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* [Bug tree-optimization/49873] New: Optimizer regression
@ 2011-07-27 16:58 abenea at gmail dot com
2011-07-27 17:00 ` [Bug tree-optimization/49873] " pinskia at gcc dot gnu.org
2011-07-28 9:56 ` rguenth at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: abenea at gmail dot com @ 2011-07-27 16:58 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49873
Summary: Optimizer regression
Product: gcc
Version: 4.6.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: abenea@gmail.com
Test code:
typedef struct {
char *ptr;
} S;
void f1(char* data) {
S s;
s.ptr = 0;
char *same_data = s.ptr + (data - s.ptr);
same_data[0] = 1;
}
int main() {
char c;
f1(&c);
if (c != 1)
return 1;
return 0;
}
Compiled with gcc version 4.4.6, the program above returns 0 regardless of the
optimization level.
Compiled with gcc versions 4.5.3 or 4.6.1, the return value is 0 only with -O0.
Using -O1, -O2, -O3 or -Os, the return value is 1.
The correct value (0) is returned if s.ptr is replaced with a local variable or
the code from the function f1 is moved inside main.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/49873] Optimizer regression
2011-07-27 16:58 [Bug tree-optimization/49873] New: Optimizer regression abenea at gmail dot com
@ 2011-07-27 17:00 ` pinskia at gcc dot gnu.org
2011-07-28 9:56 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2011-07-27 17:00 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49873
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-07-27 17:00:06 UTC ---
I think this code is undefined because you are subtracting two "arrays" which
produces an undefined result in C.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/49873] Optimizer regression
2011-07-27 16:58 [Bug tree-optimization/49873] New: Optimizer regression abenea at gmail dot com
2011-07-27 17:00 ` [Bug tree-optimization/49873] " pinskia at gcc dot gnu.org
@ 2011-07-28 9:56 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2011-07-28 9:56 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49873
Richard Guenther <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-28 09:55:58 UTC ---
char *same_data = s.ptr + (data - s.ptr);
same_data[0] = 1;
same_data now points to s.ptr plus some offset that isn't computed in any
standard compliant way. As s.ptr is NULL the store is dead.
With
char *same_data = (char *)((uintptr_t) s.ptr + (uintptr_t) data - (uintptr_t)
s.ptr));
you would get the correct answer, avoiding all the undefinedness in your code.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2011-07-27 16:58 [Bug tree-optimization/49873] New: Optimizer regression abenea at gmail dot com
2011-07-27 17:00 ` [Bug tree-optimization/49873] " pinskia at gcc dot gnu.org
2011-07-28 9:56 ` rguenth at gcc dot gnu.org
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