Re: Optimize glyph row clearing and copying routines

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 24 Sep 2013 14:10:07 +0400
> From: Dmitry Antipov <dmantipov@yandex.ru>
> CC: emacs-devel@gnu.org
> 
> On 09/24/2013 10:35 AM, Eli Zaretskii wrote:
> 
> > Does this change really speed up the code?
> 
> YMMV, as usual with benchmarks. Time is in CPU cycles, smaller is better:
> 
> It was:
> 
>                           gcc 4.8.1  gcc 4.8.1           Intel C 13.1.3   clang 3.3
>                           -O2 -g3    -O3 -march=native   -O3 -xHOST       -O3 -march=native
> ------------------------------------------------------------------------------------------
> clear_glyph_row          100        140                 90               40
> copy_row_except_pointers 90         160                 80               100
> 
> 
> Now it is:
> 
>                           gcc 4.8.1  gcc 4.8.1           Intel C 13.1.3   clang 3.3
>                           -O2 -g3    -O3 -march=native   -O3 -xHOST       -O3 -march=native
> ------------------------------------------------------------------------------------------
> clear_glyph_row          75         60                  35               35
> copy_row_except_pointers 95         150                 50               70
> 
> (Intel C and clang makes heavy use of SSE, but gcc isn't).
> 
> It would be interesting to add MSVC to this table :-).
> 
> In short, I believe that a good compiler should get more optimization opportunities from
> new code rather than from old. For this particular case, Intel C is "definitely good",
> and gcc, hm...looks controversial at least.

I think -O3 is not interesting in this case.  For -O2 with GCC, which
is the most important case (as Emacs is normally built like that), you
get net loss, because AFAIR copy_row_except_pointers is called much
more often than clear_glyph_row.  The important comparison is, of
course, as part of some redisplay scenario, not just cycle comparison.