Horton

Re: pg_verify_checksums and -fno-strict-aliasing

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Paquier <michael@paquier.xyz>
Cc: Fabien COELHO <coelho@cri.ensmp.fr>, Michael Banck <michael.banck@credativ.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-08-31T14:55:06Z
Lists: pgsql-hackers 
Michael Paquier <michael@paquier.xyz> writes:
> On Thu, Aug 30, 2018 at 07:37:37PM -0400, Tom Lane wrote:
>> Anyway, I'll work on a patch for that, unless you were on it already?

> I have begun working on that and am less than halfway through it as I
> needed a fresh problem, however I am not sure I would be able to finish
> it today, perhaps tomorrow...  If you have time and want to press on as
> 11 would release soon, of course feel free to wrap up more quickly than
> I can.

Some of these places might be performance-critical enough that adding
a palloc/pfree cycle would not be nice.  What I was considering doing
was inventing something like

typedef union PGAlignedBuffer
{
	char	data[BLCKSZ];
	double	force_align;
} PGAlignedBuffer;

and replacing plain char[] variables with that as appropriate.
We might need one for XLOG_BLCKSZ too.

Since this'd be used in frontend as well as backend, it'd likely
have to end up in c.h, which is slightly annoying but not really
a big deal as long as we pick a nonconflicting type name.

			regards, tom lane

Commits

  1. Avoid using potentially-under-aligned page buffers.

  2. Code review for pg_verify_checksums.c.

  3. Make checksum_impl.h safe to compile with -fstrict-aliasing.