Horton

Re: alignas (C11)

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Peter Eisentraut <peter@eisentraut.org>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-11-12T15:17:19Z
Lists: pgsql-hackers 
Hi,

On 2025-11-12 16:09:14 +0100, Peter Eisentraut wrote:
> On 12.11.25 15:02, Andres Freund wrote:
> > Hi,
> > 
> > On 2025-11-12 12:39:19 +0100, Peter Eisentraut wrote:
> > > - You cannot use alignas on a typedef.  So some uses of the attribute
> > > pg_attribute_aligned() like for PgAioUringContext or the whole int128
> > > business cannot be converted directly.  The solution for cases like
> > > PgAioUringContext could be to move the alignas into the struct, but I
> > > haven't studied this code closely enough, so I'm leaving it.  For int128,
> > > there is no straightforward solution, so I'm also leaving that as is.
> > 
> > Maybe I'm confused, but the aligned attribute for PgAioUringContext is on the
> > struct, not the typedef, no?
> 
> Yes, you're right.  The immediate problem there is that alignas is not
> syntactically valid at all at that position.

Argh, why couldn't C copy the C++ rules for this :(.

Just moving it to completion_lock would be fine though...

Greetings,

Andres Freund

Commits

  1. Further fix extended alignment for older g++.

  2. Disable extended alignment uses on older g++

  3. Work around buggy alignas in older g++

  4. Use C11 alignas in pg_atomic_uint64 definitions

  5. C11 alignas instead of unions -- extended alignments

  6. C11 alignas instead of unions

  7. Add <stdalign.h> to c.h