[llvm-dev] [FPEnv] FNEG instruction

[Stephen Canon via llvm-dev]

> On Aug 29, 2018, at 12:16 PM, Tim Northover via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> On Wed, 29 Aug 2018 at 07:51, Cameron McInally via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> 
>> The current thinking is that FNEG(X) and FSUB(-0.0, X) are not the same operation when X is a NaN or 0.
> 
> Do you mean denormals (when flushed) rather than 0 there? AFAIK it's
> OK for 0 itself.

Under the assumption of default rounding, yes.

>> So, the xforms in question should only be valid under Fast-Math conditions.
> 
> We could probably also "fix" the issue by taking the view that LLVM's
> fsub provides extra guarantees for NaN over the IEEE-754 one. I.e.
> that "fsub -0.0, x" behaves as negate. I believe that would still be
> conformant to IEEE-754 and not require any actual changes in LLVM
> (since it's what CPUs would do anyway).
> 
> I think that's uglier than adding FNEG though.

NaN doesn’t need special handling except in the presence of signaling NaNs, where the IEEE-754 negate operation would produce a signaling NaN rather than a quiet NaN + invalid flag. Again, under the default fenv assumption, nothing special is needed.

>> Is correcting this behavior something that the general LLVM population would like? If not, we can create constrained intrinsics for the FPEnv project.
> 
> I'm in favour of FNEG too. I remember the fsub trick being confusing
> when I first encountered it.

We should still do fneg, though, because this.

– Steve