[FFmpeg-devel] [PATCH] avutil/mathematics: make av_gcd more robust

Michael Niedermayer michael at niedermayer.cc
Thu Oct 29 21:46:46 CET 2015

On Wed, Oct 28, 2015 at 10:48:14PM -0400, Ganesh Ajjanagadde wrote:
> This ensures that no undefined behavior is invoked, while retaining
> identical return values in all cases and at no loss of performance
> (identical asm on clang and gcc).
> Essentially, this patch exchanges undefined behavior with implementation
> defined behavior, a strict improvement.
> Rationale:
> 1. The ideal solution is to have the return type a uint64_t. This
> unfortunately requires an API change.
> 2. The only pathological behavior happens if both arguments are
> INT64_MIN, to the best of my knowledge. In such a case, the
> implementation defined behavior is invoked in the sense that UINT64_MAX
> is interpreted as INT64_MIN, which any reasonable implementation will
> do. In any case, any usage where both arguments are INT64_MIN is a
> fuzzer anyway.
> 3. Alternatives of checking, etc require branching and lose performance
> for no concrete gain - no client cares about av_gcd's actual value when
> both args are INT64_MIN. Even if it did, on sane platforms (e.g all the
> ones FFmpeg cares about), it produces a correct gcd, namely INT64_MIN.
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  libavutil/mathematics.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Let us carefully observe those good qualities wherein our enemies excel us
and endeavor to excel them, by avoiding what is faulty, and imitating what
is excellent in them. -- Plutarch
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