Re: Loop unrolling
- From: user923005 <dcorbit@xxxxxxxxx>
- Date: Thu, 24 Apr 2008 21:51:44 -0700 (PDT)
On Apr 23, 2:34 am, Tim Frink <plfr...@xxxxxxxx> wrote:
Hi,
what are typical heuristics for loop unrolling, i.e. which constraints
must be satisfied to have a compiler perform unrolling on a particular
loop?
I assume that the loop must not exceed a particular size to avoid a
code size explosion. But are there any other heuristics?
Every compiler will be different. You can always unroll it yourself
and compare. E.g.:
void loopcopy(long *to, long *from, long count);
void duffcopy(long *to, long *from, long count);
void loopcopy(long *to, long *from, long count)
{
do
*to = *from++;
while (--count > 0);
return;
}
void duffcopy(long *to, long *from, long count)
{
register n = (count + 7) / 8;
switch (count % 8)
{
case 0:
do
{
*to = *from++;
case 7:
*to = *from++;
case 6:
*to = *from++;
case 5:
*to = *from++;
case 4:
*to = *from++;
case 3:
*to = *from++;
case 2:
*to = *from++;
case 1:
*to = *from++;
}
while (--n > 0);
}
return;
}
#include <stdlib.h>
#define SIZE 4096
long from[SIZE];
long to[SIZE];
long main()
{
long i;
for (i = 0; i < SIZE; i++)
from[i] = rand();
for (i = 0; i < 10000; i++)
loopcopy(to, from, SIZE);
for (i = 0; i < 10000; i++)
duffcopy(to, from, SIZE);
return 0;
}
/*
Profile: Function timing, sorted by time
Date: Wed Apr 16 23:38:59 1997
Program Statistics
------------------
Command line at 1997 Apr 16 23:38: "c:\tmp\Release\Duff"
Total time: 1187.011 millisecond
Time outside of functions: 2.139 millisecond
Call depth: 2
Total functions: 3
Total hits: 20001
Function coverage: 100.0%
Overhead Calculated 9
Overhead Average 9
Module Statistics for duff.exe
------------------------------
Time in module: 1184.872 millisecond
Percent of time in module: 100.0%
Functions in module: 3
Hits in module: 20001
Module function coverage: 100.0%
Func Func+Child Hit
Time % Time % Count Function
---------------------------------------------------------
719.321 60.7 719.321 60.7 10000 _loopcopy (duff.obj)
444.002 37.5 444.002 37.5 10000 _duffcopy (duff.obj)
21.550 1.8 1184.872 100.0 1 _main (duff.obj)
*/
.
- References:
- Loop unrolling
- From: Tim Frink
- Loop unrolling
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