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* [Bug driver/36081] New: gcc optimizations and threads (pthread)
@ 2008-04-29 18:52 snes2002 at freenet dot de
2008-04-29 18:55 ` [Bug driver/36081] " snes2002 at freenet dot de
` (10 more replies)
0 siblings, 11 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-04-29 18:52 UTC (permalink / raw)
To: gcc-bugs
gcc version 4.2.1 (Debian 4.2.1-3)
System: debian-installation from knoppix 5.1.1
When optimizations are turned on (-O, -O1, -O2, -O3),
two or more threads don't recognize when the value of a global variable
changes.
It doesn't matter if it's a global variable or a pointer to a local variable.
The change of its value by thread no 1 will not be recognized by thread no 2.
--
Summary: gcc optimizations and threads (pthread)
Product: gcc
Version: 4.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: driver
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: snes2002 at freenet dot de
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
2008-04-29 18:55 ` [Bug driver/36081] " snes2002 at freenet dot de
@ 2008-04-29 18:55 ` pinskia at gcc dot gnu dot org
2008-04-29 18:56 ` snes2002 at freenet dot de
` (8 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2008-04-29 18:55 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from pinskia at gcc dot gnu dot org 2008-04-29 18:54 -------
This is correct behavior, you need either use volatile or use locks.
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
@ 2008-04-29 18:55 ` snes2002 at freenet dot de
2008-04-29 18:55 ` pinskia at gcc dot gnu dot org
` (9 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-04-29 18:55 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from snes2002 at freenet dot de 2008-04-29 18:54 -------
Created an attachment (id=15547)
--> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15547&action=view)
gcc output
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
2008-04-29 18:55 ` [Bug driver/36081] " snes2002 at freenet dot de
2008-04-29 18:55 ` pinskia at gcc dot gnu dot org
@ 2008-04-29 18:56 ` snes2002 at freenet dot de
2008-04-29 18:57 ` snes2002 at freenet dot de
` (7 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-04-29 18:56 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from snes2002 at freenet dot de 2008-04-29 18:55 -------
Created an attachment (id=15548)
--> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15548&action=view)
c source
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
` (2 preceding siblings ...)
2008-04-29 18:56 ` snes2002 at freenet dot de
@ 2008-04-29 18:57 ` snes2002 at freenet dot de
2008-04-29 18:58 ` snes2002 at freenet dot de
` (6 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-04-29 18:57 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from snes2002 at freenet dot de 2008-04-29 18:56 -------
Created an attachment (id=15549)
--> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15549&action=view)
preprocessed file
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
` (3 preceding siblings ...)
2008-04-29 18:57 ` snes2002 at freenet dot de
@ 2008-04-29 18:58 ` snes2002 at freenet dot de
2008-04-29 19:00 ` snes2002 at freenet dot de
` (5 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-04-29 18:58 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from snes2002 at freenet dot de 2008-04-29 18:57 -------
Created an attachment (id=15550)
--> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15550&action=view)
assembler listing
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
` (4 preceding siblings ...)
2008-04-29 18:58 ` snes2002 at freenet dot de
@ 2008-04-29 19:00 ` snes2002 at freenet dot de
2008-04-29 19:06 ` snes2002 at freenet dot de
` (4 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-04-29 19:00 UTC (permalink / raw)
To: gcc-bugs
------- Comment #6 from snes2002 at freenet dot de 2008-04-29 18:59 -------
This c-code works with optimizations turned off.
With borland compiler it also works with optimizations turned on.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
` (5 preceding siblings ...)
2008-04-29 19:00 ` snes2002 at freenet dot de
@ 2008-04-29 19:06 ` snes2002 at freenet dot de
2008-04-29 19:12 ` pinskia at gcc dot gnu dot org
` (3 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-04-29 19:06 UTC (permalink / raw)
To: gcc-bugs
------- Comment #7 from snes2002 at freenet dot de 2008-04-29 19:06 -------
Subject: Re: gcc optimizations and threads (pthread)
thanks for the quick reply.
But this code works without opimizations. Even with complex constructs.
For the solution of my problem i can't use lock's.
I don't want so serialize the threads, I want to use all 2 processors of my
dual-core.
And for syncing the threads, that's the easiest way.
Works fine whithout optimizations and also works with borland c++-compiler with
all optimizations.
----- original Nachricht --------
Betreff: [Bug driver/36081] gcc optimizations and threads (pthread)
Gesendet: Di 29 Apr 2008 20:55:20 CEST
Von: "pinskia at gcc dot gnu dot org"<gcc-bugzilla@gcc.gnu.org>
>
>
> ------- Comment #2 from pinskia at gcc dot gnu dot org 2008-04-29 18:54
> -------
> This is correct behavior, you need either use volatile or use locks.
>
>
> --
>
> pinskia at gcc dot gnu dot org changed:
>
> What |Removed |Added
> ----------------------------------------------------------------------------
>
> Status|UNCONFIRMED |RESOLVED
> Resolution| |INVALID
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
>
> ------- You are receiving this mail because: -------
> You reported the bug, or are watching the reporter.
>
--- original Nachricht Ende ----
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
` (6 preceding siblings ...)
2008-04-29 19:06 ` snes2002 at freenet dot de
@ 2008-04-29 19:12 ` pinskia at gcc dot gnu dot org
2008-04-30 12:26 ` snes2002 at freenet dot de
` (2 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2008-04-29 19:12 UTC (permalink / raw)
To: gcc-bugs
------- Comment #8 from pinskia at gcc dot gnu dot org 2008-04-29 19:11 -------
>And for syncing the threads, that's the easiest way.
