Author: Dann Corbit
Date: 21:05:35 01/29/03
Go up one level in this thread
On January 29, 2003 at 23:29:36, Russell Reagan wrote:
>http://members.home.nl/matador/chess840.htm#INTRO
>
>From Ed's page...
>
>switch (piece_type) { case 0 : goto empty;
> case 1 : goto white_pawn; // evaluate white pawn
> case 2 : goto white_knight; // evaluate white knight
> case 3 : goto white_bishop;
> case 4 : goto white_rook;
> case 5 : goto white_queen;
> case 6 : goto white_king;
> case 7 : goto black_pawn; // evaluate black pawn
> case 8 : goto black_knight;
> case 9 : goto black_bishop;
> case 10 : goto black_rook;
> case 11 : goto black_queen;
> case 12 : goto black_king; }
>
>
>On one portion of Ed's discussion of Rebel (see above), he talks about using
>"indirect addressing". I get the impression from Ed's words that this method is
>supposed to fast. I understand his discussion to mean that if you create a
>switch statement like he does, you create a jump table and avoid a bunch of
>conditionals.
>
>However, in past discussions, I recall hearing that using a function pointer is
>going to be at least as slow as conditional, so I asked someone, and was told
>that Ed's example should be no different than using a function pointer or
>virtual functions.
Direct jumps (like the above) or an array of function pointers will be faster
than virtual functions.
>Ed talks about this method as if it is a good thing to use. So what is the
>advantage of it? Either someone is mistaken, or Ed and the guy I talked to are
>talking about different things.
If you use the Intel compiler, a switch statement gets translated into a jump
table (if the switch is reasonable). So the trick loses a lot of its value.
Try the following with your compiler inside of your profiler:
#include <math.h>
#include <stdio.h>
typedef double (*f_t) (double);
static f_t f[] = {log, log10, sqrt, cos, cosh, exp, sin, sinh, tan, tanh,
0};
static double accum0 = 0;
static double accum1 = 0;
static double accum2 = 0;
void arr(void)
{
int i;
double d = 0;
for (i = 0; f[i]; i++) {
d += f[i] (0.5);
}
accum0 += d;
}
void poi(void)
{
f_t *flist = f;
double d = 0;
while (*flist) {
f_t ff = *flist;
d += ff(0.5);
flist++;
}
accum1 += d;
}
void swi(void)
{
int i;
double d = 0;
for (i = 0; f[i]; i++) {
switch (i) {
case 0:
d += f[0] (0.5);
break;
case 1:
d += f[1] (0.5);
break;
case 2:
d += f[2] (0.5);
break;
case 3:
d += f[3] (0.5);
break;
case 4:
d += f[4] (0.5);
break;
case 5:
d += f[5] (0.5);
break;
case 6:
d += f[6] (0.5);
break;
case 7:
d += f[7] (0.5);
break;
case 8:
d += f[8] (0.5);
break;
case 9:
d += f[9] (0.5);
break;
default:
break;
}
}
accum2 += d;
}
int main(void)
{
long i;
for (i = 0; i < 1000000; i++) {
arr();
poi();
swi();
}
printf("%.20g, %.20g, %.20g\n", accum0, accum1, accum2);
return 0;
}
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