[AArch64] Use fneg instead of fsub -0.0, X Cin IR expansion of __builtin_neon_vfmsh_f16.

[craig.topper]

Summary

Addresses the FIXME and removes the only in tree use of
llvm::ConstantFP::getZeroValueForNegation for an FP type.

[SjoerdMeijer]

LGTM

[dmgreen]

Sounds OK to me

clang/lib/CodeGen/CGBuiltin.cpp

 //===---- CGBuiltin.cpp - Emit LLVM Code for builtins ---------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
case NEON::BI__builtin_neon_vdivh_f16:
     Ops.push_back(EmitScalarExpr(E->getArg(1)));
     return Builder.CreateFDiv(Ops[0], Ops[1], "vdivh");
   case NEON::BI__builtin_neon_vfmah_f16:
     // NEON intrinsic puts accumulator first, unlike the LLVM fma.
     return emitCallMaybeConstrainedFPBuiltin(
         *this, Intrinsic::fma, Intrinsic::experimental_constrained_fma, HalfTy,
         {EmitScalarExpr(E->getArg(1)), EmitScalarExpr(E->getArg(2)), Ops[0]});
   case NEON::BI__builtin_neon_vfmsh_f16: {
-    // FIXME: This should be an fneg instruction:
-    Value *Zero = llvm::ConstantFP::getZeroValueForNegation(HalfTy);
-    Value* Sub = Builder.CreateFSub(Zero, EmitScalarExpr(E->getArg(1)), "vsubh");
+    Value* Neg = Builder.CreateFNeg(EmitScalarExpr(E->getArg(1)), "vsubh");
 
     // NEON intrinsic puts accumulator first, unlike the LLVM fma.
     return emitCallMaybeConstrainedFPBuiltin(
         *this, Intrinsic::fma, Intrinsic::experimental_constrained_fma, HalfTy,
-        {Sub, EmitScalarExpr(E->getArg(2)), Ops[0]});
+        {Neg, EmitScalarExpr(E->getArg(2)), Ops[0]});
   }
   case NEON::BI__builtin_neon_vaddd_s64:
   case NEON::BI__builtin_neon_vaddd_u64:
     return Builder.CreateAdd(Ops[0], EmitScalarExpr(E->getArg(1)), "vaddd");
   case NEON::BI__builtin_neon_vsubd_s64:
   case NEON::BI__builtin_neon_vsubd_u64:
     return Builder.CreateSub(Ops[0], EmitScalarExpr(E->getArg(1)), "vsubd");
   case NEON::BI__builtin_neon_vqdmlalh_s16:

clang/test/CodeGen/aarch64-v8.2a-fp16-intrinsics-constrained.c

 // UNCONSTRAINED:  [[FMA:%.*]] = call half @llvm.fma.f16(half %b, half %c, half %a)
 // CONSTRAINED:    [[FMA:%.*]] = call half @llvm.experimental.constrained.fma.f16(half %b, half %c, half %a, metadata !"round.tonearest", metadata !"fpexcept.strict")
 // COMMONIR:       ret half [[FMA]]
 float16_t test_vfmah_f16(float16_t a, float16_t b, float16_t c) {
   return vfmah_f16(a, b, c);
 }
 
 // COMMON-LABEL: test_vfmsh_f16
-// UNCONSTRAINED:  [[SUB:%.*]] = fsub half 0xH8000, %b
-// CONSTRAINED:    [[SUB:%.*]] = call half @llvm.experimental.constrained.fsub.f16(half 0xH8000, half %b, metadata !"round.tonearest", metadata !"fpexcept.strict")
+// COMMONIR:  [[SUB:%.*]] = fneg half %b
 // UNCONSTRAINED:  [[ADD:%.*]] = call half @llvm.fma.f16(half [[SUB]], half %c, half %a)
 // CONSTRAINED:    [[ADD:%.*]] = call half @llvm.experimental.constrained.fma.f16(half [[SUB]], half %c, half %a, metadata !"round.tonearest", metadata !"fpexcept.strict")
 // COMMONIR:       ret half [[ADD]]
 float16_t test_vfmsh_f16(float16_t a, float16_t b, float16_t c) {
   return vfmsh_f16(a, b, c);
 }
 

clang/test/CodeGen/aarch64-v8.2a-fp16-intrinsics.c

 // CHECK-LABEL: test_vfmah_f16
 // CHECK:  [[FMA:%.*]] = call half @llvm.fma.f16(half %b, half %c, half %a)
 // CHECK:  ret half [[FMA]]
 float16_t test_vfmah_f16(float16_t a, float16_t b, float16_t c) {
   return vfmah_f16(a, b, c);
 }
 
 // CHECK-LABEL: test_vfmsh_f16
-// CHECK:  [[SUB:%.*]] = fsub half 0xH8000, %b
+// CHECK:  [[SUB:%.*]] = fneg half %b
 // CHECK:  [[ADD:%.*]] = call half @llvm.fma.f16(half [[SUB]], half %c, half %a)
 // CHECK:  ret half [[ADD]]
 float16_t test_vfmsh_f16(float16_t a, float16_t b, float16_t c) {
   return vfmsh_f16(a, b, c);
 }