[FFmpeg-devel] [PATCH] Make HAVE_FAST_UNALIGNED allow unaligned memory accesses

Måns Rullgård mans
Wed Jul 16 14:46:56 CEST 2008

Michael Niedermayer wrote:
> On Tue, Jul 15, 2008 at 08:16:21PM +0100, Mans Rullgard wrote:
>> If HAVE_FAST_UNALIGNED is defined, potentially unaligned data is
>> accessed through normal pointers.  Otherwise, compiler-specific
>> code is used to perform unaligned accesses, falling back to
>> byte-wise access if no compiler support is available.
>> ---
>>  libavutil/intreadwrite.h |  130
>> +++++++++++++++++++++++++++------------------
>>  1 files changed, 78 insertions(+), 52 deletions(-)
> [...]
>> @@ -37,7 +39,19 @@ struct unaligned_16 { uint16_t l; }
>> __attribute__((packed));
>>  #define AV_WN32(a, b) (((struct unaligned_32 *) (a))->l) = (b)
>>  #define AV_WN64(a, b) (((struct unaligned_64 *) (a))->l) = (b)
>> +#elif defined(__DECC)
>> +
>> +#define AV_RN16(a) (*((const __unaligned uint16_t*)(a)))
>> +#define AV_RN32(a) (*((const __unaligned uint32_t*)(a)))
>> +#define AV_RN64(a) (*((const __unaligned uint64_t*)(a)))
>> +
>> +#define AV_WN16(a, b) *((__unaligned uint16_t*)(a)) = (b)
>> +#define AV_WN32(a, b) *((__unaligned uint32_t*)(a)) = (b)
>> +#define AV_WN64(a, b) *((__unaligned uint64_t*)(a)) = (b)
> This change is ok, as a seperate commit (note ive edit it, the removial
> of #else on its own would break the code).
> The rest of the patch has to be split in cosmetics that are free of
> functional changes and the functional changes before it is reviewable.

Are you referring to the reordering of some macros?  That can probably
be done separately first.

Anyway, I'm not asking for a detailed review right now.  I want to know
if this is the right thing at all.  The code currently in svn has the
effect of ignoring HAVE_FAST_ALIGNED when gcc is used, since gcc turns
accesses to "attribute packed" things into byte-reads much like the
non-fast_unaligned code does.  Somehow, I don't think this was the
original intent.

Changing the semantics of fast_unaligned to allow actual unaligned
memory accesses makes the code faster on ARM versions that support
unaligned memory access.

M?ns Rullg?rd
mans at mansr.com

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