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Date: Mon, 14 Jun 2004 11:51:48 +0300
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Subject: Re: [PHP-DEV] crc32() improvements
From: valyala@tut.by ("Alexander Valyalkin")

On Sun, 13 Jun 2004 23:10:04 +0000, Curt Zirzow <curt@php.net> wrote:

> * Thus wrote Alexander Valyalkin (valyala@tut.by):
>> Here is improved version of crc32() function.
>> Features:
>> 1) Automatic initialization of crc32tab[] at first call.
>>    So, the file crc32.h with definition of this tab is not
>>    nessesary any more now.
>
> I'm not sure what advantage this is really giving you being that
> gcc will optimize where the data segment should be, vs putting the
> data directly into php's memory.
>

Yes, it's my fault.

>
>> 2) Speed is improved on large amount of data.
>
> By using a registered int with a do while() statement vs a
> foreach() statement, I'd like to see some benchmarks against this.
>

Well. Try something like this:
===================cut===================
#include <stdio.h>
#include <time.h> /* for use _rdtsc() */

#define N 10 * 1000 * 1000

int main(int argc,char *argv[])
{
	unsigned long long int t;
	unsigned int i;
	unsigned int crc = ~0;
	t = _rdtsc();
	for (i = 0; i < N; i++) {
		crc ^= i;
	}
	printf("speed of for (i++) loop: %llu\n", _rdtsc() - t);
	i = N;
	t = _rdtsc();
	for ( ; i--; ) {
		crc ^= i;
	}
	printf("speed of for (i--) loop: %llu\n", _rdtsc() - t);
	i = N;
	t = _rdtsc();
	if (i > 0) do {
		crc ^=i;
	} while (--i);
	printf("speed of do while loop: %llu\n", _rdtsc() - t);
     return 0;
}
===================cut===================

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