[LLVMdev] Commutability of X86 FMA3 instructions.

[Kay Tiong Khoo]

Hi Lang,

Unfortunately, I don't have an answer on the commutability question, but I
wanted to let you know that I filed a bug on this:
http://llvm.org/bugs/show_bug.cgi?id=17229

This also shows a memory operand variant of the fma that you may want to
consider in your patch and testcases.

Thanks!


On Thu, Dec 19, 2013 at 10:45 PM, Lang Hames <lhames at gmail.com> wrote:

> Hi all,
>
> The 213 variant of the FMA3 instructions is currently marked
> commutable (see X86InstrFMA.td). Is that safe? According to the ISA
> the FMA3 instructions aren't commutable for non-numeric results, so
> I'd have thought commuting this would only be valid in fast-math mode?
>
> For the curious, the reason that I'm asking is that we currently
> always select the 213 variant, but this introduces an extra copies in
> accumulator-style loops. Something like:
>
> while (...)
>   accumulator = x * y + accumulator;
>
> yields:
>
> loop:
>   vfmadd.213 y, x, acc
>   vmovaps acc, x
>   decl count
>   jne loop
>
> instead of
>
> loop:
>   vfmadd.231 acc, x, y
>   decl count
>   jne loop
>
> I have started writing a patch to generate the 231 variant by default,
> and I want to know whether I need to go to the trouble of adding
> custom commute logic. If these things aren't commutable then I don't
> need to worry at all. If they are commutable, but only in fast-math
> mode, then I can support that too.
>
> Thanks for the help!
>
> - Lang.
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