[FFmpeg-devel] fate : clang x86

[Eli Friedman]

On Mon, Aug 30, 2010 at 12:31 PM, Alex Converse <alex.converse at gmail.com> wrote:
> On Mon, Aug 30, 2010 at 3:29 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>> On Mon, Aug 30, 2010 at 11:56 AM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>>> 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?
>>
>> Writing dct_quantize in its current form completely in yasm is a
>> non-starter; it makes function calls, and depends on offsets into
>> structs defined in C.
>>
>
> Perhaps you should look at the yasm STRUC macro

I am not going to recode MpegEncContext into yasm.

-Eli