[FFmpeg-devel] fate : clang x86

Alex Converse alex.converse
Mon Aug 23 15:26:41 CEST 2010

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?

While "asm()" itself is standard GCC constraints are not. LLVM is free
to do whatever it likes in regard to ASM constraints. (Now clang has a
goal to be as GCC compatible as possible, so it still is an issue but)
I wouldn't say this bug makes clang not suitable for production use.


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