GNU bug report logs - #65988
30.0.50; Emacs and -O3 compiler optimization

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Arash Esbati <arash <at> gnu.org>
To: emacs-bugs <bug-gnu-emacs <at> gnu.org>
Subject: 30.0.50; Emacs and -O3 compiler optimization
Date: Fri, 15 Sep 2023 11:22:36 +0200

Hi all,

in this message[1], Eli explained why the -O3 optimization switch isn't
recommended when building Emacs:

  In a nutshell, it bloats the code (due to excessive inlining), with no
  real effect on speed.  The inner loops in Emacs are very large, and
  thus the techniques used by -O3 to speed up code (loop unrolling etc.)
  don't really work.  Moreover, they could make things worse because the
  larger loops might no longer fit into the L1 cache of the CPU.

  The -O3 is well suited to speed up relatively simple algorithms with
  tight loops.  Emacs has very few of those, in the places that matter
  for observable performance.

Presuming that the note above applies to GCC on all platforms, I suggest
to change/adjust the following example in INSTALL[2]:

  Here's an example of a 'configure' invocation, assuming a Bourne-like
  shell such as Bash, which uses these variables:

    ./configure \
      CPPFLAGS='-I/foo/myinclude' LDFLAGS='-L/bar/mylib' \
      CFLAGS='-O3' LIBS='-lfoo -lbar'

  (this is all one shell command).  This tells 'configure' to instruct the
  preprocessor to look in the '/foo/myinclude' directory for header
  files (in addition to the standard directories), instruct the linker
  to look in '/bar/mylib' for libraries, pass the -O3 optimization
  switch to the compiler, and link against libfoo and libbar
  libraries in addition to the standard ones.

IIRC this was the place where I picked up the -O3 switch for the script
I wrote for building Emacs.

Best, Arash

Footnotes:
[1]  https://lists.gnu.org/archive/html/bug-gnu-emacs/2023-06/msg01274.html

[2]  http://git.savannah.gnu.org/cgit/emacs.git/tree/INSTALL#n493

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