[FFmpeg-devel] [PATCH] Set -mfpmath=sse for cpus that support it

[Vitor Sessak]

M?ns Rullg?rd wrote:
> Vitor Sessak <vitor1001 at gmail.com> writes:
> 
>> M?ns Rullg?rd wrote:
>>> Vitor Sessak <vitor1001 at gmail.com> writes:
>>>
>>>> $subj, 10% faster TwinVQ decoding.
>>>>
>>>> As a side note, this option is not enabled even when passing
>>>> "-march=native -mtune=native" to GCC. A possible reason for it,
>>>> quoting the GCC doc, is "The resulting code should be considerably
>>>> faster [...] but may break some existing code that expects temporaries
>>>> to be 80bit.".
>>>>
>>>> -Vitor
>>>>
>>>> Index: configure
>>>> ===================================================================
>>>> --- configure	(revision 19681)
>>>> +++ configure	(working copy)
>>>> @@ -1649,22 +1649,27 @@
>>>>          cpuflags="-mcpu=cell"
>>>>          enable ldbrx
>>>>      ;;
>>>> -    # targets that do NOT support conditional mov (cmov)
>>>>      i[345]86|pentium|pentium-mmx|k6|k6-[23]|winchip-c6|winchip2|c3)
>>>>          cpuflags="-march=$cpu"
>>>>          disable cmov
>>>>      ;;
>>>> -    # targets that do support conditional mov (cmov)
>>>> -    i686|pentiumpro|pentium[23]|pentium-m|athlon|athlon-tbird|athlon-4|athlon-[mx]p|athlon64|k8|opteron|athlon-fx|core2)
>>>> +    i686|pentiumpro|pentium2|athlon|athlon-tbird)
>>>>          cpuflags="-march=$cpu"
>>>>          enable cmov
>>>>          enable fast_cmov
>>>>      ;;
>>>> +    pentium3|pentium-m|athlon-4|athlon-[mx]p|athlon64|k8|opteron|athlon-fx|core2)
>>>> +        cpuflags="-march=$cpu"
>>>> +        enable cmov
>>>> +        enable fast_cmov
>>>> +        check_cflags -mfpmath=sse
>>>> +    ;;
>>>>      # targets that do support conditional mov but on which it's slow
>>>>      pentium4|pentium4m|prescott|nocona)
>>>>          cpuflags="-march=$cpu"
>>>>          enable cmov
>>>>          disable fast_cmov
>>>> +        check_cflags -mfpmath=sse
>>>>      ;;
>>>>      sparc64)
>>>>          cpuflags="-mcpu=v9"
>>> You can't use check_cflags there.  Is there any reason to not attempt
>>> using this flag with --cpu values?  I imagine a simple check_cflags
>>> line together with the other x86 stuff (ebx checks etc) should be
>>> enough.
>> I can't just call "check_cflags -mfpmath=sse" for every x86 CPU
>> because GCC does not fail with "-march=386 -mfpmath=sse", it gives a
>> warning and uses 387 (so check_cflags would always add it).
> 
> Does any gcc version not have that flag?

Would not all this be too much GCC-specific? My original patch should be 
safe for any sane compiler (even one that happily generate SSE code with 
no warnings for -march=386 -mfpmath=sse).

-Vitor