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: unassig...@gcc.gnu.org
ReportedBy: gli...@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.
