[LLVMdev] Documentation of fmuladd intrinsic
justin.holewinski at gmail.com
Fri Jan 11 12:19:01 PST 2013
On Fri, Jan 11, 2013 at 3:16 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> ----- Original Message -----
> > From: dag at cray.com
> > To: "Justin Holewinski" <justin.holewinski at gmail.com>
> > Cc: "Hal Finkel" <hfinkel at anl.gov>, "LLVM Developers Mailing List" <
> llvmdev at cs.uiuc.edu>
> > Sent: Friday, January 11, 2013 2:13:50 PM
> > Subject: Re: [LLVMdev] Documentation of fmuladd intrinsic
> > Justin Holewinski <justin.holewinski at gmail.com> writes:
> > > Out of curiosity, what is the use-case for
> > > isFMAFasterThanMulAndAdd?
> > > If a target declares that FMA is actually slower for a given type,
> > > why not just declare it as illegal for that type? Wouldn't that
> > > accomplish the same thing without another target hook? I feel like
> > > I'm
> > > missing something here.
> > It's not expressed in the code Hal posted but I suppose a target
> > could
> > have a slow fma that the user nonetheless wants to use for precision
> > reasons.
> Yes, I believe that's right. This way you can still always get an fma with
> the intrinsic.
Now I'm confused. If a target declares that fmuladd is "slow" for a given
type, it will be lowered to mul + add in SDAG anyway (according to this
code snippet). So how could a user override this?
> > -David
> Hal Finkel
> Postdoctoral Appointee
> Leadership Computing Facility
> Argonne National Laboratory
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