Thread Local

[levy5307]

https://levy5307.github.io/blog/thread-local/

G++ now implements the C++11 thread_local keyword; this differs from the GNU __thread keyword primarily in that it allows dynamic initialization and destruction semantics. Unfortunately, this support requires a run-time penalty for references to non-function-local thread_local variables even if they don’t need dynamic initialization, so users may want to continue to use __thread for TLS variables with static initialization semantics.

What is precisely the nature and origin of this run-time penalty?

Answer
The dynamic thread_local initialization is added in commit 462819c. One of the change is:

semantics.c (finish_id_expression): Replace use of thread_local
variable with a call to its wrapper.

So the run-time penalty is that, every reference of the thread_local variable will become a function call. Let’s check with a simple test case:

// 3.cpp
extern thread_local int tls;
int main() {
tls += 37; // line 6
tls &= 11; // line 7
tls ^= 3; // line 8
return 0;
}

// 4.cpp

thread_local int tls = 42;

When compiled*, we see that every use of the tls reference becomes a function call to _ZTW3tls, which lazily initialize the the variable once:

00000000004005b0

:
main():
4005b0: 55 push rbp
4005b1: 48 89 e5 mov rbp,rsp
4005b4: e8 26 00 00 00 call 4005df <_ZTW3tls> // line 6
4005b9: 8b 10 mov edx,DWORD PTR [rax]
4005bb: 83 c2 25 add edx,0x25
4005be: 89 10 mov DWORD PTR [rax],edx
4005c0: e8 1a 00 00 00 call 4005df <_ZTW3tls> // line 7
4005c5: 8b 10 mov edx,DWORD PTR [rax]
4005c7: 83 e2 0b and edx,0xb
4005ca: 89 10 mov DWORD PTR [rax],edx
4005cc: e8 0e 00 00 00 call 4005df <_ZTW3tls> // line 8
4005d1: 8b 10 mov edx,DWORD PTR [rax]
4005d3: 83 f2 03 xor edx,0x3
4005d6: 89 10 mov DWORD PTR [rax],edx
4005d8: b8 00 00 00 00 mov eax,0x0 // line 9
4005dd: 5d pop rbp
4005de: c3 ret

00000000004005df <_ZTW3tls>:
_ZTW3tls():
4005df: 55 push rbp
4005e0: 48 89 e5 mov rbp,rsp
4005e3: b8 00 00 00 00 mov eax,0x0
4005e8: 48 85 c0 test rax,rax
4005eb: 74 05 je 4005f2 <_ZTW3tls+0x13>
4005ed: e8 0e fa bf ff call 0 // initialize the TLS
4005f2: 64 48 8b 14 25 00 00 00 00 mov rdx,QWORD PTR fs:0x0
4005fb: 48 c7 c0 fc ff ff ff mov rax,0xfffffffffffffffc
400602: 48 01 d0 add rax,rdx
400605: 5d pop rbp
400606: c3 ret
Compare it with the __thread version, which won't have this extra wrapper:

00000000004005b0

:
main():
4005b0: 55 push rbp
4005b1: 48 89 e5 mov rbp,rsp
4005b4: 48 c7 c0 fc ff ff ff mov rax,0xfffffffffffffffc // line 6
4005bb: 64 8b 00 mov eax,DWORD PTR fs:[rax]
4005be: 8d 50 25 lea edx,[rax+0x25]
4005c1: 48 c7 c0 fc ff ff ff mov rax,0xfffffffffffffffc
4005c8: 64 89 10 mov DWORD PTR fs:[rax],edx
4005cb: 48 c7 c0 fc ff ff ff mov rax,0xfffffffffffffffc // line 7
4005d2: 64 8b 00 mov eax,DWORD PTR fs:[rax]
4005d5: 89 c2 mov edx,eax
4005d7: 83 e2 0b and edx,0xb
4005da: 48 c7 c0 fc ff ff ff mov rax,0xfffffffffffffffc
4005e1: 64 89 10 mov DWORD PTR fs:[rax],edx
4005e4: 48 c7 c0 fc ff ff ff mov rax,0xfffffffffffffffc // line 8
4005eb: 64 8b 00 mov eax,DWORD PTR fs:[rax]
4005ee: 89 c2 mov edx,eax
4005f0: 83 f2 03 xor edx,0x3
4005f3: 48 c7 c0 fc ff ff ff mov rax,0xfffffffffffffffc
4005fa: 64 89 10 mov DWORD PTR fs:[rax],edx
4005fd: b8 00 00 00 00 mov eax,0x0 // line 9
400602: 5d pop rbp
400603: c3 ret

This wrapper is not needed for in every use case of thread_local though. This can be revealed from decl2.c. The wrapper is generated only when:

It is not function-local, and


It is extern (the example shown above), or


The type has a non-trivial destructor (which is not allowed for __thread variables), or


The type variable is initialized by a non-constant-expression (which is also not allowed for __thread variables).

Conclusion
In all other use cases, it behaves the same as __thread. That means, unless you have some extern __thread variables, you could replace all __thread by thread_local without any loss of performance.

[XuecongHu]

终于找到了ThreadLocal run-time penalty的介绍，不错不错