Cost of thread-safe local static variable initialization in C++11? - c++

Question

We know that local static variable initialization is thread-safe in C++11, and modern compilers fully support this. (Is local static variable initialization thread-safe in C++11?)
What is the cost of making it thread-safe? I understand that this could very well be compiler implementation dependent.
Context: I have a multi-threaded application (10 threads) accessing a singleton object pool instance via the following function at very high rates, and I'm concerned about its performance implications.
template <class T>
ObjectPool<T>* ObjectPool<T>::GetInst()
{
static ObjectPool<T> instance;
return &instance;
}

Answer 1

A look at the generated assembler code helps.
Source
#include <vector>
std::vector<int> &get(){
static std::vector<int> v;
return v;
}
int main(){
return get().size();
}
Assembler
std::vector<int, std::allocator<int> >::~vector():
movq (%rdi), %rdi
testq %rdi, %rdi
je .L1
jmp operator delete(void*)
.L1:
rep ret
get():
movzbl guard variable for get()::v(%rip), %eax
testb %al, %al
je .L15
movl get()::v, %eax
ret
.L15:
subq $8, %rsp
movl guard variable for get()::v, %edi
call __cxa_guard_acquire
testl %eax, %eax
je .L6
movl guard variable for get()::v, %edi
movq $0, get()::v(%rip)
movq $0, get()::v+8(%rip)
movq $0, get()::v+16(%rip)
call __cxa_guard_release
movl $__dso_handle, %edx
movl get()::v, %esi
movl std::vector<int, std::allocator<int> >::~vector(), %edi
call __cxa_atexit
.L6:
movl get()::v, %eax
addq $8, %rsp
ret
main:
subq $8, %rsp
call get()
movq 8(%rax), %rdx
subq (%rax), %rdx
addq $8, %rsp
movq %rdx, %rax
sarq $2, %rax
ret
Compared to
Source
#include <vector>
static std::vector<int> v;
std::vector<int> &get(){
return v;
}
int main(){
return get().size();
}
Assembler
std::vector<int, std::allocator<int> >::~vector():
movq (%rdi), %rdi
testq %rdi, %rdi
je .L1
jmp operator delete(void*)
.L1:
rep ret
get():
movl v, %eax
ret
main:
movq v+8(%rip), %rax
subq v(%rip), %rax
sarq $2, %rax
ret
movl $__dso_handle, %edx
movl v, %esi
movl std::vector<int, std::allocator<int> >::~vector(), %edi
movq $0, v(%rip)
movq $0, v+8(%rip)
movq $0, v+16(%rip)
jmp __cxa_atexit
I'm not that great with assembler, but I can see that in the first version v has a lock around it and get is not inlined whereas in the second version get is essentially gone.
You can play around with various compilers and optimization flags, but it seems no compiler is able to inline or optimize out the locks, even though the program is obviously single threaded.
You can add static to get which makes gcc inline get while preserving the lock.
To know how much these locks and additional instructions cost for your compiler, flags, platform and surrounding code you would need to make a proper benchmark.
I would expect the locks to have some overhead and be significantly slower than the inlined code, which becomes insignificant when you actually do work with the vector, but you can never be sure without measuring.

Answer 2

From my experience, this is exactly as costly as a regular mutex (critical section). If the code is called very frequently, consider using a normal global variable instead.

Answer 3

Explained extensively here https://www.youtube.com/watch?v=B3WWsKFePiM by Jason Turner.
I put a sample code to illustrate the video. Since thread-safety is the main issue, I tried to call the method from multiple threads to see its effects.
You can think that compiler is implementing double-checking lock for you even though they can do whatever they want to ensure thread-safety. But they will at least add a branch to distinguish first time initialization unless optimizer does initialization at the global scope eagerly.
https://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_C++11
#include <iostream>
#include <string>
#include <vector>
#include <thread>
struct Temp
{
// Everytime this method is called, compiler has to check whether `name` is
// constructed or not due to init-at-first-use idiom. This at least would
// involve an atomic load operation and maybe a lock acquisition.
static const std::string& name() {
static const std::string name = "name";
return name;
}
// Following does not create contention. Profiler showed little bit of
// performance improvement.
const std::string& ref_name = name();
const std::string& get_name_ref() const {
return ref_name;
}
};
int main(int, char**)
{
Temp tmp;
constexpr int num_worker = 8;
std::vector<std::thread> threads;
for (int i = 0; i < num_worker; ++i) {
threads.emplace_back([&](){
for (int i = 0; i < 10000000; ++i) {
// name() is almost 5s slower
printf("%zu\n", tmp.get_name_ref().size());
}
});
}
for (int i = 0; i < num_worker; ++i) {
threads[i].join();
}
return 0;
}
The name() version is 5s slower than get_name_ref() on my machine.
$ time ./test > /dev/null
Also I used compiler explorer to see what gcc generates. Following proves double checking lock pattern: Pay attention to atomic loads and guards acquired.
name ()
{
bool retval.0;
bool retval.1;
bool D.25443;
struct allocator D.25437;
const struct string & D.29013;
static const struct string name;
_1 = __atomic_load_1 (&_ZGVZL4namevE4name, 2);
retval.0 = _1 == 0;
if (retval.0 != 0) goto <D.29003>; else goto <D.29004>;
<D.29003>:
_2 = __cxa_guard_acquire (&_ZGVZL4namevE4name);
retval.1 = _2 != 0;
if (retval.1 != 0) goto <D.29006>; else goto <D.29007>;
<D.29006>:
D.25443 = 0;
try
{
std::allocator<char>::allocator (&D.25437);
try
{
try
{
std::__cxx11::basic_string<char>::basic_string (&name, "name", &D.25437);
D.25443 = 1;
__cxa_guard_release (&_ZGVZL4namevE4name);
__cxa_atexit (__dt_comp , &name, &__dso_handle);
}
finally
{
std::allocator<char>::~allocator (&D.25437);
}
}
finally
{
D.25437 = {CLOBBER};
}
}
catch
{
if (D.25443 != 0) goto <D.29008>; else goto <D.29009>;
<D.29008>:
goto <D.29010>;
<D.29009>:
__cxa_guard_abort (&_ZGVZL4namevE4name);
<D.29010>:
}
goto <D.29011>;
<D.29007>:
<D.29011>:
goto <D.29012>;
<D.29004>:
<D.29012>:
D.29013 = &name;
return D.29013;
}