面试官不讲武德,偷袭我一个大学生：当"我知道答案"遇上"请解释原理"

https://compiler-explorer.com/

.LC0:        .string "Hello"func[abi:cxx11]():        push    rbp        mov     rbp, rsp        push    r12        push    rbx        sub     rsp, 32        mov     QWORD PTR [rbp-40], rdi        movzx   eax, BYTE PTR guard variable for func[abi:cxx11]()::key[rip]        test    al, al        sete    al        test    al, al        je      .L6        mov     edi, OFFSET FLAT:guard variable for func[abi:cxx11]()::key        call    __cxa_guard_acquire        test    eax, eax        setne   al        test    al, al        je      .L6        mov     r12d, 0        lea     rax, [rbp-25]        mov     QWORD PTR [rbp-24], rax        nop        nop        lea     rax, [rbp-25]        mov     rdx, rax        mov     esi, OFFSET FLAT:.LC0        mov     edi, OFFSET FLAT:func[abi:cxx11]()::key        call    std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string<std::allocator<char> >(char const*, std::allocator<char> const&)        mov     edx, OFFSET FLAT:__dso_handle        mov     esi, OFFSET FLAT:func[abi:cxx11]()::key        mov     edi, OFFSET FLAT:std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() [complete object destructor]        call    __cxa_atexit        mov     edi, OFFSET FLAT:guard variable for func[abi:cxx11]()::key        call    __cxa_guard_release        lea     rax, [rbp-25]        mov     rdi, rax        call    std::__new_allocator<char>::~__new_allocator() [base object destructor]        nop.L6:        mov     rax, QWORD PTR [rbp-40]        mov     esi, OFFSET FLAT:func[abi:cxx11]()::key        mov     rdi, rax        call    std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) [complete object constructor]        jmp     .L11        mov     rbx, rax        lea     rax, [rbp-25]        mov     rdi, rax        call    std::__new_allocator<char>::~__new_allocator() [base object destructor]        nop        test    r12b, r12b        jne     .L9        mov     edi, OFFSET FLAT:guard variable for func[abi:cxx11]()::key        call    __cxa_guard_abort.L9:        mov     rax, rbx        mov     rdi, rax        call    _Unwind_Resume.L11:        mov     rax, QWORD PTR [rbp-40]        add     rsp, 32        pop     rbx        pop     r12        pop     rbp        ret

 movzx   eax, BYTE PTR guard variable for func[abi:cxx11]()::key[rip]        test    al, al        sete    al        test    al, al        je      .L6

mov     edi, OFFSET FLAT:guard variable for func[abi:cxx11]()::keycall    __cxa_guard_acquiretest    eax, eaxsetne   altest    al, alje      .L6

使用原子操作检查guard变量如果已初始化，返回0如果未初始化，获取互斥锁双重检查(double-check)guard变量返回1表示获得了初始化权限

初始化静态变量调用__cxa_guard_release设置guard变量释放互斥锁

mov     edi, OFFSET FLAT:guard variable for func[abi:cxx11]()::keycall    __cxa_guard_abort

https://github.com/llvm/llvm-project/blob/main/libcxxabi/src/cxa_guard.cpphttps://github.com/llvm/llvm-project/blob/main/libcxxabi/src/cxa_guard_impl.h

The first "guard byte" (which is checked by the compiler) is set only upon * the completion of cxa release. * * The second "init byte" does the rest of the bookkeeping. It tracks if * initialization is complete or pending, and if there are waiting threads. * * If the guard variable is 64-bits and the platforms supplies a 32-bit thread * identifier, it is used to detect recursive initialization. The thread ID of * the thread currently performing initialization is stored in the second word. * *  Guard Object Layout: * --------------------------------------------------------------------------- * | a+0: guard byte | a+1: init byte | a+2: unused ... | a+4: thread-id ... | * ---------------------------------------------------------------------------

struct GuardByte {    GuardByte(uint8_t* const guard_byte_address) : guard_byte(guard_byte_address) {}        bool acquire() {        // 如果guard_byte非0，说明初始化已完成        return guard_byte.load(std::_AO_Acquire) != UNSET;    }        void release() {         guard_byte.store(COMPLETE_BIT, std::_AO_Release);     }        void abort() {} // 终止时不需要做任何事
private:    AtomicInt<uint8_t> guard_byte;};

static constexpr uint8_t UNSET = 0;static constexpr uint8_t COMPLETE_BIT = (1 << 0);static constexpr uint8_t PENDING_BIT = (1 << 1);static constexpr uint8_t WAITING_BIT = (1 << 2);

template<class T, T (*Init)()>struct LazyValue {    LazyValue() : is_init(false) {}    T& get() {        if (!is_init) {            value = Init();            is_init = true;        }        return value;    }private:    T value;    bool is_init = false;};