| Summary: | gcc optimizations and threads (pthread) | ||
|---|---|---|---|
| Product: | gcc | Reporter: | snes2002 |
| Component: | driver | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED INVALID | ||
| Severity: | normal | CC: | gcc-bugs |
| Priority: | P3 | ||
| Version: | 4.2.1 | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | ||
| Attachments: |
gcc output
c source preprocessed file assembler listing |
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Description
snes2002
2008-04-29 18:51:39 UTC
Created attachment 15547 [details]
gcc output
This is correct behavior, you need either use volatile or use locks. Created attachment 15548 [details]
c source
Created attachment 15549 [details]
preprocessed file
Created attachment 15550 [details]
assembler listing
This c-code works with optimizations turned off. With borland compiler it also works with optimizations turned on. 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 ---- >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 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 ---- This is definitely a bug. The same source-code must do the same thing, no matter what optimize-options are enabled or disabled. No it must not. If your program is bogus then it is bogus. |