[FFmpeg-devel] fate : clang x86

Ronald S. Bultje rsbultje
Mon Aug 30 18:43:00 CEST 2010


Hi,

On Tue, Aug 24, 2010 at 11:56 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Tue, Aug 24, 2010 at 11:48:45AM -0400, Jason Garrett-Glaser wrote:
>> On Tue, Aug 24, 2010 at 11:19 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> > On Mon, Aug 23, 2010 at 06:57:20PM -0700, Eli Friedman wrote:
>> >> 2010/8/23 M?ns Rullg?rd <mans at mansr.com>:
>> >> > Eli Friedman <eli.friedman at gmail.com> writes:
>> >> >
>> >> >> 2010/8/21 M?ns Rullg?rd <mans at mansr.com>:
>> >> >>> Eli Friedman <eli.friedman at gmail.com> writes:
>> >> >>>
>> >> >>>> 2010/8/21 M?ns Rullg?rd <mans at mansr.com>:
>> >> >>>>> castet.matthieu at free.fr writes:
>> >> >>>>>
>> >> >>>>>> Hi,
>> >> >>>>>>
>> >> >>>>>> on freebsd "-mllvm -regalloc=fast" cflags are used to make clang/llvm accept
>> >> >>>>>> some inline asm.
>> >> >>>>>>
>> >> >>>>>> May be we should do the same on linux ?
>> >> >>>>>
>> >> >>>>> I tried and failed to figure out what that flag does. ?I assume it
>> >> >>>>> does something with the register allocator, but I'd like to know what.
>> >> >>>>
>> >> >>>> It's a workaround of sorts for
>> >> >>>> http://llvm.org/bugs/show_bug.cgi?id=4668 . ?LLVM essentially has two
>> >> >>>> register allocator implementations: one is the "fast" allocator, which
>> >> >>>> is a local register allocator used for -O0, and the other is the
>> >> >>>> "linear scan" allocator, which is the slower global register allocator
>> >> >>>> used for -O1+. ?"-mllvm -regalloc=fast" forces the use of the "fast"
>> >> >>>> allocator, which leads to slower generated code, but isn't affected by
>> >> >>>> the bug in question.
>> >> >>>
>> >> >>> Sounds like it's not suitable for production use. ?Any chance they'll
>> >> >>> fix the bug?
>> >> >>
>> >> >> In the near future? ?Not very likely... from what I understand, it's a
>> >> >> relatively difficult issue to solve, and bugs rejecting valid inline
>> >> >> asm are generally considered low priority for the people who know the
>> >> >> register allocator well enough to fix this.
>> >> >
>> >> > That leaves two options:
>> >> >
>> >
>> >> > 1. Declare clang officially unsupported for x86_32.
>> >
>> > ok with me
>> >
>> >
>> >> > 2. Fix our code.
>> >>
>> >> Attached attempts option 2. ?It's essentially a straight port of the
>> >> inline asm to yasm, with a couple minor changes to reduce the number
>> >> of arguments. ?This removes all the inline asm blocks from
>> >> mpegvideo_mmx_template.c, which allows clang to successfully build
>> >> ffmpeg. ?Passes regression tests.
>> >
>> > and is slower due to additional call overhead
>>
>> Why would it have additional overhead? ?Both are called by the same
>> function pointer.
>
> - ? ? ? ? ? ?: "+a" (last_non_zero_p1)
> - ? ? ? ? ? ?: "r" (block+64), "r" (qmat), "r" (bias),
> - ? ? ? ? ? ? ?"r" (inv_zigzag_direct16+64), "r" (temp_block+64)
> - ? ? ? ?);
> + ? ? ? ?last_non_zero_p1 =
> + ? ? ? ? ? ?RENAMEcore(ff_dct_quantize_core_h263)(block+64, qmat, bias,
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? inv_zigzag_direct16+64,
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? temp_block+64, overflow);
> ? ? }else{ // FMT_H263

Eli, are you still working on this?

Ronald



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