c++ 对于传递引用和传递值的理解

首先先上一段c++代码, 可以看出foo函数参数是引用类型，bar函数参数是值类型

typedef struct A{
    int x;
    int y;
} A;

void foo(A& a) {
r    a.x++;
}

void bar(A a) {
    a.x++;
}

int main() {
    A a = {1, 2};
    foo(a);
    bar(a);
    return 0;
}

在vscode中开启debug模式，然后在debug console页面查看三个函数的汇编代码

-exec disassemble foo
Dump of assembler code for function foo(A&):
   0x00005590410681a9 <+0>:	endbr64 
   0x00005590410681ad <+4>:	push   rbp
   0x00005590410681ae <+5>:	mov    rbp,rsp
   0x00005590410681b1 <+8>:	mov    QWORD PTR [rbp-0x8],rdi
   0x00005590410681b5 <+12>:	mov    rax,QWORD PTR [rbp-0x8]
=> 0x00005590410681b9 <+16>:	mov    eax,DWORD PTR [rax]
   0x00005590410681bb <+18>:	lea    edx,[rax+0x1]
   0x00005590410681be <+21>:	mov    rax,QWORD PTR [rbp-0x8]
   0x00005590410681c2 <+25>:	mov    DWORD PTR [rax],edx
   0x00005590410681c4 <+27>:	nop
   0x00005590410681c5 <+28>:	pop    rbp
   0x00005590410681c6 <+29>:	ret    
End of assembler dump.
-exec disassemble bar
Dump of assembler code for function bar(A):
   0x00005590410681c7 <+0>:	endbr64 
   0x00005590410681cb <+4>:	push   rbp
   0x00005590410681cc <+5>:	mov    rbp,rsp
   0x00005590410681cf <+8>:	mov    QWORD PTR [rbp-0x8],rdi
   0x00005590410681d3 <+12>:	mov    eax,DWORD PTR [rbp-0x8]
   0x00005590410681d6 <+15>:	add    eax,0x1
   0x00005590410681d9 <+18>:	mov    DWORD PTR [rbp-0x8],eax
   0x00005590410681dc <+21>:	nop
   0x00005590410681dd <+22>:	pop    rbp
   0x00005590410681de <+23>:	ret    
End of assembler dump.
-exec disassemble main
Dump of assembler code for function main():
   0x00005590410681df <+0>:	endbr64 
   0x00005590410681e3 <+4>:	push   rbp
   0x00005590410681e4 <+5>:	mov    rbp,rsp
   0x00005590410681e7 <+8>:	sub    rsp,0x10
   0x00005590410681eb <+12>:	mov    rax,QWORD PTR fs:0x28
   0x00005590410681f4 <+21>:	mov    QWORD PTR [rbp-0x8],rax
   0x00005590410681f8 <+25>:	xor    eax,eax
   0x00005590410681fa <+27>:	mov    DWORD PTR [rbp-0x10],0x1  //rbp-0x10 ~ rbp-0xc 存放的是0x1
   0x0000559041068201 <+34>:	mov    DWORD PTR [rbp-0xc],0x2   //rbp-0xc ~ rbp-0x8 这4个字节存放的是0x2
   0x0000559041068208 <+41>:	lea    rax,[rbp-0x10]    //rbp-10是一个内存地址， 把这个内存地址存放在rax寄存器中
   0x000055904106820c <+45>:	mov    rdi,rax           //rax寄存器内容移动到rdi中
   0x000055904106820f <+48>:	call   0x5590410681a9 <foo(A&)>  //call 指令会把下一条指令地址0x0000559041068214 push到栈顶
   0x0000559041068214 <+53>:	mov    eax,DWORD PTR [rbp-0x10]
   0x0000559041068217 <+56>:	mov    esi,eax
   0x0000559041068219 <+58>:	lea    rdi,[rip+0x2e20]        # 0x55904106b040 <_ZSt4cout@@GLIBCXX_3.4>
   0x0000559041068220 <+65>:	call   0x5590410680b0 <_ZNSolsEi@plt>
   0x0000559041068225 <+70>:	mov    rax,QWORD PTR [rbp-0x10] //这个是mov指令， 会把[rpb-0x10] 引用的内存里面的值传递给被调用函数
   0x0000559041068229 <+74>:	mov    rdi,rax  
   0x000055904106822c <+77>:	call   0x5590410681c7 <bar(A)>
   0x0000559041068231 <+82>:	mov    eax,DWORD PTR [rbp-0x10]
   0x0000559041068234 <+85>:	mov    esi,eax
   0x0000559041068236 <+87>:	lea    rdi,[rip+0x2e03]        # 0x55904106b040 <_ZSt4cout@@GLIBCXX_3.4>
   0x000055904106823d <+94>:	call   0x5590410680b0 <_ZNSolsEi@plt>
   0x0000559041068242 <+99>:	mov    eax,0x0
   0x0000559041068247 <+104>:	mov    rdx,QWORD PTR [rbp-0x8]
   0x000055904106824b <+108>:	xor    rdx,QWORD PTR fs:0x28
   0x0000559041068254 <+117>:	je     0x55904106825b <main()+124>
   0x0000559041068256 <+119>:	call   0x559041068090 <__stack_chk_fail@plt>
   0x000055904106825b <+124>:	leave  
   0x000055904106825c <+125>:	ret    
End of assembler dump.