C++11要求对函数本地的static
变量的初始化是线程安全的。因此,在遵循规范的编译器中,通常会使用某些同步原语来确保每次进入函数时都需要检查。
例如,这个程序的代码汇编清单如下:
#include <string>
void foo() {
static std::string str("Hello, Stack Overflow!");
}
int main() {}
.LC0:
.string "Hello, Stack Overflow!"
foo():
cmpb $0, guard variable for foo()::str(%rip)
je .L14
ret
.L14:
pushq %rbx
movl guard variable for foo()::str, %edi
subq $16, %rsp
call __cxa_guard_acquire
testl %eax, %eax
jne .L15
.L1:
addq $16, %rsp
popq %rbx
ret
.L15:
leaq 15(%rsp), %rdx
movl $.LC0, %esi
movl foo()::str, %edi
call std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)
movl guard variable for foo()::str, %edi
call __cxa_guard_release
movl $__dso_handle, %edx
movl foo()::str, %esi
movl std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string(), %edi
call __cxa_atexit
jmp .L1
movq %rax, %rbx
movl guard variable for foo()::str, %edi
call __cxa_guard_abort
movq %rbx, %rdi
call _Unwind_Resume
main:
xorl %eax, %eax
ret
__cxa_guard_acquire
、
__cxa_guard_release
等函数用于保护静态变量的初始化过程。