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Re: Optimize glyph row clearing and copying routines

From: Dmitry Antipov
Subject: Re: Optimize glyph row clearing and copying routines
Date: Tue, 24 Sep 2013 16:42:57 +0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.0

On 09/24/2013 03:50 PM, Eli Zaretskii wrote:

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.

1) I do not believe in 5 CPU cycles difference too much. This is
   just ~5%, which may be explained by some irregular inaccuracy.

2) I'm talking about GCC 4.8.1, but people from this list still
   uses 4.2.x. or even older. Unfortunately GCC is known to have
   some serious regressions from time to time.

3) As it's proven by icc/clang, good compiler _really_ gets more
   optimization opportunities from new code rather than from old.
   IMHO we should provide workaround if it's known that gcc-X.Y.Z
   has serious bug; but we should not reject generally better
   code if we have just an unconvincing suspicion that gcc-P.Q.R
   introduces some miserable regression.


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