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* [Bug c++/18842] New: Weak optimization on global references
@ 2004-12-05 8:55 felix dot nawothnig at t-online dot de
2004-12-05 8:58 ` [Bug c++/18842] " felix dot nawothnig at t-online dot de
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: felix dot nawothnig at t-online dot de @ 2004-12-05 8:55 UTC (permalink / raw)
To: gcc-bugs
int x, &y = x;
int main() { y = 42; }
produces
x:
.zero 4
...
y:
.long x
...
movl y, %eax
movl $42, (%eax)
Since a non-arg reference is always (?) constant it would be safe to omit
movl $42, x
--
Summary: Weak optimization on global references
Product: gcc
Version: 3.4.3
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: felix dot nawothnig at t-online dot de
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18842
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/18842] Weak optimization on global references
2004-12-05 8:55 [Bug c++/18842] New: Weak optimization on global references felix dot nawothnig at t-online dot de
@ 2004-12-05 8:58 ` felix dot nawothnig at t-online dot de
2004-12-05 14:53 ` steven at gcc dot gnu dot org
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: felix dot nawothnig at t-online dot de @ 2004-12-05 8:58 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From felix dot nawothnig at t-online dot de 2004-12-05 08:58 -------
> Since a non-arg reference is always (?) constant it would be safe to omit
Err... safe to emit.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18842
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/18842] Weak optimization on global references
2004-12-05 8:55 [Bug c++/18842] New: Weak optimization on global references felix dot nawothnig at t-online dot de
2004-12-05 8:58 ` [Bug c++/18842] " felix dot nawothnig at t-online dot de
@ 2004-12-05 14:53 ` steven at gcc dot gnu dot org
2004-12-05 15:00 ` pinskia at gcc dot gnu dot org
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: steven at gcc dot gnu dot org @ 2004-12-05 14:53 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From steven at gcc dot gnu dot org 2004-12-05 14:53 -------
This is not a bug. You need IPA to figure out that y is never changed. For
example:
int x, &y = x, z;
int bar() { y = z; }
int foo() { y = 42; }
--
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18842
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/18842] Weak optimization on global references
2004-12-05 8:55 [Bug c++/18842] New: Weak optimization on global references felix dot nawothnig at t-online dot de
2004-12-05 8:58 ` [Bug c++/18842] " felix dot nawothnig at t-online dot de
2004-12-05 14:53 ` steven at gcc dot gnu dot org
@ 2004-12-05 15:00 ` pinskia at gcc dot gnu dot org
2004-12-05 15:14 ` [Bug tree-optimization/18842] " pinskia at gcc dot gnu dot org
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2004-12-05 15:00 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-05 15:00 -------
Actually references cannot change where they point to so this is something which we can do.
--
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|INVALID |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18842
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/18842] Weak optimization on global references
2004-12-05 8:55 [Bug c++/18842] New: Weak optimization on global references felix dot nawothnig at t-online dot de
` (2 preceding siblings ...)
2004-12-05 15:00 ` pinskia at gcc dot gnu dot org
@ 2004-12-05 15:14 ` pinskia at gcc dot gnu dot org
2004-12-05 19:11 ` felix dot nawothnig at t-online dot de
2004-12-05 19:15 ` pinskia at gcc dot gnu dot org
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2004-12-05 15:14 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-05 15:14 -------
Consider these two TUs:
extern int *y, x, z;
int foo();
extern "C" void abort();
int main()
{
foo();
if (*y!=42)
abort();
if (y!=&x)
abort();
y = &z;
foo ();
if (*y!=42)
abort();
if (y!=&z)
abort();
return 0;
}
-----
int x, &y = x, z;
int foo() { y = 42; }
-----
Now I think the above is invalid C++ but we don't have to diagnostic since the standard allows us not
to but since the standard allows the correct way, we can optimizate global references now.
--
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Component|c++ |tree-optimization
Ever Confirmed| |1
Keywords| |missed-optimization
Last reconfirmed|0000-00-00 00:00:00 |2004-12-05 15:14:15
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18842
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/18842] Weak optimization on global references
2004-12-05 8:55 [Bug c++/18842] New: Weak optimization on global references felix dot nawothnig at t-online dot de
` (3 preceding siblings ...)
2004-12-05 15:14 ` [Bug tree-optimization/18842] " pinskia at gcc dot gnu dot org
@ 2004-12-05 19:11 ` felix dot nawothnig at t-online dot de
2004-12-05 19:15 ` pinskia at gcc dot gnu dot org
5 siblings, 0 replies; 7+ messages in thread
From: felix dot nawothnig at t-online dot de @ 2004-12-05 19:11 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From felix dot nawothnig at t-online dot de 2004-12-05 19:10 -------
Diagnostics? Both files are 100% valid C++ - but since the C++ standard doesn't
enforce implementation of references as pointers they cause ABI-defined (?)
behaviour at/after linkage, just as...
extern int x;
void main() { foo(); assert(x == 5); }
---
float x;
void foo() { x = 5; }
...isn't guaranteed to "work" either. (but is still valid C)
Reading the reference over a pointer would still work because we can't remove
the reference itself from the object since someone might do "extern int &y;" in
another file - but it's evil. (non-portable)
*And* they are guaranteed to be const by the standard.
So, yes, safe to optimize. I think. :)
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18842
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/18842] Weak optimization on global references
2004-12-05 8:55 [Bug c++/18842] New: Weak optimization on global references felix dot nawothnig at t-online dot de
` (4 preceding siblings ...)
2004-12-05 19:11 ` felix dot nawothnig at t-online dot de
@ 2004-12-05 19:15 ` pinskia at gcc dot gnu dot org
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2004-12-05 19:15 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-05 19:15 -------
(In reply to comment #5)
> Diagnostics? Both files are 100% valid C++ - but since the C++ standard doesn't
> enforce implementation of references as pointers they cause ABI-defined (?)
> behaviour at/after linkage, just as...
Yes both files are valid C++ but when you link them together they become invalid C++ and yes
the C++ defines this as invalid with no diagnostic required. In fact your example is also invalid C++
but since the standard says we don't have to provide diagnostic as it is a hard problem to actually
provide the diagnostic.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18842
^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2004-12-05 19:15 UTC | newest]
Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2004-12-05 8:55 [Bug c++/18842] New: Weak optimization on global references felix dot nawothnig at t-online dot de
2004-12-05 8:58 ` [Bug c++/18842] " felix dot nawothnig at t-online dot de
2004-12-05 14:53 ` steven at gcc dot gnu dot org
2004-12-05 15:00 ` pinskia at gcc dot gnu dot org
2004-12-05 15:14 ` [Bug tree-optimization/18842] " pinskia at gcc dot gnu dot org
2004-12-05 19:11 ` felix dot nawothnig at t-online dot de
2004-12-05 19:15 ` pinskia at gcc dot gnu dot org
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