Re: pg_verify_checksums and -fno-strict-aliasing

Michael Banck <michael(dot)banck(at)credativ(dot)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