From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Banck <michael.banck@credativ.de>
Cc: Andres Freund <andres@anarazel.de>
Cc: Magnus Hagander <magnus@hagander.net>
Cc: Fabien COELHO <coelho@cri.ensmp.fr>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: pg_verify_checksums and -fno-strict-aliasing
Date: Fri, 31 Aug 2018 12:02:04 -0400
Message-ID: <366.1535731324@sss.pgh.pa.us> (raw)
In-Reply-To: <1535703067.1286.10.camel@credativ.de>
References: <1535618100.1286.3.camel@credativ.de>
<alpine.DEB.2.21.1808301325070.32764@lancre>
<7321.1535639966@sss.pgh.pa.us>
<CABUevEz+3g4-tKLgcjTNK7EwgHm95NSYSKTNyEjcnQEGRzie0A@mail.gmail.com>
<1535660901.1286.8.camel@credativ.de>
<25968.1535663968@sss.pgh.pa.us>
<20180830214606.xpm4bho5nmhfuzlx@alap3.anarazel.de>
<20180830220409.4zhdka6ibs66enbk@alap3.anarazel.de>
<28544.1535667100@sss.pgh.pa.us>
<20180830223315.m2lc57oay3kztjz4@alap3.anarazel.de>
<7502.1535670135@sss.pgh.pa.us>
<1535703067.1286.10.camel@credativ.de>
Michael Banck <michael.banck@credativ.de> writes:
> Am Donnerstag, den 30.08.2018, 19:02 -0400 schrieb Tom Lane:
>> 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)'.
Ah-hah. I still can't replicate any warning with gcc 8.1.1, but
I do have a Raspbian installation at hand, with
gcc version 6.3.0 20170516 (Raspbian 6.3.0-18+rpi1+deb9u1)
and on that I get
$ gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fwrapv -fexcess-precision=standard -g -O2 -I../../../src/include -D_GNU_SOURCE -c -o pg_verify_checksums.o pg_verify_checksums.c
pg_verify_checksums.c: In function 'scan_file':
pg_verify_checksums.c:112:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
if (PageIsNew(buf))
^~
Adding -Wstrict-aliasing=2 makes it slightly more verbose:
$ gcc -Wstrict-aliasing=2 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fwrapv -fexcess-precision=standard -g -O2 -I../../../src/include -D_GNU_SOURCE -c -o pg_verify_checksums.o pg_verify_checksums.c
pg_verify_checksums.c: In function 'scan_file':
pg_verify_checksums.c:82:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
PageHeader header = (PageHeader) buf;
^~~~~~~~~~
pg_verify_checksums.c:112:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
if (PageIsNew(buf))
^~
Compiling this way also causes pg_verify_checksums to compute wrong
checksums. Installing the patched version of checksum_impl.h fixes
that, but doesn't change the warnings, meaning they have absolutely
nothing to do with the actual problem :-(
So, even aside from the difficulty of getting rid of aliasing-unsafe
code in Postgres, the state of the compiler warning technology is
still years short of where we could sanely consider dispensing with
-fno-strict-aliasing in general. Still, as I said before, it'd be
a good idea for checksum_impl.h to not depend on that. I'll go
ahead and push the fix.
regards, tom lane
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, michael.banck@credativ.de, andres@anarazel.de, magnus@hagander.net, coelho@cri.ensmp.fr
Subject: Re: pg_verify_checksums and -fno-strict-aliasing
In-Reply-To: <366.1535731324@sss.pgh.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox