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From: Eli Zaretskii <eliz@gnu.org>
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Subject: Re: The Emacs master is much slower than the emacs-27 branch.
Date: Sat, 05 Dec 2020 09:44:14 +0200
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Cc: acm@muc.de, stephen.berman@gmx.net, emacs-devel@gnu.org
To: Gregory Heytings <ghe@sdf.org>
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 (message from Gregory Heytings on Fri, 04 Dec 2020 21:20:33 +0000)
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> Date: Fri, 04 Dec 2020 21:20:33 +0000
> From: Gregory Heytings <ghe@sdf.org>
> cc: acm@muc.de, stephen.berman@gmx.net, emacs-devel@gnu.org
> 
> Sorry, I meant: it only improves compilation with -Og _and -O0_.  I don't 
> know if -O0 is important.

Given my measurements (see upthread), the effect on -O0 build is
negligible.  And yes, -O0 _is_ important.