[LLVMdev] Commutability of X86 FMA3 instructions.

[Lang Hames]

Hi Elena,

Thank you very much for looking in to that.

I'll go ahead and remove the isCommutable flag from those
instructions, since it sounds like that's the right thing to do. I
would still like to change the default from the 231 variant to 213
too, as this will reduce code-size for accumulator-style loops. I have
at least one benchmark that shows significant speedups when this
change is made (presumably the smaller loop fits within the decode
buffer, though I haven't verified this). Please let me know if there's
any reason to keep the 231 variant.

Cheers,
Lang.


On Sat, Dec 21, 2013 at 11:42 PM, Demikhovsky, Elena
<elena.demikhovsky at intel.com> wrote:
> 1) Our architects say that vmovaps %reg, %reg causes only register renaming in HW and does not  affect performance.
>
> 2) The non-numeric result of FMA is not commutative for multiplicand and the multiplier. You are right. This property should be removed.
>
>
> -  Elena
>
> -----Original Message-----
> From: Lang Hames [mailto:lhames at gmail.com]
> Sent: Friday, December 20, 2013 23:03
> To: Kay Tiong Khoo
> Cc: LLVM Developers Mailing List; Demikhovsky, Elena; Craig Topper
> Subject: Re: [LLVMdev] Commutability of X86 FMA3 instructions.
>
> Hi Kay,
>
> My patch will partially address your bug. For now I'm just looking to switch the default FMA from vfmadd213xx to vfmadd231xx. That will cause the code in PR17229 to compile as desired, but would regress code like:
>
> double foo(double a, double b, double c) {
>   return a * b + c;
> }
>
> Which will now require a vmovaps + vfmadd231.
>
> If this impacts real benchmarks we could add an optimization to change the FMA variant based on how it's used.
>
> - Lang.
>
> On Fri, Dec 20, 2013 at 8:29 AM, Kay Tiong Khoo <kkhoo at perfwizard.com> wrote:
>> 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.
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
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