I am studying how c++ invoke the right member functions through assembly language. The simple program that I come with is as the following:
class A
{
public:
virtual void show() {}
};
class B : public A
{
public:
void show() {}
};
int main()
{
A* pA = new B;
pA->show();
return 0;
}
its assembly is as the following:
main:
.LFB2:
.cfi_startproc
.cfi_personality 0x3,__gxx_personality_v0
pushq %rbp
.cfi_def_cfa_offset 16
movq %rsp, %rbp
.cfi_offset 6, -16
.cfi_def_cfa_register 6
pushq %rbx
subq $24, %rsp
movl $8, %edi
.cfi_offset 3, -24
call _Znwm <<<================ 1
movq %rax, %rbx
movq %rbx, %rax
movq %rax, %rdi
call _ZN1BC1Ev
.L11:
movq %rbx, %rax
movq %rax, -24(%rbp)
movq -24(%rbp), %rax
movq (%rax), %rax
movq (%rax), %rdx
movq -24(%rbp), %rax
movq %rax, %rdi
call *%rdx <<<=== 2
movl $0, %eax
addq $24, %rsp
popq %rbx
leave
ret
.cfi_endproc
my questions are:
The *
is used before absolute addresses in AT&T assembly syntax for call or jump instructions. This means it will jump to the address contained in the register. The alternative is a relative jump, which is relative to the current instruction.
From the GNU as
manual:
AT&T absolute (as opposed to PC relative) jump/call operands are prefixed by `*'; they are undelimited in Intel syntax.
In your code it makes sense for there to be a call to an address in a register. Calling pA->show()
requires a look up to see what the right function to be called is. This is because it is a virtual function on class A.
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