[FFmpeg-devel] [PATCHv2] avutil/mathematics: speed up av_gcd by using Stein's binary GCD algorithm

[Ganesh Ajjanagadde]

On Thu, Oct 22, 2015 at 8:33 AM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> On Thu, Oct 22, 2015 at 07:04:41AM -0400, Ganesh Ajjanagadde wrote:
>> On Thu, Oct 22, 2015 at 5:49 AM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:
>> > Ganesh Ajjanagadde <gajjanagadde <at> gmail.com> writes:
>> >
>> >> > This broke fate with -ftrapv
>> >
>> >> > The problem seems to be in av_gcd() and not the De-Bruijn version of
>> >> > ff_ctzll since the gcc builtin is being used.
>> >>
>> >> Don't have the time to investigate right now - revert for now unless
>> >> someone can fix it quickly.
>> >
>> > Ping?
>> >
>> > Please revert if nobody can fix this.
>>
>> "Fixing" this at the moment is highly dubious - read the thread. It
>> does not even occur in a reproducible way, does not trigger on ubsan,
>> I can't even see it on ftrapv. Show me why it is not a toolchain
>> issue.
>
> IMO never just assume that a problem that appears after a change you do
> is not caused by your code.
> Because if you do and you are wrong the bug might never be fixed
> while OTOH, if you assume its in your code you will sooner or later
> by debuging find the bug or more details, and that may be a
> code generation issue in the toolchain outside your code in case
> you where wrong.
> Its the safer default assumtation as the failure path if you are wrong
> is less bad.
>
> either way, bug fixed
> the reason for non reproduceablility likely was that the x86 asm
> overrode the C code in which the bug was

Ok, go ahead and revert. I just did not see the hurry for reverting;
since it does not break normal builds.

>
>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> I have often repented speaking, but never of holding my tongue.
> -- Xenocrates
>
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