[FFmpeg-devel] [PATCH] ff_h[yc]scale_fast_mmext always clobbers %rbx

Carl Eugen Hoyos cehoyos at ag.or.at
Wed May 6 22:08:41 CEST 2015

Nick Lewycky <nlewycky <at> google.com> writes:

> On 6 May 2015 at 11:58, Carl Eugen Hoyos:
> > Nick Lewycky <nlewycky <at> google.com> writes:
> >
> > > This occurs when building ffmpeg with clang
> > > -fsanitize=address -O1 -fPIE.
> >
> > What is the usecase for -O1?
> It's very important for asan. ASan instruments 
> every memory access (outside inline asm, for now) 
> and if we don't run the optimizer then every local
> variable is a stack allocation with loads and 
> stores, and all those loads and stores get 
> instrumented leading to both crazy slowdown and 
> binary size increase.

What I meant was (and I believe I wasn't clear):
What is the usecase for -O1 as opposed to default 
Or in other words: Is the issue with clang also 
reproducible with -O2 and -O3?

> > Did you check performance with and without 
> > your patch?
> No.

I believe such a performance test is a precondition 
for an asm patch (but I am not the maintainer).

Carl Eugen

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