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* [Bug tree-optimization/55812] New: Unnecessary TLS accesses @ 2012-12-26 12:53 glisse at gcc dot gnu.org 2012-12-26 15:44 ` [Bug tree-optimization/55812] " glisse at gcc dot gnu.org ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: glisse at gcc dot gnu.org @ 2012-12-26 12:53 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55812 Bug #: 55812 Summary: Unnecessary TLS accesses Classification: Unclassified Product: gcc Version: 4.8.0 Status: UNCONFIRMED Keywords: missed-optimization Severity: normal Priority: P3 Component: tree-optimization AssignedTo: unassigned@gcc.gnu.org ReportedBy: glisse@gcc.gnu.org Target: x86_64-linux-gnu Hello, TLS accesses are expensive, so as much as possible gcc should copy the address to a local variable and use that instead. The following example may not be very good, I am just trying to illustrate the issue. #include <vector> thread_local std::vector<int> v; int main(){ for(long i=0;i<400000000;++i){ v.push_back(i); } return v.size(); } compiled with g++ -std=c++11 -O2 -Wall -DNDEBUG. If I remove "thread_local", the speed-up is about 20%. It seems to me that the compiler should get the address of v once at the beginning of main and use that for the rest of the function, and thus the performance difference should be negligible. If I add "static" in front of "thread_local", the program fails to link, but my gcc snapshot is a bit old (Nov 20) and I think I've already seen that reported. I was surprised not to find any compiler option that would disable threads, so I could write thread_local but not pay the price when compiling a single-threaded program. Without -pthread, glibc uses cheap thread-unsafe functions, but I still pay for TLS. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/55812] Unnecessary TLS accesses 2012-12-26 12:53 [Bug tree-optimization/55812] New: Unnecessary TLS accesses glisse at gcc dot gnu.org @ 2012-12-26 15:44 ` glisse at gcc dot gnu.org 2012-12-26 21:55 ` [Bug target/55812] " pinskia at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: glisse at gcc dot gnu.org @ 2012-12-26 15:44 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55812 --- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> 2012-12-26 15:43:50 UTC --- More precisely, the following seems equivalent to me and gets back all the performance, so it would be good if gcc could turn the original code into this one. #include <vector> thread_local std::vector<int> v; int main(){ std::vector<int>*vp=&v; for(long i=0;i<400000000;++i){ vp->push_back(i); } return vp->size(); } ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/55812] Unnecessary TLS accesses 2012-12-26 12:53 [Bug tree-optimization/55812] New: Unnecessary TLS accesses glisse at gcc dot gnu.org 2012-12-26 15:44 ` [Bug tree-optimization/55812] " glisse at gcc dot gnu.org @ 2012-12-26 21:55 ` pinskia at gcc dot gnu.org 2012-12-26 21:57 ` [Bug tree-optimization/55812] " pinskia at gcc dot gnu.org 2012-12-27 9:38 ` [Bug tree-optimization/55812] thread_local with either a ctor or dtor causes a function call every time through a loop glisse at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2012-12-26 21:55 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55812 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Component|tree-optimization |target --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-12-26 21:54:57 UTC --- >TLS accesses are expensive They should not be except for the -fPIC case. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/55812] Unnecessary TLS accesses 2012-12-26 12:53 [Bug tree-optimization/55812] New: Unnecessary TLS accesses glisse at gcc dot gnu.org 2012-12-26 15:44 ` [Bug tree-optimization/55812] " glisse at gcc dot gnu.org 2012-12-26 21:55 ` [Bug target/55812] " pinskia at gcc dot gnu.org @ 2012-12-26 21:57 ` pinskia at gcc dot gnu.org 2012-12-27 9:38 ` [Bug tree-optimization/55812] thread_local with either a ctor or dtor causes a function call every time through a loop glisse at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2012-12-26 21:57 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55812 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Last reconfirmed| |2012-12-26 Component|target |tree-optimization Ever Confirmed|0 |1 Severity|normal |enhancement --- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-12-26 21:57:12 UTC --- The issue is rather the call to _ZTH1v ( __tls_init) than anything else. So it is not TLS accesses which are expensive but rather making sure the thread_local variable has been initialized. I think Jason had proposed an attribute for these function calls but it was rejected IIRC. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/55812] thread_local with either a ctor or dtor causes a function call every time through a loop 2012-12-26 12:53 [Bug tree-optimization/55812] New: Unnecessary TLS accesses glisse at gcc dot gnu.org ` (2 preceding siblings ...) 2012-12-26 21:57 ` [Bug tree-optimization/55812] " pinskia at gcc dot gnu.org @ 2012-12-27 9:38 ` glisse at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: glisse at gcc dot gnu.org @ 2012-12-27 9:38 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55812 --- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> 2012-12-27 09:38:15 UTC --- (In reply to comment #3) > I think Jason had proposed an attribute for these function calls but it was > rejected IIRC. http://gcc.gnu.org/ml/gcc/2012-10/msg00024.html ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2012-12-27 9:38 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2012-12-26 12:53 [Bug tree-optimization/55812] New: Unnecessary TLS accesses glisse at gcc dot gnu.org 2012-12-26 15:44 ` [Bug tree-optimization/55812] " glisse at gcc dot gnu.org 2012-12-26 21:55 ` [Bug target/55812] " pinskia at gcc dot gnu.org 2012-12-26 21:57 ` [Bug tree-optimization/55812] " pinskia at gcc dot gnu.org 2012-12-27 9:38 ` [Bug tree-optimization/55812] thread_local with either a ctor or dtor causes a function call every time through a loop glisse at gcc dot gnu.org
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