Feedback-ID: id4a34324:Fastmail
Date: Wed, 7 Feb 2024 18:06:37 -0800
From: Andres Freund <andres@anarazel.de>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Mats Kindahl <mats@timescale.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@lists.postgresql.org
Subject: Re: glibc qsort() vulnerability
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Hi,

On 2024-02-07 19:52:11 -0600, Nathan Bossart wrote:
> On Wed, Feb 07, 2024 at 04:42:07PM -0800, Andres Freund wrote:
> > On 2024-02-07 16:21:24 -0600, Nathan Bossart wrote:
> >> The assembly for that looks encouraging, but I still need to actually test
> >> it...
> > 
> > Possible. For 16bit upcasting to 32bit is clearly the best way. For 32 bit
> > that doesn't work, given the 32bit return, so we need something more.
> 
> For the same compASC() test, I see an ~8.4% improvement with your int64
> code

Just to be clear, that code unfortuntely isn't correct, the return value is a
32 bit integer, so the 64bit difference doesn't help. In contrast to the 16bit
case.


> and a ~3.4% improvement with this:

I guess that's still something.

Another branchless variant is (a > b) - (a < b). It seems to get a similar
improvement as the overflow-checking version.

Greetings,

Andres Freund