[llvm-commits] [llvm] r41967 - in /llvm/trunk: include/llvm/ADT/APInt.h include/llvm/ADT/FoldingSet.h include/llvm/Constants.h lib/CodeGen/SelectionDAG/LegalizeDAG.cpp lib/CodeGen/SelectionDAG/SelectionDAG.cpp lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp lib/Support/APInt.cpp lib/Support/FoldingSet.cpp lib/Target/CBackend/CBackend.cpp lib/Target/X86/X86ISelLowering.cpp lib/Target/X86/X86RegisterInfo.cpp lib/Transforms/Scalar/InstructionCombining.cpp lib/VMCore/Constants.cpp

[Dale Johannesen]

On Sep 14, 2007, at 11:12 PM, Chris Lattner wrote:
>> +ConstantFP *ConstantFP::getNegativeZero(const Type *Ty) {
>> +  APFloat apf = cast <ConstantFP>(Constant::getNullValue(Ty))-
>>> getValueAPF();
>> +  apf.changeSign();
>> +  return ConstantFP::get(Ty, apf);
>> +}
>
> This seems fairly expensive.  Why not just start by creating an
> APFloat of the appropriate zero type, changing its sign, then
> returning it.  Why start with a Constant?

Because there's no easy way to make "an APFloat of the appropriate  
zero type", you need a switch like the one in getNullValue.  This is  
cleaner.  (It's true I'm getting tired of converting from MVT to Type  
to fpSemantics all over the place, maybe I'll do something about  
that...)