[FFmpeg-devel] Compiling with Sun cc

Loren Merritt lorenm
Fri Oct 3 09:04:20 CEST 2008

On Thu, 2 Oct 2008, Roman V. Shaposhnik wrote:
> On Fri, 2008-10-03 at 06:46 +0300, Uoti Urpala wrote:
>> On Thu, 2008-10-02 at 20:10 -0700, Roman V. Shaposhnik wrote:
>>> Well, it is very controversial thing, for one. The trouble is, that even 
>>> gcc can not really guarantee it, since initial stack assumptions are not 
>>> controlled by the compiler, but rather by the combination of an OS 
>>> and how linking was done.
>> The initial stack state is irrelevant.
>> GCC will always align the stack at the start of main().
> Huh? Meaning that it does so in crt*.o? And tough luck if you link 
> with a different crt* modules?

main() is in ffmpeg.o not crt*.o
I confirm that gcc compiles main() as if it had force_align_arg_pointer.

>> The only way stack alignment can fail is if 
>> GCC-generated code is called from a function compiled with another 
>> compiler
> Or written by hand, or generate by JIT compiler, or...

There isn't a single asm statement in ffmpeg that calls back to C. And if 
we ever need that, someone handwriting asm can preserve whatever they want, 
including alignment.

>> FFmpeg uses that attribute for functions that can be called from outside 
>> the library and require the stack to be aligned.
> Tracing all the entry points in a library such as libavcodec could 
> be quite tricky. And arbitrary force_align_arg_pointer is 
> expensive.

Forbidding SSE is expensive too.
Even if the only way to guarantee alignment was to force_align_arg_pointer 
every nonstatic function (or every function that declares aligned data), 
that would still be a necessary cost.

--Loren Merritt

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