Horton

Re: pg_verify_checksums and -fno-strict-aliasing

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

View thread View on postgr.es
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: 2018-08-31T19:55:51Z
Lists: pgsql-hackers

Attachments

I wrote:
> 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;

Here's a proposed patch using that approach.

Although some of the places that were using "char buf[BLCKSZ]" variables
weren't doing anything that really requires better alignment, I made
almost all of them use PGAlignedBuffer variables anyway, on the grounds
that you typically get better memcpy speed for aligned than unaligned
transfers.

I also fixed a few places that were using the palloc solution, and
one that was actually doing hand-alignment of the pointer (ugh).
I didn't try to be thorough about getting rid of such pallocs,
just fix places that seemed likely to be performance-critical.

This needs to be back-patched as relevant, of course.

			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.