[LLVMdev] Documentation of fmuladd intrinsic

[Justin Holewinski]

On Fri, Jan 11, 2013 at 3:23 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Justin Holewinski" <justin.holewinski at gmail.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "David A. Greene" <dag at cray.com>, "LLVM Developers Mailing List" <
> llvmdev at cs.uiuc.edu>
> > Sent: Friday, January 11, 2013 2:19:01 PM
> > Subject: Re: [LLVMdev] Documentation of fmuladd intrinsic
> >
> >
> > 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?
>
> The user should use the fma intrinsic directly. There are now two
> intrinsics, fma and fmuladd.
>

Ah, alright.  I missed that there are actually two different intrinsics.


>
>  -Hal
>
> >
> >
> >
> > -Hal
> >
> > >
> > > -David
> >
> >
> > >
> >
> > --
> > Hal Finkel
> > Postdoctoral Appointee
> > Leadership Computing Facility
> > Argonne National Laboratory
> >
> >
> >
> >
> > --
> >
> >
> > Thanks,
> >
> >
> > Justin Holewinski
>
> --
> Hal Finkel
> Postdoctoral Appointee
> Leadership Computing Facility
> Argonne National Laboratory
>



-- 

Thanks,

Justin Holewinski
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