GNU bug report logs - #65491
[PATCH] Improve performance allocating vectors

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Message #115 received at 65491 <at> debbugs.gnu.org (full text, mbox):

From: Po Lu <luangruo <at> yahoo.com>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 65491 <at> debbugs.gnu.org,
 Mattias Engdegård <mattias.engdegard <at> gmail.com>,
 Eli Zaretskii <eliz <at> gnu.org>
Subject: Re: bug#65491: [PATCH] Improve performance allocating vectors
Date: Mon, 18 Sep 2023 10:19:09 +0800

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:

> Even converting them back to their original pointer (which is what we do
> with tag/untag pairs) is documented to be well-defined if you compile
> using GCC.

Only if the consequent pointer designates the same object as the initial
pointer (of which I see no scrutable definition within GCC's
documentation.)  But in practice, this doesn't matter, or so I thought.

> In contrast the pointer arithmetic on NULL pointers appears to be
> something which compilers have started to (ab)use as an assumption for
> their optimizations.  Hence the need to update our code.

Hmm?  Where in this thread was that fact established?

Thanks.

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