c++ 函数 forward

c++ forward 一般来说减少数据拷贝，能提升程序性能；用在function上会有什么效果； 预期应该是 use_func 更高效

代码比较

#include<array>
#include<stdio.h>
#include<thread>
template <typename Fn>
void use_func(Fn&& fn) {
    std::thread t2(std::forward<Fn>(fn)); 
    t2.join();
}

template <typename Fn>
void use_func2(const Fn* fn) {
    std::thread t2((*fn)); 
    t2.join();
}

template <typename Fn>
void use_func3(Fn fn) {
    std::thread t2((fn)); 
    t2.join();
}

int main(int argc, char* argv[]) {
    std::array<int, 100> blob;
    int x = argc;
    
    auto lambda = [blob, x](){ 
        printf("This lambda is invoked from %d, %d", blob.size(), x); 
    };
    use_func(lambda);
    auto lambda2 = [blob, x](){ 
        printf("This lambda is invoked from %d, %d", blob.size(), x); 
    };
    use_func2(&lambda2);

    auto lambda3 = [blob, x](){ 
        printf("This lambda is invoked from %d, %d", blob.size(), x); 
    };
    use_func3(lambda3);
}

放到godbolt 查看汇编代码

• 汇编代码

使用gcc 13.2 -O0

.LC0:
        .string "This lambda is invoked from %d, %d"
main::{lambda()#1}::operator()() const:
        push    rbp
        mov     rbp, rsp
        sub     rsp, 32
        mov     QWORD PTR [rbp-24], rdi
        mov     rax, QWORD PTR [rbp-24]
        mov     eax, DWORD PTR [rax+400]
        mov     rdx, QWORD PTR [rbp-24]
        mov     QWORD PTR [rbp-8], rdx
        mov     ecx, 100
        mov     edx, eax
        mov     rsi, rcx
        mov     edi, OFFSET FLAT:.LC0
        mov     eax, 0
        call    printf
        nop
        leave
        ret
main::{lambda()#2}::operator()() const:
        push    rbp
        mov     rbp, rsp
        sub     rsp, 32
        mov     QWORD PTR [rbp-24], rdi
        mov     rax, QWORD PTR [rbp-24]
        mov     eax, DWORD PTR [rax+400]
        mov     rdx, QWORD PTR [rbp-24]
        mov     QWORD PTR [rbp-8], rdx
        mov     ecx, 100
        mov     edx, eax
        mov     rsi, rcx
        mov     edi, OFFSET FLAT:.LC0
        mov     eax, 0
        call    printf
        nop
        leave
        ret
main::{lambda()#3}::operator()() const:
        push    rbp
        mov     rbp, rsp
        sub     rsp, 32
        mov     QWORD PTR [rbp-24], rdi
        mov     rax, QWORD PTR [rbp-24]
        mov     eax, DWORD PTR [rax+400]
        mov     rdx, QWORD PTR [rbp-24]
        mov     QWORD PTR [rbp-8], rdx
        mov     ecx, 100
        mov     edx, eax
        mov     rsi, rcx
        mov     edi, OFFSET FLAT:.LC0
        mov     eax, 0
        call    printf
        nop
        leave
        ret
main:
        push    rbp
        mov     rbp, rsp
        sub     rsp, 1680
        mov     DWORD PTR [rbp-1668], edi
        mov     QWORD PTR [rbp-1680], rsi
        mov     eax, DWORD PTR [rbp-1668]
        mov     DWORD PTR [rbp-4], eax
        lea     rax, [rbp-832]
        lea     rdx, [rbp-416]
        mov     ecx, 50
        mov     rdi, rax
        mov     rsi, rdx
        rep movsq
        mov     eax, DWORD PTR [rbp-4]
        mov     DWORD PTR [rbp-432], eax
        lea     rax, [rbp-832]
        mov     rdi, rax
        call    void use_func<main::{lambda()#1}&>(main::{lambda()#1}&)
        lea     rax, [rbp-1248]
        lea     rdx, [rbp-416]
        mov     ecx, 50
        mov     rdi, rax
        mov     rsi, rdx
        rep movsq
        mov     eax, DWORD PTR [rbp-4]
        mov     DWORD PTR [rbp-848], eax
        lea     rax, [rbp-1248]
        mov     rdi, rax
        call    void use_func2<main::{lambda()#2}>(main::{lambda()#2} const*)
        lea     rax, [rbp-1664]
        lea     rdx, [rbp-416]
        mov     ecx, 50
        mov     rdi, rax
        mov     rsi, rdx
        rep movsq
        mov     eax, DWORD PTR [rbp-4]
        mov     DWORD PTR [rbp-1264], eax
        sub     rsp, 8
        sub     rsp, 408
        mov     rax, rsp
        mov     rdx, rax
        lea     rax, [rbp-1664]
        mov     ecx, 50
        mov     rdi, rdx
        mov     rsi, rax
        rep movsq
        mov     rax, rsi
        mov     rdx, rdi
        mov     ecx, DWORD PTR [rax]
        mov     DWORD PTR [rdx], ecx
        call    void use_func3<main::{lambda()#3}>(main::{lambda()#3})
        add     rsp, 416
        mov     eax, 0
        leave
        ret
void use_func<main::{lambda()#1}&>(main::{lambda()#1}&):
        push    rbp
        mov     rbp, rsp
        push    rbx
        sub     rsp, 40
        mov     QWORD PTR [rbp-40], rdi
        mov     rax, QWORD PTR [rbp-40]
        mov     rdi, rax
