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* [Bug c/47670] New: Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic
@ 2011-02-09 20:56 argondsgn at gmail dot com
2011-02-09 21:14 ` [Bug c/47670] " rguenth at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: argondsgn at gmail dot com @ 2011-02-09 20:56 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47670
Summary: Gcc 4.5.1 inconsistent optimisation of loop with mixed
32bit and 64bit arithmetic
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: argondsgn@gmail.com
Created attachment 23290
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23290
Output of gcc -v save-temps -O2 bug.c
int main(int argc, char *argv[])
{
int i;
long long count=0;
int ret;
for (i=0;i<9;i++)
{
count+=i*0x40000000;
}
ret=count>>32;
return ret;
}
produces different answers when compiled with
gcc -O1 bug.c (returns -1)
or
gcc -O2 bug.c (returns 9)
Judging from the assembler output with -S, this loop is optimised to a
different constant expression in the two cases.
It works with gcc 3.4.4, but not with gcc 4.5.1 (I haven't tested any other
versions yet)
(First time I've attempted to submit a bug report to an open source project, so
many apologies if this is already fixed, an issue with my understanding of C,
or in the wrong category!)
^ permalink raw reply [flat|nested] 5+ messages in thread* [Bug c/47670] Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic 2011-02-09 20:56 [Bug c/47670] New: Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic argondsgn at gmail dot com @ 2011-02-09 21:14 ` rguenth at gcc dot gnu.org 2011-02-09 21:20 ` rguenth at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: rguenth at gcc dot gnu.org @ 2011-02-09 21:14 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47670 --- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-02-09 21:13:46 UTC --- Singed overflow invokes undefined behavior, use count += ((long long)i) * 0x40000000 instead. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/47670] Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic 2011-02-09 20:56 [Bug c/47670] New: Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic argondsgn at gmail dot com 2011-02-09 21:14 ` [Bug c/47670] " rguenth at gcc dot gnu.org @ 2011-02-09 21:20 ` rguenth at gcc dot gnu.org 2011-02-09 21:54 ` jakub at gcc dot gnu.org 2011-02-09 22:00 ` argondsgn at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: rguenth at gcc dot gnu.org @ 2011-02-09 21:20 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47670 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> 2011-02-09 21:14:01 UTC --- . ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/47670] Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic 2011-02-09 20:56 [Bug c/47670] New: Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic argondsgn at gmail dot com 2011-02-09 21:14 ` [Bug c/47670] " rguenth at gcc dot gnu.org 2011-02-09 21:20 ` rguenth at gcc dot gnu.org @ 2011-02-09 21:54 ` jakub at gcc dot gnu.org 2011-02-09 22:00 ` argondsgn at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: jakub at gcc dot gnu.org @ 2011-02-09 21:54 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47670 Jakub Jelinek <jakub at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |jakub at gcc dot gnu.org --- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-02-09 21:20:51 UTC --- Or count+=i*0x40000000LL; ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/47670] Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic 2011-02-09 20:56 [Bug c/47670] New: Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic argondsgn at gmail dot com ` (2 preceding siblings ...) 2011-02-09 21:54 ` jakub at gcc dot gnu.org @ 2011-02-09 22:00 ` argondsgn at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: argondsgn at gmail dot com @ 2011-02-09 22:00 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47670 Peter de Rivaz <argondsgn at gmail dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|RESOLVED |CLOSED --- Comment #4 from Peter de Rivaz <argondsgn at gmail dot com> 2011-02-09 21:54:15 UTC --- Thanks! (And sorry for the invalid report). C99 specification 6.5 para 5 agrees that this is undefined behaviour. Both suggestions solve the problem. I've closed the bug now. ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2011-02-09 21:54 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2011-02-09 20:56 [Bug c/47670] New: Gcc 4.5.1 inconsistent optimisation of loop with mixed 32bit and 64bit arithmetic argondsgn at gmail dot com 2011-02-09 21:14 ` [Bug c/47670] " rguenth at gcc dot gnu.org 2011-02-09 21:20 ` rguenth at gcc dot gnu.org 2011-02-09 21:54 ` jakub at gcc dot gnu.org 2011-02-09 22:00 ` argondsgn at gmail dot com
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