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

[Michael Niedermayer]

On Fri, Jul 18, 2008 at 02:23:29AM +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 |   34 +++++++++++++++++++++++++++++-----
>  1 files changed, 29 insertions(+), 5 deletions(-)
> 
> diff --git a/libavutil/intreadwrite.h b/libavutil/intreadwrite.h
> index 72ad5b3..12dec54 100644
> --- a/libavutil/intreadwrite.h
> +++ b/libavutil/intreadwrite.h
> @@ -23,6 +23,8 @@
>  #include "config.h"
>  #include "bswap.h"
>  
> +#if !defined(HAVE_FAST_UNALIGNED)
> +
>  #ifdef __GNUC__
>  
>  struct unaligned_64 { uint64_t l; } __attribute__((packed));
> @@ -47,7 +49,9 @@ struct unaligned_16 { uint16_t l; } __attribute__((packed));
>  #define AV_WN32(a, b) *((__unaligned uint32_t*)(a)) = (b)
>  #define AV_WN64(a, b) *((__unaligned uint64_t*)(a)) = (b)
>  
> -#else
> +#endif
> +
> +#else /* !HAVE_FAST_UNALIGNED */
>  
>  #define AV_RN16(a) (*((const uint16_t*)(a)))
>  #define AV_RN32(a) (*((const uint32_t*)(a)))
> @@ -57,7 +61,7 @@ struct unaligned_16 { uint16_t l; } __attribute__((packed));
>  #define AV_WN32(a, b) *((uint32_t*)(a)) = (b)
>  #define AV_WN64(a, b) *((uint64_t*)(a)) = (b)
>  
> -#endif /* !__GNUC__ */
> +#endif /* !HAVE_FAST_UNALIGNED */
>  
>  /* endian macros */
>  #define AV_RB8(x)     (((const uint8_t*)(x))[0])

ok


> @@ -66,7 +70,8 @@ struct unaligned_16 { uint16_t l; } __attribute__((packed));
>  #define AV_RL8(x)     AV_RB8(x)
>  #define AV_WL8(p, d)  AV_WB8(p, d)
>  
> -#ifdef HAVE_FAST_UNALIGNED
> +#ifdef AV_RN16
> +
>  # ifdef WORDS_BIGENDIAN
>  #  define AV_RB16(x)    AV_RN16(x)
>  #  define AV_WB16(p, d) AV_WN16(p, d)

Platforms where HAVE_FAST_UNALIGNED is not set would generally have AV_RN16
set thus use things like:
#  define AV_RL16(x)    bswap_16(AV_RN16(x))
#  define AV_WL16(p, d) AV_WN16(p, bswap_16(d))
#  define AV_RL32(x)    bswap_32(AV_RN32(x))
#  define AV_WL32(p, d) AV_WN32(p, bswap_32(d))

These may be slower than the naive bytewise reading macros on such platforms.


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No human being will ever know the Truth, for even if they happen to say it
by chance, they would not even known they had done so. -- Xenophanes
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