[FFmpeg-devel] [PATCH] Fix for DECLARE_ALIGNED_8 macro on ARM (also prevent gcc 3.4.x ICE)
Sat Feb 16 14:49:13 CET 2008
Siarhei Siamashka <siarhei.siamashka at gmail.com> writes:
> 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:
> 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).
I agree. Since some ARM variants might require strict alignment for
64-bit loads, we must provide this.
> 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*/);
> Index: libavcodec/dsputil.h
> --- libavcodec/dsputil.h (revision 12120)
> +++ libavcodec/dsputil.h (working copy)
> @@ -535,10 +535,8 @@
> #elif defined(ARCH_ARMV4L)
> -/* 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
> +#define DECLARE_ALIGNED_8(t, v) DECLARE_ALIGNED(8, t, v)
> +#define STRIDE_ALIGN 8
Looks good to me.
mans at mansr.com
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