[llvm-dev] FMA canonicalization in IR

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Hal J. via llvm-dev Finkel" <llvm-dev at lists.llvm.org>
> To: "Sanjay Patel" <spatel at rotateright.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Saturday, November 19, 2016 10:58:27 AM
> Subject: Re: [llvm-dev] FMA canonicalization in IR
> 
> 
> Sent from my Verizon Wireless 4G LTE DROID
> On Nov 19, 2016 10:26 AM, Sanjay Patel < spatel at rotateright.com >
> wrote:
> > 
> > If I have my FMA intrinsics story straight now (thanks for the
> > explanation, Hal!), I think it raises another question about IR
> > canonicalization (and may affect the proposed revision to IR FMF):
> 
> 
> No, I think that we specifically don't want to canonicalize to
> fmuladd at the IR level at all. If the backend has the freedom to
> form FMAs as it sees fit, then we should delay the decision until
> whenever the backend finds most appropriate. Some backends, for
> example, form FMAs using the MachineCombiner pass which considers
> critical path, latency, throughputs, etc. in order to find the best
> fusion opportunities. We only use fmuladd when required to restrict
> the backend to certain choices due to source-language semantics.

I'll also add that, in general, we canonicalize in order to enable other transformations (and reduce the number of input forms those transformations need to match in order to be effective). Forming @llvm.fmulall at the IR level does not seem to further this goal. Did you have something in mind that this canonicalization would help?

Thanks again,
Hal

> 
> 
> Thanks again,
> Hal
> 
> 
> > 
> > define float @foo(float %a, float %b, float %c) {
> > %mul = fmul fast float %a, %b ; using 'fast' because there is no
> > 'fma' flag
> > %add = fadd fast float %mul, %c
> > ret float %add
> > }
> > 
> > Should this be:
> > 
> > define float @goo(float %a, float %b, float %c) {
> > % maybe.fma = call fast float @llvm.fmuladd.f32(float %a, float %b,
> > float %c)
> > ret float % maybe.fma
> > }
> > declare float @llvm.fmuladd.f32(float %a, float %b, float %c)
> > 
> > 
> 
> 
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory