-NEXT:    ret float fsub (float 4.000000e+00, float undef)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = fsub float 4.0, undef
   ret float %r
 }
 
 define float @fmul_undef_op0_nnan_constant(float %x) {
 ; CHECK-LABEL: @fmul_undef_op0_nnan_constant(
-; CHECK-NEXT:    ret float fmul (float undef, float 5.000000e+00)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = fmul nnan float undef, 5.0
   ret float %r
 }
 
 define float @fmul_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fmul_undef_op1_constant(
-; CHECK-NEXT:    ret float fmul (float 6.000000e+00, float undef)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = fmul float 6.0, undef
   ret float %r
 }
 
 define float @fdiv_undef_op0_fast_constant(float %x) {
 ; CHECK-LABEL: @fdiv_undef_op0_fast_constant(
-; CHECK-NEXT:    ret float fdiv (float undef, float 7.000000e+00)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = fdiv fast float undef, 7.0
   ret float %r
 }
 
 define float @fdiv_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fdiv_undef_op1_constant(
-; CHECK-NEXT:    ret float fdiv (float 8.000000e+00, float undef)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = fdiv float 8.0, undef
   ret float %r
 }
 
 define float @frem_undef_op0_nnan_constant(float %x) {
 ; CHECK-LABEL: @frem_undef_op0_nnan_constant(
-; CHECK-NEXT:    ret float frem (float undef, float 9.000000e+00)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = frem nnan float undef, 9.0
   ret float %r
 }
 
 define float @frem_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @frem_undef_op1_constant(
-; CHECK-NEXT:    ret float frem (float 1.000000e+01, float undef)
+; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %r = frem float 10.0, undef
   ret float %r
 }
 
 ; Constant folding - special constants: NaN.
 
 define double @fadd_undef_op0_constant_nan(double %x) {
 ; CHECK-LABEL: @fadd_undef_op0_constant_nan(
-; CHECK-NEXT:    ret double fadd (double undef, double 0x7FF8000000000000)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fadd double undef, 0x7FF8000000000000
   ret double %r
 }
 
 define double @fadd_undef_op1_fast_constant_nan(double %x) {
 ; CHECK-LABEL: @fadd_undef_op1_fast_constant_nan(
-; CHECK-NEXT:    ret double fadd (double 0xFFF0000000000001, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fadd fast double 0xFFF0000000000001, undef
   ret double %r
 }
 
 define double @fsub_undef_op0_constant_nan(double %x) {
 ; CHECK-LABEL: @fsub_undef_op0_constant_nan(
-; CHECK-NEXT:    ret double fsub (double undef, double 0xFFF8000000000010)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fsub double undef, 0xFFF8000000000010
   ret double %r
 }
 
 define double @fsub_undef_op1_nnan_constant_nan(double %x) {
 ; CHECK-LABEL: @fsub_undef_op1_nnan_constant_nan(
-; CHECK-NEXT:    ret double fsub (double 0x7FF0000000000011, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fsub nnan double 0x7FF0000000000011, undef
   ret double %r
 }
 
 define double @fmul_undef_op0_constant_nan(double %x) {
 ; CHECK-LABEL: @fmul_undef_op0_constant_nan(
-; CHECK-NEXT:    ret double fmul (double undef, double 0x7FF8000000000100)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fmul double undef, 0x7FF8000000000100
   ret double %r
 }
 
 define double @fmul_undef_op1_fast_constant_nan(double %x) {
 ; CHECK-LABEL: @fmul_undef_op1_fast_constant_nan(
-; CHECK-NEXT:    ret double fmul (double 0xFFF0000000000101, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fmul fast double 0xFFF0000000000101, undef
   ret double %r
 }
 
 define double @fdiv_undef_op0_constant_nan(double %x) {
 ; CHECK-LABEL: @fdiv_undef_op0_constant_nan(
-; CHECK-NEXT:    ret double fdiv (double undef, double 0xFFF8000000000110)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fdiv double undef, 0xFFF8000000000110
   ret double %r
 }
 
 define double @fdiv_undef_op1_nnan_constant_nan(double %x) {
 ; CHECK-LABEL: @fdiv_undef_op1_nnan_constant_nan(
-; CHECK-NEXT:    ret double fdiv (double 0x7FF0000000000111, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fdiv nnan double 0x7FF0000000000111, undef
   ret double %r
 }
 
