[llvm-dev] [FPEnv] FNEG instruction

[Cameron McInally via llvm-dev]

On Tue, Oct 2, 2018 at 3:11 PM Roman Lebedev <lebedev.ri at gmail.com> wrote:

> On Tue, Oct 2, 2018 at 10:06 PM Cameron McInally via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > On Mon, Oct 1, 2018 at 5:41 PM Sanjay Patel <spatel at rotateright.com>
> wrote:
> >>
> >> I don't see any controversy for the preliminary requirement of removing
> BinaryOperator::isFNeg() and friends, so start with that?
> >> That work may reveal other potential regressions that we can patch in
> advance too.
> >
> >
> > This is true and I will agree to do this work...
> >
> >>
> >> Other than that, I think there's really only a question of do we want 1
> or both of fneg and fneg_constrained (and if we choose both, then I assume
> we'd also add fabs_constrained and copysign_constrained).
> >
> >
> > but this is the real goal. Doing the BinaryOperator::isFNeg() work is in
> vain if we don't have at least a conditional approval of an explicit FNEG
> IR instruction.
> >
> > Would it be possible to obtain that conditional approval before work
> begins? That seems most prudent.

Will this affect (regress, pessimize) the current optimizations for
> non-strict cases?
> What about -ffast-math?
>

The goal is for no regressions, but there may be problems to work through
to get there.

-ffast-math allows for unsafe transformations, so should remain unchanged.
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