On 12/24/2010 08:32 AM, Parmenides wrote:
I have modified the code as follows:
#include<stdio.h>
#define barrier() __asm__ __volatile__("": : :"memory")
int main()
{
int i, n;
int s = 0;
int *p;
scanf("%d",&n);
for (i = 0; i< n; i++)
s += i;
barrier();
p =&s;<--- for s, the& operator is used
printf("%d %d %d\n", i, n, *p);<--- the s is referenced through a pointer
return 0;
}
and the corresponding asm code is as follows:
leal -8(%ebp), %eax
pushl %eax
pushl $.LC0
call scanf
movl -8(%ebp), %edx
addl $16, %esp
testl %edx, %edx
jg .L2
movl $0, %eax
movl $0, %ecx
jmp .L4
.L2:
movl $0, %eax
movl $0, %ecx
.L5:
addl %eax, %ecx
incl %eax
cmpl %edx, %eax
jne .L5
.L4:
<--- after the barrier(()
pushl %ecx<--- In spite of using the '&' operator, the
value of s is still gotten from the register rather than the
corresponding memory location.
pushl -8(%ebp)<--- the value of n is gotten from its
memory location indeed
pushl %eax
pushl $.LC1
call printf
Alough the '&' operator is used for both 's' and 'n', gcc deals with
them differently. The former's value is in a register, while the
latter's value is retrieved from the corresponding memory location. I
wonder the difference.
Please don't top-post.
I don't know why you'd expect a difference, really. Try this:
#include <stdio.h>
#define barrier() __asm__ __volatile__(";": : :"memory")
int s = 0;
int main()
{
int i, n;
int *p;
scanf("%d", &n);
for (i = 0; i < n; i++)
s += i;
p = &s; // <--- for s, the & operator is used
barrier();
printf("%d %d %d\n", i, n, *p); // <--- the s is referenced through a pointer
return 0;
}
Andrew.
