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* [Bug c++/59324] New: C++11: -Wsequence-point
@ 2013-11-28 8:23 mathieu.malaterre at gmail dot com
2015-03-16 16:20 ` [Bug c++/59324] " manu at gcc dot gnu.org
0 siblings, 1 reply; 2+ messages in thread
From: mathieu.malaterre at gmail dot com @ 2013-11-28 8:23 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59324
Bug ID: 59324
Summary: C++11: -Wsequence-point
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mathieu.malaterre at gmail dot com
It would be nice to remove the following warning about UB when compiling in
C++11 mode:
$ cat t.cxx
int main()
{
int b = 1;
int m = (b++) + (++b);
(void)m;
return 0;
}
$ g++ -std=c++11 -Wsequence-point t.cxx
t.cxx: In function 'int main()':
t.cxx:4:23: warning: operation on 'b' may be undefined [-Wsequence-point]
int m = (b++) + (++b);
^
$ g++ --version
g++ (Debian 4.8.2-1) 4.8.2
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug c++/59324] C++11: -Wsequence-point
2013-11-28 8:23 [Bug c++/59324] New: C++11: -Wsequence-point mathieu.malaterre at gmail dot com
@ 2015-03-16 16:20 ` manu at gcc dot gnu.org
0 siblings, 0 replies; 2+ messages in thread
From: manu at gcc dot gnu.org @ 2015-03-16 16:20 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59324
Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |manu at gcc dot gnu.org
--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
Clang also thinks this is UB:
warning: multiple unsequenced modifications to 'b' [-Wunsequenced]
>From gcc-bugs-return-480478-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Mon Mar 16 16:25:15 2015
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From: "vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/65443] Don't peel last iteration from loop in transform_to_exit_first_loop
Date: Mon, 16 Mar 2015 16:25:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide443
--- Comment #1 from vries at gcc dot gnu.org ---
Consider test.c, compiled with -O2 -tree-parallelize-loops=2:
...
#include <stdio.h>
extern unsigned int *a;
void
f (unsigned int n)
{
int i;
unsigned int sum = 1;
#pragma omp parallel
{
#pragma omp for
for (i = 0; i < n; ++i)
sum += a[i];
}
printf ("%u\n", sum);
}
...
Before tranform_to_exit_first_loop, the loop looks like this:
...
<bb 4>:
# sum_18 = PHI <1(11), sum_11(6)>
# ivtmp_25 = PHI <0(11), ivtmp_6(6)>
i_17 = (int) ivtmp_25;
_7 = (long unsigned int) i_17;
_8 = _7 * 4;
_9 = pretmp_24 + _8;
_10 = *_9;
sum_11 = _10 + sum_18;
i_12 = i_17 + 1;
i.1_3 = (unsigned int) i_12;
if (ivtmp_25 < _20)
goto <bb 6>;
else
goto <bb 5>;
<bb 5>:
# sum_21 = PHI <sum_11(4), sum_26(8)>
goto <bb 7>;
<bb 6>:
ivtmp_6 = ivtmp_25 + 1;
goto <bb 4>;
...
You might say that the transformation applied by tranform_to_exit_first_loop is
that all the statements in bb4 before the if are moved past the if, into both
bb5 and bb6.
After, it looks like:
...
<bb 4>:
# sum_28 = PHI <1(11), sum_11(6)>
# ivtmp_29 = PHI <0(11), ivtmp_6(6)>
if (ivtmp_29 < _20)
goto <bb 13>;
else
goto <bb 14>;
<bb 13>:
# sum_18 = PHI <sum_28(4)>
# ivtmp_25 = PHI <ivtmp_29(4)>
i_17 = (int) ivtmp_25;
_7 = (long unsigned int) i_17;
_8 = _7 * 4;
_9 = pretmp_24 + _8;
_10 = *_9;
sum_11 = _10 + sum_18;
i_12 = i_17 + 1;
i.1_3 = (unsigned int) i_12;
goto <bb 6>;
<bb 14>:
# sum_30 = PHI <sum_28(4)>
ivtmp_31 = _20;
i_32 = (int) ivtmp_31;
_33 = (long unsigned int) i_32;
_34 = _33 * 4;
_35 = pretmp_24 + _34;
_36 = *_35;
sum_37 = _36 + sum_30;
i_38 = i_32 + 1;
i.1_39 = (unsigned int) i_38;
<bb 5>:
# sum_21 = PHI <sum_37(14), sum_26(8)>
goto <bb 7>;
<bb 6>:
ivtmp_6 = ivtmp_25 + 1;
goto <bb 4>;
...
^ permalink raw reply [flat|nested] 2+ messages in thread
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2013-11-28 8:23 [Bug c++/59324] New: C++11: -Wsequence-point mathieu.malaterre at gmail dot com
2015-03-16 16:20 ` [Bug c++/59324] " manu at gcc dot gnu.org
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