        call    main::{lambda()#1}& std::forward<main::{lambda()#1}&>(std::remove_reference<main::{lambda()#1}&>::type&)
        mov     rdx, rax
        lea     rax, [rbp-24]
        mov     rsi, rdx
        mov     rdi, rax
        call    std::thread::thread<main::{lambda()#1}&, , void>(main::{lambda()#1}&)
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::join()
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::~thread() [complete object destructor]
        jmp     .L21
        mov     rbx, rax
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::~thread() [complete object destructor]
        mov     rax, rbx
        mov     rdi, rax
        call    _Unwind_Resume
.L21:
        mov     rbx, QWORD PTR [rbp-8]
        leave
        ret
void use_func2<main::{lambda()#2}>(main::{lambda()#2} const*):
        push    rbp
        mov     rbp, rsp
        push    rbx
        sub     rsp, 40
        mov     QWORD PTR [rbp-40], rdi
        mov     rdx, QWORD PTR [rbp-40]
        lea     rax, [rbp-24]
        mov     rsi, rdx
        mov     rdi, rax
        call    std::thread::thread<main::{lambda()#2} const&, , void>(main::{lambda()#2} const&)
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::join()
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::~thread() [complete object destructor]
        jmp     .L25
        mov     rbx, rax
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::~thread() [complete object destructor]
        mov     rax, rbx
        mov     rdi, rax
        call    _Unwind_Resume
.L25:
        mov     rbx, QWORD PTR [rbp-8]
        leave
        ret
void use_func3<main::{lambda()#3}>(main::{lambda()#3}):
        push    rbp
        mov     rbp, rsp
        push    rbx
        sub     rsp, 24
        lea     rax, [rbp-24]
        lea     rsi, [rbp+16]
        mov     rdi, rax
        call    std::thread::thread<main::{lambda()#3}&, , void>(main::{lambda()#3}&)
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::join()
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::~thread() [complete object destructor]
        jmp     .L29
        mov     rbx, rax
        lea     rax, [rbp-24]
        mov     rdi, rax
        call    std::thread::~thread() [complete object destructor]
        mov     rax, rbx
        mov     rdi, rax
        call    _Unwind_Resume
.L29:
        mov     rbx, QWORD PTR [rbp-8]
        leave
        ret
vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >:
        .quad   0
        .quad   typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >::~_State_impl() [complete object destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >::~_State_impl() [deleting destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >::_M_run()
vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >:
        .quad   0
        .quad   typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >::~_State_impl() [complete object destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >::~_State_impl() [deleting destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >::_M_run()
vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >:
        .quad   0
        .quad   typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >::~_State_impl() [complete object destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >::~_State_impl() [deleting destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >::_M_run()
typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >:
        .string "*NSt6thread11_State_implINS_8_InvokerISt5tupleIJZ4mainEUlvE1_EEEEEE"
typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >:
        .quad   vtable for __cxxabiv1::__si_class_type_info+16
        .quad   typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >
        .quad   typeinfo for std::thread::_State
typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >:
        .string "*NSt6thread11_State_implINS_8_InvokerISt5tupleIJZ4mainEUlvE0_EEEEEE"
typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >:
        .quad   vtable for __cxxabiv1::__si_class_type_info+16
        .quad   typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >
        .quad   typeinfo for std::thread::_State
typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >:
        .string "*NSt6thread11_State_implINS_8_InvokerISt5tupleIJZ4mainEUlvE_EEEEEE"
typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >:
        .quad   vtable for __cxxabiv1::__si_class_type_info+16
        .quad   typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >
        .quad   typeinfo for std::thread::_State

