Bug#195834: marked as done ([PR 11371] bogus warning and/or mis-optimization on ia64)
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>From james@ruari-quinn.demon.co.uk Mon Jun 02 12:44:25 2003
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Subject: gcc: bogus warning and/or mis-optimization on ia64
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From: James Troup <james@nocrew.org>
Date: Mon, 02 Jun 2003 18:40:43 +0100
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Package: gcc-3.3
Version: 3.3-2
| troup@merulo:~/gdbm-1.8.3$ gcc -O2 -Wall -c gdbmopen.c
| gdbmopen.c: In function `gdbm_open':
| gdbmopen.c:15: warning: `lock_val' might be used uninitialized in this function
| fstat (dbf->desc, &file_stat);
|
| if ((flags & GDBM_OPENMASK) == GDBM_READER)
| {
| if (dbf->file_locking)
| {
| struct flock flock;
| flock.l_type = F_RDLCK;
| flock.l_whence = SEEK_SET;
| flock.l_start = flock.l_len = 0L;
| lock_val = fcntl (dbf->desc, F_SETLK, &flock);
| }
| }
| else if (dbf->file_locking)
| {
| struct flock flock;
| flock.l_type = F_WRLCK;
| flock.l_whence = SEEK_SET;
| flock.l_start = flock.l_len = 0L;
| lock_val = fcntl (dbf->desc, F_SETLK, &flock);
| }
| if (dbf->file_locking && (lock_val != 0))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| {
| return NULL;
| }
I think this warning is bogus because a) it only happens on ia64
(AFAICT, it doesn't happen on arm, i386 or hppa, at least), b) it only
happens with optimization turned on (disappears with -O0) and
c) if the (entirely unrelated) fstat call is commented out, the
warning disappears. I also think that it's clear from the code above
that lock_val couldn't be used uninitialized unless I'm missing
something embarrassingly obvious. Unfortunately I haven't been able
to determine if it's just a bogus warning or if the code's actually
being mis-compiled.
This isn't a regression, all previous versions of gcc (2.96, 3.0 and
3.2) for ia64 have the same problem and gcc-snapshot (20030531-2)
doesn't fix it.
http://people.debian.org/~troup/gcc/gdbm/ contains gdbmopen.{c,i},
fixed-gdbmopen.{c,i} (i.e. with the fstat commented out) and
orig-gdbmopen.{c,i} (the unreduced original file from gdbm 1.8.3).
--
James
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Received: (at 195834-done) by bugs.debian.org; 20 Sep 2003 04:17:43 +0000
>From rmurray@cyberhqz.com Fri Sep 19 23:17:41 2003
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Date: Fri, 19 Sep 2003 21:17:40 -0700
To: 195834-done@bugs.debian.org
Subject: documented defect
Message-ID: <20030920041740.GA32188@cyberhqz.com>
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From: rmurray@cyberhqz.com (Ryan Murray)
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Upstream has closed this bug report with the following message:
------- Additional Comment #1 From Andrew Pinski 2003-06-29 13:47 -------
This is a know documented defect of gcc's warning about uninitialized
variables. In fact gcc should warn about it on all platforms but it
does not.
Here is the sample from the documenation:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
which is almost like your code but slightly different so this is not a bug.
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