AMDGPU: Fold rcp/rsq of undef to undef

[kzhuravl]

LGTM.

[arsenm]

r298725

lib/Transforms/InstCombine/InstCombineCalls.cpp

if (Constant *CV1 = dyn_cast<Constant>(Arg1))
       if (ConstantInt *Splat =
               dyn_cast_or_null<ConstantInt>(CV1->getSplatValue()))
         if (Splat->isOne())
           return CastInst::CreateIntegerCast(Arg0, II->getType(),
                                              /*isSigned=*/!Zext);
 
     break;
   }
-
   case Intrinsic::amdgcn_rcp: {
-    if (const ConstantFP *C = dyn_cast<ConstantFP>(II->getArgOperand(0))) {
+    Value *Src = II->getArgOperand(0);
+
+    // TODO: Move to ConstantFolding/InstSimplify?
+    if (isa<UndefValue>(Src))
+      return replaceInstUsesWith(CI, Src);
+
+    if (const ConstantFP *C = dyn_cast<ConstantFP>(Src)) {
       const APFloat &ArgVal = C->getValueAPF();
       APFloat Val(ArgVal.getSemantics(), 1.0);
       APFloat::opStatus Status = Val.divide(ArgVal,
                                             APFloat::rmNearestTiesToEven);
       // Only do this if it was exact and therefore not dependent on the
       // rounding mode.
       if (Status == APFloat::opOK)
         return replaceInstUsesWith(CI, ConstantFP::get(II->getContext(), Val));
     }
 
     break;
   }
+  case Intrinsic::amdgcn_rsq: {
+    Value *Src = II->getArgOperand(0);
+
+    // TODO: Move to ConstantFolding/InstSimplify?
+    if (isa<UndefValue>(Src))
+      return replaceInstUsesWith(CI, Src);
+    break;
+  }
   case Intrinsic::amdgcn_frexp_mant:
   case Intrinsic::amdgcn_frexp_exp: {
     Value *Src = II->getArgOperand(0);
     if (const ConstantFP *C = dyn_cast<ConstantFP>(Src)) {
       int Exp;
       APFloat Significand = frexp(C->getValueAPF(), Exp,
                                   APFloat::rmNearestTiesToEven);
 

test/Transforms/InstCombine/amdgcn-intrinsics.ll

 ; RUN: opt -instcombine -S < %s | FileCheck %s
 
 ; --------------------------------------------------------------------
 ; llvm.amdgcn.rcp
 ; --------------------------------------------------------------------
 
 declare float @llvm.amdgcn.rcp.f32(float) nounwind readnone
 declare double @llvm.amdgcn.rcp.f64(double) nounwind readnone
 
+; CHECK-LABEL: @test_constant_fold_rcp_f32_undef
+; CHECK-NEXT: ret float undef
+define float @test_constant_fold_rcp_f32_undef() nounwind {
+  %val = call float @llvm.amdgcn.rcp.f32(float undef) nounwind readnone
+  ret float %val
+}
 
 ; CHECK-LABEL: @test_constant_fold_rcp_f32_1
 ; CHECK-NEXT: ret float 1.000000e+00
 define float @test_constant_fold_rcp_f32_1() nounwind {
   %val = call float @llvm.amdgcn.rcp.f32(float 1.0) nounwind readnone
   ret float %val
 }
 

 
 ; CHECK-LABEL: @test_constant_fold_rcp_f64_43
 ; CHECK-NEXT: call double @llvm.amdgcn.rcp.f64(double 4.300000e+01)
 define double @test_constant_fold_rcp_f64_43() nounwind {
   %val = call double @llvm.amdgcn.rcp.f64(double 4.300000e+01) nounwind readnone
   ret double %val
 }
 
+; --------------------------------------------------------------------
+; llvm.amdgcn.rsq
+; --------------------------------------------------------------------
+
+declare float @llvm.amdgcn.rsq.f32(float) nounwind readnone
+
+; CHECK-LABEL: @test_constant_fold_rsq_f32_undef
+; CHECK-NEXT: ret float undef
+define float @test_constant_fold_rsq_f32_undef() nounwind {
+  %val = call float @llvm.amdgcn.rsq.f32(float undef) nounwind readnone
+  ret float %val
+}
 
 ; --------------------------------------------------------------------
 ; llvm.amdgcn.frexp.mant
 ; --------------------------------------------------------------------
 
 declare float @llvm.amdgcn.frexp.mant.f32(float) nounwind readnone
 declare double @llvm.amdgcn.frexp.mant.f64(double) nounwind readnone