[FFmpeg-devel] fate : clang x86

[Eli Friedman]

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.

The option isn't documented in the clang documentation because it's an
internal switch which is really only meant to be used by developers.

-Eli