[FFmpeg-devel] [PATCH 3/5] startcode: Stop overreading

[Michael Niedermayer]

On Sun, Jun 02, 2019 at 12:47:17AM +0200, Andreas Rheinhardt wrote:
> Up until now ff_startcode_find_candidate_c could overread; it relied on
> zero-padding after the buffer in order to function correctly. This has
> been changed: No overreads occur any more.
> The ultimate goal behind all this is to create a high-performance
> function for searching of startcodes that can be applied even in
> scenarios where the buffer is not padded.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> ---
>  libavcodec/startcode.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/libavcodec/startcode.c b/libavcodec/startcode.c
> index 373572365b..b027c191c0 100644
> --- a/libavcodec/startcode.c
> +++ b/libavcodec/startcode.c
> @@ -41,10 +41,7 @@ int ff_startcode_find_candidate_c(const uint8_t *buf, int size)
>  
>  #define READ(bitness) AV_RN ## bitness ## A
>  #define MAIN_LOOP(bitness, mask1, mask2) do {                              \
> -        /* we check p < end instead of p + 3 / 7 because it is
> -         * simpler and there must be AV_INPUT_BUFFER_PADDING_SIZE
> -         * bytes at the end. */                                            \

> -        for (; buf < end; buf += bitness / 8)                              \
> +        for (; buf <= end - bitness / 8; buf += bitness / 8)               \

is this faster than subtracting  bitness / 8 from end outside the loop ?
if not then i would suggest to do it outside as that means 1 thing less
the compiler has to optimize out

thx

[...]

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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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