When passing an object Bar as a value to a function foo(), I noticed that the compiler (gcc) would construct a copy of the object outside of foo’s stack frame and then pass the newly constructed object’s reference to foo(). This behavior seems counter intuitive, what’s the reason for it?

Bellow are the C++ code I used and its correspondent assembly snippet from main:

struct Bar
{
    Bar(){}
    Bar(const Bar& obj){}
};

Bar obj;

void foo(Bar obj){}

int main() {
    foo(obj);
}

sub     esp, 8
push    OFFSET FLAT:obj
lea     eax, [ebp-9]
push    eax
call    Bar::Bar(Bar const&) [complete object constructor]
add     esp, 16
sub     esp, 12
lea     eax, [ebp-9]
push    eax
call    foo(Bar)
add     esp, 16