| Re: Optimising A = B+C? [message #6341 is a reply to message #6257] |
Mon, 03 June 1996 00:00   |
davis
Messages: 15
Registered: March 1995
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Junior Member |
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On 2 Jun 1996 16:53:52 GMT, John E. Davis <davis@space.mit.edu>
wrote:
: On Fri, 31 May 1996 02:24:03 GMT, Karl Glazebrook <kgb@aaoepp.aao.gov.au>
: wrote:
: : Does anyone know how IDL optimises A=B+C where
: : A,B and C are arrays?
: Also, if IDL arrays are allocated with an even number of elements (with the
: last one padded for odd sized arrays), this is faster:
:
: for (i = 0; i < n; i++)
: {
: a[i] = b[i] + c[i];
: i++;
: a[i] = b[i] + c[i];
: }
This really does appear to what IDL does. For example, below is an IDL
procedure and equivalent C code:
pro test, nloops, val
a = make_array (1024, 1024, /FLOAT, value=0)
b = make_array (1024, 1024, /FLOAT, value=val)
for i=1,nloops do begin
a = a + b
print, a(10, 10)
endfor
end
The C code follows at the end of this message. Here is the result of simply
compiling the C code and running it:
Script started on Mon Jun 3 17:49:46 1996
# acc -O t.c
# time ./a.out 10 1.3
1.300000
2.600000
3.900000
5.200000
6.500000
7.800000
9.100000
10.400001
11.700001
13.000001
18.848u 1.079s 0:20.03 99.4% 0+4061k 0+0io 0pf+0w
It turns out that this is much slower than the IDL routine executes.
However, if is is compiled to use extra padding in arrays, it runs 3 times
faster:
# acc -DEXTRA=1 -O t.c
# time ./a.out 10 1.3
1.300000
2.600000
3.900000
5.200000
6.500000
7.800000
9.100000
10.400001
11.700001
13.000001
6.679u 1.109s 0:07.95 97.7% 0+3793k 0+0io 0pf+0w
# acc -DEXTRA=2 -O t.c
# time ./a.out 10 1.3
1.300000
2.600000
3.900000
5.200000
6.500000
7.800000
9.100000
10.400001
11.700001
13.000001
6.329u 1.119s 0:07.60 97.7% 0+3764k 0+0io 0pf+0w
# acc -DEXTRA=8 -O t.c
# time ./a.out 10 1.3
1.300000
2.600000
3.900000
5.200000
6.500000
7.800000
9.100000
10.400001
11.700001
13.000001
5.099u 0.969s 0:06.19 97.7% 0+3656k 0+0io 0pf+0w
# exit
exit
script done on Mon Jun 3 17:53:43 1996
Here is the file t.c:
#include <stdio.h>
#include <stdlib.h>
#define MAX_SIZE 1024
#ifndef EXTRA
# define EXTRA 0
#endif
int main (int argc, char **argv)
{
unsigned int dim = MAX_SIZE * MAX_SIZE;
unsigned int i;
float a[MAX_SIZE+EXTRA][MAX_SIZE+EXTRA], b[MAX_SIZE+EXTRA][MAX_SIZE+EXTRA];
float *ap, *bp;
int nloops, loop;
float val;
if ((argc != 3)
|| (1 != sscanf (argv[1], "%d", &nloops))
|| (1 != sscanf (argv[2], "%f", &val)))
{
fprintf (stderr, "Usage: %s NLOOPS VALUE\n",
argv[0]);
return 1;
}
ap = (float *) a;
bp = (float *) b;
for (i = 0; i < dim; i++)
{
ap[i] = 0.0;
bp[i] = val;
#if EXTRA > 0
i++;
ap[i] = 0.0;
bp[i] = val;
#endif
#if EXTRA > 1
i++;
ap[i] = 0.0;
bp[i] = val;
#endif
#if EXTRA > 2
i++;
ap[i] = 0.0;
bp[i] = val;
#endif
#if EXTRA > 3
i++;
ap[i] = 0.0;
bp[i] = val;
#endif
#if EXTRA > 4
i++;
ap[i] = 0.0;
bp[i] = val;
#endif
#if EXTRA > 5
i++;
ap[i] = 0.0;
bp[i] = val;
#endif
#if EXTRA > 6
i++;
ap[i] = 0.0;
bp[i] = val;
#endif
#if EXTRA > 7
i++;
ap[i] = 0.0;
bp[i] = val;
#endif
}
for (loop = 0; loop < nloops; loop++)
{
for (i = 0; i < dim; i++)
{
ap[i] += bp[i];
#if EXTRA > 0
i++;
ap[i] += bp[i];
#endif
#if EXTRA > 1
i++;
ap[i] += bp[i];
#endif
#if EXTRA > 2
i++;
ap[i] += bp[i];
#endif
#if EXTRA > 3
i++;
ap[i] += bp[i];
#endif
#if EXTRA > 4
i++;
ap[i] += bp[i];
#endif
#if EXTRA > 5
i++;
ap[i] += bp[i];
#endif
#if EXTRA > 6
i++;
ap[i] += bp[i];
#endif
#if EXTRA > 7
i++;
ap[i] += bp[i];
#endif
}
fprintf (stdout, "%f\n", a[9][9]);
}
return 0;
}
--
John E. Davis Center for Space Research/AXAF Science Center
617-258-8119 MIT 37-662c, Cambridge, MA 02139
http://space.mit.edu/~davis
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