GNU bug report logs - #36597
27.0.50; rehash hash tables eagerly in pdumper

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

From: Pip Cet <pipcet <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org, Daniel Colascione <dancol <at> dancol.org>
Subject: 27.0.50; rehash hash tables eagerly in pdumper
Date: Thu, 11 Jul 2019 14:05:14 +0000

[Message part 1 (text/plain, inline)]

This is a follow-up to bug#36447, which has been fixed.

Lazy rehashing for hash tables should be removed. This patch does that.

Lazy rehashing makes all code that accesses hash tables a little more
complicated; in at least one case, we forgot to do that, resulting in
bug#36596.

The sole benefit of lazy rehashing to be mentioned so far is that
start-up time is reduced a little (by less than a millisecond on this
system), and that for certain usage patterns (many short Emacs
sessions that don't do very much, I think), this might outweigh the
negative consequences of lazy rehashing.

Lazy rehashing means less maintainable code, more code, and, at
run-time, slightly slower code.

In particular, it means that we have to have a comment in lisp.h which
distracts from the code and has to be marked as "IMPORTANT!!!!!!!",
which I think is nearly as annoying as having to jump through special
hoops before accessing a hash table.

I'm posting this as a separate bug so we can have a, hopefully, brief
discussion about it and then either move away from lazy rehashing or
continue using it; if we decide to continue using it, it should be
documented in much more detail.

(We should keep in mind that conditional branches, even ones that are
well-predicted and don't cause cache misses, aren't free: they use
execution units, and preparing their arguments in registers increases
register pressure, and of course they increase code size.)

[0001-Rehash-hash-tables-eagerly-after-loading-a-portable-.patch (text/x-patch, attachment)]

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