From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: pgsql-hackers@lists.postgresql.org, Antonin Houska <ah@cybertec.at>
Subject: Re: xl_heap_header alignment?
In-reply-to: <5A0F3128-D990-4431-9E97-793CD99FF11E@anarazel.de>
References: <21643.1595353537@antos>
 <5A0F3128-D990-4431-9E97-793CD99FF11E@anarazel.de>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Tue, 21 Jul 2020 11:02:11 -0700"
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Date: Tue, 21 Jul 2020 14:33:23 -0400
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Andres Freund <andres@anarazel.de> writes:
> On July 21, 2020 10:45:37 AM PDT, Antonin Houska <ah@cybertec.at> wrote:
>> I don't quite understand this part of the comment of the xl_heap_header
>> structure:
>> * NOTE: t_hoff could be recomputed, but we may as well store it because
>> * it will come for free due to alignment considerations.

> Unless you declare them as packed, structs will add padding to align mem=
bers correctly (if, and only if, the whole struct is stored well aligned).

I think that comment may be out of date, because what's there now is

 * NOTE: t_hoff could be recomputed, but we may as well store it because
 * it will come for free due to alignment considerations.
 */
typedef struct xl_heap_header
{
	uint16		t_infomask2;
	uint16		t_infomask;
	uint8		t_hoff;
} xl_heap_header;

I find it hard to see how tacking t_hoff onto what would have been a
4-byte struct is "free".  Maybe sometime in the dim past there was
another field in this struct?  (But I checked back as far as 7.4
without finding one.)

I don't particularly want to remove the field, but we ought to
change or remove the comment.

			regards, tom lane