Use mutexes then, they are designed exactly for this.
>I don't want so serialize the threads, I want to use all 2 processors of my
dual-core.
You are serializing them by doing a busy loop. In fact using a busy loop will
just make the CPUs spin without doing anything except for checking a condition.
Using a mutex allows for other stuff to happen while that thread is waiting
for more work to do.
I guess you need to read up on threading programming more because right now you
are causing extra work to happen on thread for no reason.
-- Pinski
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
` (7 preceding siblings ...)
2008-04-29 19:12 ` pinskia at gcc dot gnu dot org
@ 2008-04-30 12:26 ` snes2002 at freenet dot de
2008-05-03 10:27 ` snes2002 at freenet dot de
2008-05-03 11:04 ` rguenth at gcc dot gnu dot org
10 siblings, 0 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-04-30 12:26 UTC (permalink / raw)
To: gcc-bugs
------- Comment #9 from snes2002 at freenet dot de 2008-04-30 12:26 -------
Subject: Re: gcc optimizations and threads (pthread)
Hello,
I read about mutexes, cond_wait and cond_signal.
When I use these things instead of "busy loop" there is no performance gain at
all for my problem (=generating the first 50000 prime numbers).
I tested the execution-times of all variants:
Single-thread (compiled with -O2) : 17,5 s
Double-thread with busy-loop (compiled with -O0) : 11 s
Double-thread with mutexes (compiled with -O2) : 47,1 s !!!
But all this is not the problem or the reason why I'm reporting this.
The problem is, that the same code (whatever it does) compiled with "gcc -O0"
or "bcc32 -O0" (with wine) or "bcc32 -O2" works.
But if compiled with "gcc -O2" it doesn't work.
In this case the second thread doesn't read or dereference the actual pointer.
It seems to be cached somehow in a register and not read realtime.
If there are additional exit-conditions in loops (because of
speed-optimization)
they won't be recognized.
----- original Nachricht --------
Betreff: [Bug driver/36081] gcc optimizations and threads (pthread)
Gesendet: Di 29 Apr 2008 21:11:58 CEST
Von: "pinskia at gcc dot gnu dot org"<gcc-bugzilla@gcc.gnu.org>
>
>
> ------- Comment #8 from pinskia at gcc dot gnu dot org 2008-04-29 19:11
> -------
> >And for syncing the threads, that's the easiest way.
>
> Use mutexes then, they are designed exactly for this.
>
> >I don't want so serialize the threads, I want to use all 2 processors of
> my
> dual-core.
>
> You are serializing them by doing a busy loop. In fact using a busy loop
> will
> just make the CPUs spin without doing anything except for checking a
> condition.
> Using a mutex allows for other stuff to happen while that thread is
> waiting
> for more work to do.
>
> I guess you need to read up on threading programming more because right now
> you
> are causing extra work to happen on thread for no reason.
>
> -- Pinski
>
>
> --
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
>
> ------- You are receiving this mail because: -------
> You reported the bug, or are watching the reporter.
>
--- original Nachricht Ende ----
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
` (8 preceding siblings ...)
2008-04-30 12:26 ` snes2002 at freenet dot de
@ 2008-05-03 10:27 ` snes2002 at freenet dot de
2008-05-03 11:04 ` rguenth at gcc dot gnu dot org
10 siblings, 0 replies; 12+ messages in thread
From: snes2002 at freenet dot de @ 2008-05-03 10:27 UTC (permalink / raw)
To: gcc-bugs
------- Comment #10 from snes2002 at freenet dot de 2008-05-03 10:26 -------
This is definitely a bug.
The same source-code must do the same thing, no matter what optimize-options
are enabled or disabled.
--
snes2002 at freenet dot de changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|INVALID |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug driver/36081] gcc optimizations and threads (pthread)
2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
` (9 preceding siblings ...)
2008-05-03 10:27 ` snes2002 at freenet dot de
@ 2008-05-03 11:04 ` rguenth at gcc dot gnu dot org
10 siblings, 0 replies; 12+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2008-05-03 11:04 UTC (permalink / raw)
To: gcc-bugs
------- Comment #11 from rguenth at gcc dot gnu dot org 2008-05-03 11:03 -------
No it must not. If your program is bogus then it is bogus.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
^ permalink raw reply [flat|nested] 12+ messages in thread
end of thread, other threads:[~2008-05-03 11:04 UTC | newest]
Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed)
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2008-04-29 18:52 [Bug driver/36081] New: gcc optimizations and threads (pthread) snes2002 at freenet dot de
2008-04-29 18:55 ` [Bug driver/36081] " snes2002 at freenet dot de
2008-04-29 18:55 ` pinskia at gcc dot gnu dot org
2008-04-29 18:56 ` snes2002 at freenet dot de
2008-04-29 18:57 ` snes2002 at freenet dot de
2008-04-29 18:58 ` snes2002 at freenet dot de
2008-04-29 19:00 ` snes2002 at freenet dot de
2008-04-29 19:06 ` snes2002 at freenet dot de
2008-04-29 19:12 ` pinskia at gcc dot gnu dot org
2008-04-30 12:26 ` snes2002 at freenet dot de
2008-05-03 10:27 ` snes2002 at freenet dot de
2008-05-03 11:04 ` rguenth at gcc dot gnu dot org
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