[FFmpeg-devel] [PATCH] vp9: change order of operations in adapt_prob().

[James Zern]

On Fri, Oct 14, 2016 at 11:31 AM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> On Fri, Oct 14, 2016 at 08:29:37PM +0200, Michael Niedermayer wrote:
>> On Fri, Oct 14, 2016 at 11:09:30AM -0700, James Zern wrote:
>> > Ronald,
>> >
>> > On Fri, Oct 14, 2016 at 10:01 AM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>> > > This is intended to workaround bug "665 Integer Divide Instruction May
>> > > Cause Unpredictable Behavior" on some early AMD CPUs, which causes a
>> > > div-by-zero in this codepath, such as reported in Mozilla bug #1293996.
>> > >
>> > > Note that this isn't guaranteed to fix the bug, since a compiler is free
>> > > to reorder instructions that don't depend on each other. However, it
>> > > appears to fix the bug in Firefox, and a similar patch was applied to
>> > > libvpx also (see Chrome bug #599899).
>> > >
>> >
>> > I recently made a few additional changes as this regressed in chrome
>> > [1][2], but just like this change there's no guarantee it won't occur
>> > again.
>>
>> maybe you can use empty "asm volatile(:::"memory")" statments to
>> prevent unwanted instruction reordering by the compiler
>> never tried something like this so dunno, also it would be specific
>> to gcc compatible compilers but should not be architecture specific
>
> thinking again, why dont you write the function in asm for x86 ?
> this would take the compiler out of the equation
>

That could work, I started with a portion in assembly before making
the C changes, but didn't follow through with it.

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