 define double @frem_undef_op0_constant_nan(double %x) {
 ; CHECK-LABEL: @frem_undef_op0_constant_nan(
-; CHECK-NEXT:    ret double frem (double undef, double 0x7FF8000000001000)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = frem double undef, 0x7FF8000000001000
   ret double %r
 }
 
 define double @frem_undef_op1_fast_constant_nan(double %x) {
 ; CHECK-LABEL: @frem_undef_op1_fast_constant_nan(
-; CHECK-NEXT:    ret double frem (double 0xFFF0000000001001, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = frem fast double 0xFFF0000000001001, undef
   ret double %r
 }
 
 ; Constant folding - special constants: Inf.
 
 define double @fadd_undef_op0_constant_inf(double %x) {
 ; CHECK-LABEL: @fadd_undef_op0_constant_inf(
-; CHECK-NEXT:    ret double fadd (double undef, double 0x7FF0000000000000)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fadd double undef, 0x7FF0000000000000
   ret double %r
 }
 
 define double @fadd_undef_op1_fast_constant_inf(double %x) {
 ; CHECK-LABEL: @fadd_undef_op1_fast_constant_inf(
-; CHECK-NEXT:    ret double fadd (double 0xFFF0000000000000, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fadd fast double 0xFFF0000000000000, undef
   ret double %r
 }
 
 define double @fsub_undef_op0_constant_inf(double %x) {
 ; CHECK-LABEL: @fsub_undef_op0_constant_inf(
-; CHECK-NEXT:    ret double fsub (double undef, double 0xFFF0000000000000)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fsub double undef, 0xFFF0000000000000
   ret double %r
 }
 
 define double @fsub_undef_op1_ninf_constant_inf(double %x) {
 ; CHECK-LABEL: @fsub_undef_op1_ninf_constant_inf(
-; CHECK-NEXT:    ret double fsub (double 0x7FF0000000000000, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fsub ninf double 0x7FF0000000000000, undef
   ret double %r
 }
 
 define double @fmul_undef_op0_constant_inf(double %x) {
 ; CHECK-LABEL: @fmul_undef_op0_constant_inf(
-; CHECK-NEXT:    ret double fmul (double undef, double 0x7FF0000000000000)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fmul double undef, 0x7FF0000000000000
   ret double %r
 }
 
 define double @fmul_undef_op1_fast_constant_inf(double %x) {
 ; CHECK-LABEL: @fmul_undef_op1_fast_constant_inf(
-; CHECK-NEXT:    ret double fmul (double 0xFFF0000000000000, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fmul fast double 0xFFF0000000000000, undef
   ret double %r
 }
 
 define double @fdiv_undef_op0_constant_inf(double %x) {
 ; CHECK-LABEL: @fdiv_undef_op0_constant_inf(
-; CHECK-NEXT:    ret double fdiv (double undef, double 0xFFF0000000000000)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fdiv double undef, 0xFFF0000000000000
   ret double %r
 }
 
 define double @fdiv_undef_op1_ninf_constant_inf(double %x) {
 ; CHECK-LABEL: @fdiv_undef_op1_ninf_constant_inf(
-; CHECK-NEXT:    ret double fdiv (double 0x7FF0000000000000, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = fdiv ninf double 0x7FF0000000000000, undef
   ret double %r
 }
 
 define double @frem_undef_op0_constant_inf(double %x) {
 ; CHECK-LABEL: @frem_undef_op0_constant_inf(
-; CHECK-NEXT:    ret double frem (double undef, double 0x7FF0000000000000)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = frem double undef, 0x7FF0000000000000
   ret double %r
 }
 
 define double @frem_undef_op1_fast_constant_inf(double %x) {
 ; CHECK-LABEL: @frem_undef_op1_fast_constant_inf(
-; CHECK-NEXT:    ret double frem (double 0xFFF0000000000000, double undef)
+; CHECK-NEXT:    ret double 0x7FF8000000000000
 ;
   %r = frem fast double 0xFFF0000000000000, undef
   ret double %r
 }