------- Additional Comments From tkoenig at gcc dot gnu dot org 2005-07-17
19:45 -------
I don't think the timing issue is valid.
Look at these benchmarks:
The first one simulates copying 12-byte values to 10-bit values,
the second one a compact memcpy of a larger field.
$ cat foo.c
#include <string.h>
#define NELEM 10000000
#define SA 12
#define SB 10
void foo(char *a, char *b, int n);
int main()
{
char *a, *b;
int n;
a = malloc(NELEM*SA);
b = malloc(NELEM*SB);
for (n=0; n<10; n++)
foo(a,b,NELEM);
return 0;
}
void foo(char *a, char *b, int n)
{
int i;
for (i=0; i<n; i++)
{
memcpy(a, b, 10);
a += SA;
b += SB;
}
}
$ gcc -O3 foo.c
$ time ./a.out
real 0m2.628s
user 0m2.523s
sys 0m0.096s
$ cat foo2.c
#include <string.h>
#define NELEM 10000000
#define SA 12
void foo(char *a, char *b, int n);
int main()
{
char *a, *b;
int n;
a = malloc(NELEM*SA);
b = malloc(NELEM*SA);
for (n=0; n<10; n++)
memcpy(a, b, NELEM*SA);
return 0;
}
$ gcc foo2.c
foo2.c: In function 'main':
foo2.c:13: warning: incompatible implicit declaration of built-in function
'malloc'
$ time ./a.out
real 0m2.876s
user 0m2.777s
sys 0m0.093s
We also have slow disk I/O to deal with.
This is on i686-pc-linux-gnu. Timings on other systems may differ,
of course.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22519
