[CodeGen] Use IRBuilder::CreateFNeg for __builtin_conj

[craig.topper]

Summary

This replaces the fsub -0.0 idiom with an fneg instruction. We didn't see to have a test that showed the current codegen. Just some tests for constant folding and a test that was only checking the declare lines for libcalls. The latter just checked that we did not have a declare for @conj when using __builtin_conj.

clang/lib/CodeGen/CGBuiltin.cpp

case Builtin::BI__builtin_llabs: {
     return RValue::get(Result);
   }
   case Builtin::BI__builtin_conj:
   case Builtin::BI__builtin_conjf:
   case Builtin::BI__builtin_conjl: {
     ComplexPairTy ComplexVal = EmitComplexExpr(E->getArg(0));
     Value *Real = ComplexVal.first;
     Value *Imag = ComplexVal.second;
-    Value *Zero =
-      Imag->getType()->isFPOrFPVectorTy()
-        ? llvm::ConstantFP::getZeroValueForNegation(Imag->getType())
-        : llvm::Constant::getNullValue(Imag->getType());
-
-    Imag = Builder.CreateFSub(Zero, Imag, "sub");
+    Imag = Builder.CreateFNeg(Imag, "neg");
     return RValue::getComplex(std::make_pair(Real, Imag));
   }
   case Builtin::BI__builtin_creal:
   case Builtin::BI__builtin_crealf:
   case Builtin::BI__builtin_creall:
   case Builtin::BIcreal:
   case Builtin::BIcrealf:
   case Builtin::BIcreall: {

clang/test/CodeGen/complex-builtins-2.c

+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -w -S -o - -emit-llvm              %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -w -S -o - -emit-llvm -fmath-errno %s | FileCheck %s
+
+float _Complex test__builtin_conjf(float _Complex x) {
+// CHECK-LABEL: @test__builtin_conjf(
+// CHECK: fneg float %x.imag
+  return __builtin_conjf(x);
+}
+
+double _Complex test__builtin_conj(double _Complex x) {
+// CHECK-LABEL: @test__builtin_conj(
+// CHECK: fneg double %x.imag
+  return __builtin_conj(x);
+}
+
+long double _Complex test__builtin_conjl(long double _Complex x) {
+// CHECK-LABEL: @test__builtin_conjl(
+// CHECK: fneg x86_fp80 %x.imag
+  return __builtin_conjl(x);
+}