Here is a simple function
#include <stdio.h>
int foo() {
int a = 3;
int b = 4;
int c = 5;
return a * b * c;
}
int main() {
int a = foo();
}
And the assembly for foo() looks like
foo:
push rbp
mov rbp, rsp
mov DWORD PTR [rbp-4], 3
mov DWORD PTR [rbp-8], 4
mov DWORD PTR [rbp-12], 5
mov eax, DWORD PTR [rbp-4]
imul eax, DWORD PTR [rbp-8]
imul eax, DWORD PTR [rbp-12]
pop rbp
ret
as seen with the rbp - N
, the inner stack frame is being modified. So, why are there no leave
s or add rsp, n
?
CodePudding user response:
It "isn't modified" in the sense that the stack pointer does not change. Since we never subtracted any offset from rsp
on entry, of course we should not add any on exit.
And leave
is simply the equivalent of mov rsp, rbp ; pop rbp
. We did include pop rbp
, and mov rsp, rbp
would be redundant since as you can see, rsp
and rbp
are still equal at that point in the code.
In fact, this function stores its local variables on the stack without adjusting the stack pointer to "make space" for them, so they end up below the stack pointer. This is legal because the x86-64 SysV ABI mandates a red zone; code that does not make any function calls may safely use 128 bytes below the stack pointer. Signal handlers are guaranteed not to overwrite it.
Why does the x86-64 GCC function prologue allocate less stack than the local variables?