https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84323
Bug ID: 84323
Summary: call_once uses TLS even when once_flag is set
Product: gcc
Version: 7.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Disassembly of the following code:
#include <mutex>
std::once_flag once;
int* foo() {
static int* p{};
std::call_once(once,[](){
p = 0;
});
return p;
}
shows that a lot of work is going on the hot path. TLS is used (twice) and
there is a function call:
mov QWORD PTR [rsp+8], rax
mov rax, QWORD PTR std::__once_callable@gottpoff[rip]
mov QWORD PTR fs:[rax], rdx
mov rax, QWORD PTR std::__once_call@gottpoff[rip]
mov QWORD PTR fs:[rax], OFFSET FLAT:void
std::call_once<foo()::{lambda()#1}>(std::once_flag&,
foo()::{lambda()#1}&&)::{lambda()#2}::_FUN()
mov eax, OFFSET FLAT:__gthrw___pthread_key_create(unsigned int*, void
(*)(void*))
test rax, rax
je .L6
mov esi, OFFSET FLAT:__once_proxy
mov edi, OFFSET FLAT:once
call __gthrw_pthread_once(int*, void (*)())
This seems to be suboptimal, as double-checked-like locking could be used
without TLS + 'call' usage on a hot path. std::call_once could be implemented
just like thread safe static local variables resulting in a much better
disassembly on a hot path:
movzx eax, BYTE PTR once
test al, al
je .L9 ; not called
