[FFmpeg-devel] fate : clang x86

[Eli Friedman]

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?

-Eli