[FFmpeg-devel] fate : clang x86

[Alex Converse]

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