[FFmpeg-devel] [PATCH] Fix for DECLARE_ALIGNED_8 macro on ARM (also prevent gcc 3.4.x ICE)

[Siarhei Siamashka]

Hello All,

Currently DECLARE_ALIGNED_8 macro is quite misleading on ARM (it tries to
enforce 4 byte alignment contarary to what a developer reading the 
ffmpeg sources could naturally expect):

...
/* This is to use 4 bytes read to the IDCT pointers for some 'zero'
   line optimizations */
#define DECLARE_ALIGNED_8(t, v) DECLARE_ALIGNED(4, t, v)
#define STRIDE_ALIGN 4
...

And it really causes problems with gcc 3.4.x:
http://lists.mplayerhq.hu/pipermail/ffmpeg-user/2005-August/000930.html
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22177

The offending line is the following:
    DECLARE_ALIGNED_8 (uint64_t, aligned_bak[stride]);

Apparently gcc got confused when asked to align uint64_t array at 4 byte
boundary and died with ICE. Earlier I just thought 'WTF?' and removed
DECLARE_ALIGNED_8 around that array (gcc should be clever enough to align 
that array properly without any help anyway). Now it is clear that
DECLARE_ALIGNED_8 macro definition triggered this problem.

Modern ARM cores have instructions for reading/writing 64-bit data and require
strict 8 byte alignment for using them. According to ARM manual, using
LDRD instruction with the data that is not aligned at 8 byte boundary is
UNPREDICTABLE and IMPLEMENTATION DEFINED. Unfortunately, both ARM cores that I
have available (ARM9E and ARM11) require just 4 byte alignment for these
instructions (ARM11 is faster when used with 8 byte aligned data though). So
if there were any other problems related to the use of this macro, I could not
observe them in my tests.

Anyway, I think it is better to fix this macro even though the immediate
visible effect is only the "out of the box" support for legacy compiler (gcc
3.4.x).

The comment in the code was removed by patch as I don't see direct relation of
this macro and IDCT, 16 byte alignment is supposedly guaranteed for IDCT
according to the comments from dsputil.h, anyway feel free to prove me wrong:
    void (*idct)(DCTELEM *block/* align 16*/);
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