[FFmpeg-devel] fate : clang x86

Ronald S. Bultje rsbultje
Mon Aug 30 20:56:23 CEST 2010


Hi,

On Mon, Aug 30, 2010 at 2:20 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Mon, Aug 30, 2010 at 9:43 AM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>> 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?
>
> Do you have any suggestions for putting the patch into an acceptable form?

Make the called function a macro and inline it manually in yasm?

Ronald



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