Horton

Re: pg_verify_checksums and -fno-strict-aliasing

Michael Banck <michael.banck@credativ.de>

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From: Michael Banck <michael.banck@credativ.de>
To: Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>
Cc: Magnus Hagander <magnus@hagander.net>, Fabien COELHO <coelho@cri.ensmp.fr>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-08-31T08:11:07Z
Lists: pgsql-hackers 
Hi,

Am Donnerstag, den 30.08.2018, 19:02 -0400 schrieb Tom Lane:
> Andres Freund <andres@anarazel.de> writes:
> > On 2018-08-30 18:11:40 -0400, Tom Lane wrote:
> > > I suspect people will complain about the added cost of doing that.
> > I think the compiler will just optimize it away.
> 
> Maybe.  In any case, the attached version avoids any need for memcpy
> and is, I think, cleaner code anyhow.  It fixes the problem for me
> with Fedora's gcc, though I'm not sure that it'd be enough to get rid
> of the warning Michael sees (I wonder what compiler he's using).

This fixes the bogus checksums, thanks!

I am on Debian stable, i.e. 'gcc version 6.3.0 20170516 (Debian 6.3.0-
18+deb9u1)'. 

The warning shows up only if I add -Wall *and* -O2 to CFLAGS, I think I
did not mention that explictly before.

As it is in pg_verify_checksums.c I need Magnus' patch as well to make
it go away. But the warning is independent of the runtime issue so less
of an issue (but probably worth fixing).


Michael 

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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.