Feedback-ID: id4a34324:Fastmail
Date: Thu, 2 Apr 2026 10:31:48 -0400
From: Andres Freund <andres@anarazel.de>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
	Devrim =?utf-8?B?R8O8bmTDvHo=?= <devrim@gunduz.org>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Matheus Alcantara <matheusssilv97@gmail.com>,
 Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>
Subject: Re: LLVM 22
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Hi,

On 2026-04-02 16:20:41 +1300, Thomas Munro wrote:
> On Wed, Apr 1, 2026 at 4:25 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> > Working on this, more shortly...  I'm trying to figure out if Anthonin
> > and Andres's feedback means the poison approach does nothing useful
> > and we might as well just #ifdef out the lifetime.end stuff for LLVM
> > >= 22 to fix the breakage today.
> 
> Done.  Hopefully midge and Devrim will now turn green :-)

Thanks!


> > Either way it looks like we need a patch to use alloca instead, which
> > I'll also look into...
> 
> I see a few options, but I need to hack on them for a while to figure
> out the tradeoffs, or what I'm missing...  after the freeze.

I've experimented a bunch with this, it seems we need the larger changes done
as part of the patchset for removing pointers from the expressions to actually
allow recent-ish LLVM to optimize this.  I did verify that what we did didn't
have an effect with any other recent LLVM either.

The real fix here might be to have a separate calling convention for the very
common case of a scalar stable function with 1-3 arguments.  We loose a fair
bit of efficiency even in interpreted execution due to ferrying arguments,
their nullness, and the nullness of the return value through memory.

Greetings,

Andres Freund