[CodeGen] Use CreateFNeg in buildFMulAdd

[craig.topper]

Summary

We have an fneg instruction now and should use it instead of the fsub -0.0 idiom. Looks like we had no test that showed that we handled the negation cases here so I've added new tests.

[craig.topper]

clang/lib/CodeGen/CGExprScalar.cpp

3350

I removed the 'else' here because logically it didn't make sense that whether we looked at negAdd should be dependent on negMul being false. The assert at the beginning of the function still assures they are mutex. But the code shouldn't need that assumption.

[rjmccall]

clang/lib/CodeGen/CGExprScalar.cpp

3350

Fine by me, although it's not clear why the assertion's restriction is required, then.

clang/lib/CodeGen/CGExprScalar.cpp

 // efficient operations.
 static Value* buildFMulAdd(llvm::BinaryOperator *MulOp, Value *Addend,
                            const CodeGenFunction &CGF, CGBuilderTy &Builder,
                            bool negMul, bool negAdd) {
   assert(!(negMul && negAdd) && "Only one of negMul and negAdd should be set.");
 
   Value *MulOp0 = MulOp->getOperand(0);
   Value *MulOp1 = MulOp->getOperand(1);
-  if (negMul) {
-    MulOp0 =
-      Builder.CreateFSub(
-        llvm::ConstantFP::getZeroValueForNegation(MulOp0->getType()), MulOp0,
-        "neg");
-  } else if (negAdd) {
-    Addend =
-      Builder.CreateFSub(
-        llvm::ConstantFP::getZeroValueForNegation(Addend->getType()), Addend,
-        "neg");
-  }
+  if (negMul)
+    MulOp0 = Builder.CreateFNeg(MulOp0, "neg");
+  if (negAdd)
+    Addend = Builder.CreateFNeg(Addend, "neg");
 
   Value *FMulAdd = Builder.CreateCall(
       CGF.CGM.getIntrinsic(llvm::Intrinsic::fmuladd, Addend->getType()),
       {MulOp0, MulOp1, Addend});
    MulOp->eraseFromParent();
 
    return FMulAdd;
 }

clang/test/CodeGen/fp-contract-pragma.cpp

 float fp_contract_7(float a, float b, float c) {
 // CHECK: _Z13fp_contract_7fff
 // CHECK:  %[[M:.+]] = fmul float %b, 2.000000e+00
 // CHECK-NEXT: fsub float %[[M]], %c
   #pragma STDC FP_CONTRACT ON
   return (a = 2 * b) - c;
 }
 
+float fp_contract_8(float a, float b, float c) {
+// CHECK: _Z13fp_contract_8fff
+// CHECK: fneg float %c
+// CHECK: tail call float @llvm.fmuladd
+  #pragma STDC FP_CONTRACT ON
+  return a * b - c;
+}
+
+float fp_contract_9(float a, float b, float c) {
+// CHECK: _Z13fp_contract_9fff
+// CHECK: fneg float %a
+// CHECK: tail call float @llvm.fmuladd
+  #pragma STDC FP_CONTRACT ON
+  return c - a * b;
+}