Re: [Qemu-devel] [PATCH] tcg: Remove stack protection from helper functions

[Avi Kivity]

On 09/26/2011 11:15 AM, Laurent Desnogues wrote:
> On Mon, Sep 26, 2011 at 10:01 AM, Mulyadi Santosa
> <address@hidden>  wrote:
> >  Hi...
> >
> >  On Mon, Sep 26, 2011 at 14:46, Jan Kiszka<address@hidden>  wrote:
> >>  This increases the overhead of frequently executed helpers.
> >>
> >>  Signed-off-by: Jan Kiszka<address@hidden>
> >
> >  IMHO, stack protector setup put more stuffs during epilogue, but quite
> >  likely it is negligible unless it cause too much L1 cache misses. So,
> >  I think this micro tuning is somewhat unnecessary but still okay.
>
> The impact of stack protection is very high for instance running
> FFmpeg ARM with NEON optimizations:  a few months ago I
> measured that removing stack protection improved the run time
> by more than 10%.  Of course it's extreme since the proportion
> of NEON instructions (and hence of helper calls) is very high.

I saw a lot of helper calls for sse in ordinary x86_64 code, likely for 
memcpy/cmp and friends.  Native tcg ops for common vector instructions 
would probably be quite a speedup.

--
error compiling committee.c: too many arguments to function