> From: Richard A. Smith [mailto:[EMAIL PROTECTED]]
>
> On Sat, 16 Oct 1999 22:54:32 +0100, Mathew Hendry wrote:
>
> >With gcc, using array notation is usually faster
> (particularly, I found,
> >when the dataset exceeds the level 2 cache size - couldn't figure out
> >why...)
>
> Perhaps we should examine the compiler assembly output of the 2 test
> cases.
/* "array" version */
int
sum_array_ar(int *array, long len) {
int sum = 0;
long i;
for (i = len; i--;)
sum += array[i];
return sum;
}
/*
; gcc 2.95/Cygwin -O9 -fomit-frame-pointer
_sum_array_ar:
movl 4(%esp),%ecx
movl 8(%esp),%edx
xorl %eax,%eax
decl %edx
cmpl $-1,%edx
je L17
.p2align 4,,7
L19:
addl (%ecx,%edx,4),%eax
subl $1,%edx
jnc L19
L17:
ret
*/
/* "pointer" version */
int
sum_array_pr(int *array, long len) {
int sum = 0, *p;
long i;
for (i = len, p = array + i; i--;)
sum += *--p;
return sum;
}
/*
; gcc 2.95/Cygwin -O9 -fomit-frame-pointer
_sum_array_pr:
movl 8(%esp),%edx
xorl %eax,%eax
leal 0(,%edx,4),%ecx
addl 4(%esp),%ecx
subl $1,%edx
jc L23
.p2align 4,,7
L25:
addl $-4,%ecx
addl (%ecx),%eax
subl $1,%edx
jnc L25
L23:
ret
*/
-- Mat.
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