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 Banck <michael.banck@credativ.de>
Cc: Magnus Hagander <magnus@hagander.net>, Fabien COELHO <coelho@cri.ensmp.fr>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-08-30T21:19:28Z
Lists: pgsql-hackers 
Michael Banck <michael.banck@credativ.de> writes:
> Could well be I'm doing something wrong, so it would be cool if somebody
> could reproduce this first. In principle, it should be enough to run
> 'make clean && make CLFAGS=-O2' in the src/bin/pg_verify_checksums
> subdirectory in order to get a broken executable.

I can reproduce it on my Fedora 28 machine (gcc 8.1.1) by compiling
pg_verify_checksums with all the normal flags except -fno-strict-aliasing.
I don't see any warning, not even with -Wstrict-aliasing=2 :-(, but the
generated code malfunctions as Michael describes.  As best I can tell,
the problem is in the inlined code from checksum_impl.h rather than
in pg_verify_checksums.c itself.  I think what is happening is gcc
is concluding that it can optimize away the code that temporarily
sets the page's checksum field to zero.

Although, as Alvaro mentioned upthread, there's basically no hope of
getting away from -fno-strict-aliasing for Postgres-at-large, I think it'd
be a good thing if we could get checksum_impl.h to not depend on it.  The
whole point of that header, according to its own self-description, is to
export the checksum calculation for use by external programs --- and we
can't really control what compiler flags will be used by those.  The fact
that Michael even ran into this problem seems to be an instance of that.

So, I've been fooling around trying to get it to work without
-fno-strict-aliasing, but with little luck so far.

			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.