[x264-devel] gcc stack alignment

[Jason Garrett-Glaser]

On Fri, Dec 19, 2008 at 12:03 AM, aaron <aaron_ppus at fmad.co.jp> wrote:
> Hi,
>
> Found a kind of interesting but wrt to stack aligned variables. so in
>
> encoder/slicetype.c :  x264_slicetype_mb_cost() line 194
> (x264-snapshot-20081207-2245)
>
> it declares
>
> DECLARE_ALLIGNED_16( uint8_t edge[33]);
>
> with gcc 4.3.1 edge ends up being on an 8B boundary, and thus the SSE2 asm
>
> predict_8x8_ddl_sse2:
>    movdqa   xmm3, [r1+16]
>
> will fault, non 128b aligned address.
>
> My understanding was gcc could not gaurentee the alignment of variables
> on the stack? it would only gaurentee the alignment of static/globals
> variables?
>
> Quick fix  is either
>
> 1) move edge[] to be a static var
>
> 2) use movdqu & take the perf hit.

or use x264_stack_align like one is supposed to and suffer no penalty at all.

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