Re: [PATCH,i386] fma,fma4 and xop flags

[Richard Guenther <richard dot guenther at gmail dot com>]

On Thu, Aug 9, 2012 at 7:55 AM, Gopalasubramanian, Ganesh
<Ganesh.Gopalasubramanian@amd.com> wrote:
>> Otherwise, what does -mno-fma4 -mxop do?
>> (it should enable both xop and fma4!)  what should -mfma4 -mno-xop do
>> (it should disable both xop and fma4!).
>
> Yes! that's what GCC does now.
> Some flags are coupled (atleast for now).
> For ex, -mno-sse4.2 -mavx enables both sse4.2 and avx
> whereas -mavx -mno-sse4.2 disables both.
>
> Setting of the following are clubbed.
> 1) 3DNow sets MMX
> 2) SSE2 sets SSE
> 3) SSE3 sets SSE2
> 4) SSE4_1 sets SSE3
> 5) SSE4_2 sets SSE4_1
> 6) FMA sets AVX
> 7) AVX2 sets AVX
> 8) SSE4_A sets SSE3
> 9) FMA4 set SSE4_A and AVX
> 10) XOP sets FMA4
> 11) AES sets SSE2
> 12) PCLMUL sets SSE2
> 13) ABM sets POPCNT
>
> Resetting is done in reversely (MMX resets 3DNOW).
>
> IMO, if we have different cpuid flags, enabling\disabling
> the compiler flags depends on these cpuid flags directly.
> Adding subsets to them or tangling them together may give
> wrong results.

Uh, ok ... it's messier than I anticipated ;)

> Please let me know your opinion.

Well, your patch looks reasonable then.  I'll defer to x86 maintainers for
approval though.

Thanks,
Richard.

> Regards
> Ganesh
>
> -----Original Message-----
> From: Richard Guenther [mailto:richard.guenther@gmail.com]
> Sent: Wednesday, August 08, 2012 5:12 PM
> To: Gopalasubramanian, Ganesh
> Cc: gcc-patches@gcc.gnu.org; ubizjak@gmail.com
> Subject: Re: [PATCH,i386] fma,fma4 and xop flags
>
> On Wed, Aug 8, 2012 at 1:31 PM,  <Ganesh.Gopalasubramanian@amd.com> wrote:
>> Hello,
>>
>> Bdver2 cpu supports both fma and fma4 instructions.
>> Previous to patch, option "-mno-xop" removes "-mfma4".
>> Similarly, option "-mno-fma4" removes "-mxop".
>
> Eh?  Why's that?  I think we should disentangle -mxop and -mfma4
> instead.  Otherwise, what does -mno-fma4 -mxop do?
> (it should enable both xop and fma4!)  what should -mfma4 -mno-xop do
> (it should disable both xop and fma4!).  All this is just confusing to
> the user, even if in AMD documents XOP includes FMA4.
>
> Richard.
>