• 汇编代码

使用gcc 13.2 -O3

.LC0:
        .string "This lambda is invoked from %d, %d"
std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >::_M_run():
        mov     edx, DWORD PTR [rdi+408]
        mov     esi, 100
        mov     edi, OFFSET FLAT:.LC0
        xor     eax, eax
        jmp     printf
std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >::_M_run():
        mov     edx, DWORD PTR [rdi+408]
        mov     esi, 100
        mov     edi, OFFSET FLAT:.LC0
        xor     eax, eax
        jmp     printf
std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >::_M_run():
        mov     edx, DWORD PTR [rdi+408]
        mov     esi, 100
        mov     edi, OFFSET FLAT:.LC0
        xor     eax, eax
        jmp     printf
main:
        push    rbx
        mov     ebx, edi
        mov     edi, 416
        sub     rsp, 32
        ;use use_func
        mov     QWORD PTR [rsp+16], 0
        call    operator new(unsigned long)
        mov     edx, OFFSET FLAT:_ZNSt6thread24_M_thread_deps_never_runEv
        lea     rsi, [rsp+24]
        lea     rdi, [rsp+16]
        mov     QWORD PTR [rax], OFFSET FLAT:vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >+16
        mov     DWORD PTR [rax+408], ebx
        mov     QWORD PTR [rsp+24], rax
        call    std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)())
        lea     rdi, [rsp+24]
        call    std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >::~unique_ptr() [complete object destructor]
        lea     rdi, [rsp+16]
        call    std::thread::join()
        cmp     QWORD PTR [rsp+16], 0
        jne     .L23
        ;use use_func2
        mov     QWORD PTR [rsp+8], 0
        mov     edi, 416
        call    operator new(unsigned long)
        mov     edx, OFFSET FLAT:_ZNSt6thread24_M_thread_deps_never_runEv
        lea     rsi, [rsp+24]
        lea     rdi, [rsp+8]
        mov     QWORD PTR [rax], OFFSET FLAT:vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >+16
        mov     DWORD PTR [rax+408], ebx
        mov     QWORD PTR [rsp+24], rax
        call    std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)())
        lea     rdi, [rsp+24]
        call    std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >::~unique_ptr() [complete object destructor]
        lea     rdi, [rsp+8]
        call    std::thread::join()
        cmp     QWORD PTR [rsp+8], 0
        jne     .L23
        ;use use_func3
        mov     edi, 416
        mov     QWORD PTR [rsp], 0
        call    operator new(unsigned long)
        mov     edx, OFFSET FLAT:_ZNSt6thread24_M_thread_deps_never_runEv
        lea     rsi, [rsp+24]
        mov     rdi, rsp
        mov     QWORD PTR [rax], OFFSET FLAT:vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >+16
        mov     DWORD PTR [rax+408], ebx
        mov     QWORD PTR [rsp+24], rax
        call    std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)())
        lea     rdi, [rsp+24]
        call    std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >::~unique_ptr() [complete object destructor]
        mov     rdi, rsp
        call    std::thread::join()
        cmp     QWORD PTR [rsp], 0
        jne     .L23
        add     rsp, 32
        xor     eax, eax
        pop     rbx
        ret
.L23:
        call    std::terminate()
        mov     rdi, rax
        jmp     .L22
        mov     rbx, rax
        jmp     .L19
        mov     rdi, rax
        jmp     .L30
        mov     rbx, rax
        jmp     .L29
        mov     rdi, rax
        jmp     .L27
        mov     rbx, rax
        jmp     .L25
main.cold:
typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >:
        .quad   vtable for __cxxabiv1::__si_class_type_info+16
        .quad   typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >
        .quad   typeinfo for std::thread::_State
typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >:
        .string "*NSt6thread11_State_implINS_8_InvokerISt5tupleIJZ4mainEUlvE_EEEEEE"
typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >:
        .quad   vtable for __cxxabiv1::__si_class_type_info+16
        .quad   typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >
        .quad   typeinfo for std::thread::_State
typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >:
        .string "*NSt6thread11_State_implINS_8_InvokerISt5tupleIJZ4mainEUlvE0_EEEEEE"
typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >:
        .quad   vtable for __cxxabiv1::__si_class_type_info+16
        .quad   typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >
        .quad   typeinfo for std::thread::_State
typeinfo name for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >:
        .string "*NSt6thread11_State_implINS_8_InvokerISt5tupleIJZ4mainEUlvE1_EEEEEE"
vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >:
        .quad   0
        .quad   typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >::~_State_impl() [complete object destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >::~_State_impl() [deleting destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#1}> > >::_M_run()
vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >:
        .quad   0
        .quad   typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >::~_State_impl() [complete object destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >::~_State_impl() [deleting destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#2}> > >::_M_run()
vtable for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >:
        .quad   0
        .quad   typeinfo for std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >::~_State_impl() [complete object destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >::~_State_impl() [deleting destructor]
        .quad   std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::{lambda()#3}> > >::_M_run()

结论

从上面可以看到，

• 比较 O0 所有的汇编代码， 在使用function 值传递（use_func3）反而更高效;
• 比较 O2 所有的汇编代码，都是一样的效率;

总结:

• 只要不涉及大块内存数据分配，通常情况使用值传递能更高效。
• 涉及大块内存数据分配使用std::ref(lambda)
• O2 级别优化后效率是等同的

参考

how-to-pass-lambda-functions-in-C++/

C++ best practice: Pass use-only (not stored) lambda argument to function by const-reference or by forwarding-reference (a.k.a. universal-reference)