[patch] test failure on sparc64 due to undefined division by zero behaviour
Hello PostgreSQL hackers,
I recently got the following bug report about a test suite failure on
sparc64:
----- Forwarded message from Aurelien Jarno <aurel32@debian.org> -----
Date: Wed, 02 Mar 2011 23:49:05 +0100
From: Aurelien Jarno <aurel32@debian.org>
To: Debian Bug Tracking System <submit@bugs.debian.org>
Subject: Bug#616180: postgresql-9.0: FTBFS on sparc64, testsuite issues with int8
postgresql-9.0 fails to build on sparc64 due to testsuite errors with
int8. The division by 0 is not trapped and a SIGFPE is issued instead of
an error.
Postgresql people claims it's a gcc bug, while gcc people says the code
is incorrect [1]http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29968. Whatever the real issue is, the fix is very simple
(actually taken in another part from the very same file), so it's
probably the best to simply apply it.
[1]: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29968
----- End forwarded message -----
The C99 standard [2]http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf -- Martin Pitt | http://www.piware.de Ubuntu Developer (www.ubuntu.com) | Debian Developer (www.debian.org), section 6.5.5 paragraph 5 actually says:
"The result of the / operator is the quotient from the division of
the first operand by the second; the result of the % operator is the
remainder. In both operations, if the value of the second operand is
zero, the behavior is undefined."
so the gcc folk's claim that this isn't a gcc bug looks justified.
Aurelien sent a straightforward patch for this, I updated it to apply
to current git head, updated the comments, and git-formatted it.
Thanks for considering!
Martin
[2]: http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf -- Martin Pitt | http://www.piware.de Ubuntu Developer (www.ubuntu.com) | Debian Developer (www.debian.org)
--
Martin Pitt | http://www.piware.de
Ubuntu Developer (www.ubuntu.com) | Debian Developer (www.debian.org)
Attachments:
0001-Avoid-undefined-division-by-zero.patchtext/x-diff; charset=us-asciiDownload+14-3
Martin Pitt <mpitt@debian.org> writes:
The C99 standard [2], section 6.5.5 paragraph 5 actually says:
"The result of the / operator is the quotient from the division of
the first operand by the second; the result of the % operator is the
remainder. In both operations, if the value of the second operand is
zero, the behavior is undefined."
so the gcc folk's claim that this isn't a gcc bug looks justified.
The gcc guys are full of it. The issue that is relevant here is the C
standard's definition of sequence points, and in particular the
requirement that visible side effects of a later statement cannot happen
before the execution of an earlier function call. The last time I
pestered them about this, I got some lame claim that a SIGFPE wasn't a
side effect within the definitions of the spec. At that point useful
discussion stopped, because it's impossible to negotiate with someone
who's willing to claim that.
Aurelien sent a straightforward patch for this, I updated it to apply
to current git head, updated the comments, and git-formatted it.
Hmm ... I'm willing to put in the extra return statements, but not to
remove the comments that we're working around a gcc bug. In particular,
given the gcc folks' claim that they need not consider the timing of
program-generated signals, there is really nothing to stop them from
trying to push the division up before the return too. This is nothing
but a band-aid for a non-standards-compliant optimizer.
regards, tom lane