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From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/51834] -Wsequence-point fails when convoluted expressions with multiple side effects are used
Date: Thu, 19 Apr 2012 15:07:00 -0000 [thread overview]
Message-ID: <bug-51834-4-hnynAPnLb4@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-51834-4@http.gcc.gnu.org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51834
--- Comment #4 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> 2012-04-19 15:06:58 UTC ---
(In reply to comment #3)
> (i++, i) + i is undefined. The sequence point only orders i++ and i inside the
> parens, but not the operands of +. The third example is not undefined.
The example is not (i++, i) + i, but (i, i++, i) + i, which is different
because there is a sequence point before and after the i++. Still, there seem
to be disagreements on how to interpret the standard.
There's a discussion "On sequence points and evaluation order" [1] in
comp.std.c in 1995-12 (though that's a bit old), from which there are arguments
to see the above expressions as UB. But "sequence points and evaluation order"
[2] in comp.lang.c in 2006-09 and a message from Keith Thompson [3] in
comp.std.c in 2010-10 both contradict it: they both say something like sin(x) +
cos(x) has defined behavior even if sin() and cos() both modify errno (and that
these functions can be implemented by a macro, as long as it has a sequence
point).
[1]
http://groups.google.com/group/comp.std.c/browse_thread/thread/d133e9c51bef572b/0b6545278c23d37f
[2]
http://groups.google.com/group/comp.lang.c/browse_thread/thread/c4bc836b783b91be/d807a3ad7202b45b
[3] http://groups.google.com/group/comp.std.c/msg/2dc8d2e8a0f4e572
What's strange is that GCC (4.4 to 4.7 at least) complains on
(i ? (j |= 1, 0) : 0) | (i ? (j |= 1, 0) : 0);
but not on
(j |= 1, 0) | (j |= 1, 0);
Contrary to GCC, I would say that the latter is UB (because from the root of
the expression, one can evaluate both j |= 1 without getting a sequence point
yet -- GCC should have output a warning, and that's bug 51562), but not the
former (similar to the errno case).
Here's a simple testcase I've used, with more tests:
int i, j;
static inline int set_flag (void)
{
j |= 1;
return 0;
}
#define FOO (i ? (j |= 1, 0) : 0)
#define BAR (i ? set_flag () : 0)
void fct (void)
{
FOO || FOO;
FOO | FOO;
BAR | BAR;
set_flag () + set_flag ();
j = (++i, j) + (j, ++i);
return;
}
GCC 4.7.0 warns only for "FOO | FOO;" (and I think that's incorrect, as said
above).
prev parent reply other threads:[~2012-04-19 15:07 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-01-12 6:29 [Bug c/51834] New: " prasoonsaurav.nit at gmail dot com
2012-01-12 8:51 ` [Bug c/51834] " rguenth at gcc dot gnu.org
2012-01-12 9:08 ` prasoonsaurav.nit at gmail dot com
2012-01-12 9:20 ` schwab@linux-m68k.org
2012-04-19 15:07 ` vincent-gcc at vinc17 dot net [this message]
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