[FFmpeg-devel] fate : clang x86

[Jason Garrett-Glaser]

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.

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