[Ffmpeg-devel] DECLARE_ALIGNED_16 not honoured correctly in MinGW

Steve Lhomme slhomme
Fri Oct 27 14:54:24 CEST 2006


We are encountering a crash when calling DrFFMPEG from DrDivX on 
Windows. The crash occurs when decoding WMA content. After some 
investigation it appears that this declaration in wmadec.c doesn't 
produce what it's supposed to do:


When the DLL (could be libavcodec) is called from MSVC code 'output' is 
not aligned to 16 bits (0x01F36D28 in this case for example). And it 
crashes the SSE code used later. But it works fine when everything is 
built with MinGW. It appears that gcc assumes the stack is already 
aligned a certain way when it does the alignment of this variable in the 
stack. And this is not true when the stack is given by MSVC code.

Now I understand there's no support for MSVC here, but on the other hand 
libavcodec is a library, no matter how it's being called. Plus the bug 
is not in MSVC (who has the right to do whatever they want with the 
stack) but in gcc.

So my question is: does anybody know a way to make gcc behave correctly 
in such case ?

The other possibility I see to avoid the crash in my case is to add a 
test in ff_imdct_calc_sse() and vector_fmul_add_add_sse() to use the C 
code if one of the parameter passed is not aligned as it should. (I 
could make the patch if the solution is OK)


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