Xref: news2.mv.net comp.os.msdos.djgpp:4983
From: davis AT space DOT mit DOT edu (John E. Davis)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Speed optimization: memcpy() or for loop ??
Date: 14 Jun 1996 17:17:39 GMT
Organization: Center for Space Research
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On Thu, 13 Jun 1996 23:13:22 +0100, Shawn Hargreaves <slh100 AT york DOT ac DOT uk>
wrote:
 : I think you people are seriously underestimating the power of gcc's 
 : optimiser. It is _good_ :-) Tricks like this may occasionally gain a slight 
 : speed increase, but in most cases gcc will do them for you if you just 
 : write your loop in the simple and obvious way. Making something this

Many of us write code for more than just gcc.  Some of the code occurs in
inner loops and must be as fast as possible as well as portable.  For
example, some compilers do not have a memchr function or do but it is slow.
I have found the following to be very fast without sacrificing too much
portability:

char *jed_memchr(register char *p, register char c, register int n)
{
   int n2;
   register char *pmax;
   
   pmax = p + (n - 32);
	
   while (p <= pmax)
     {
	if (*p == c) return p;
	if (*(p + 1) == c) return p + 1;
	if (*(p + 2) == c) return p + 2;
	if (*(p + 3) == c) return p + 3;
	if (*(p + 4) == c) return p + 4;
	if (*(p + 5) == c) return p + 5;
	if (*(p + 6) == c) return p + 6;
	if (*(p + 7) == c) return p + 7;
	if (*(p + 8) == c) return p + 8;
	if (*(p + 9) == c) return p + 9;
	if (*(p + 10) == c) return p + 10;
	if (*(p + 11) == c) return p + 11;
	if (*(p + 12) == c) return p + 12;
	if (*(p + 13) == c) return p + 13;
	if (*(p + 14) == c) return p + 14;
	if (*(p + 15) == c) return p + 15;
	if (*(p + 16) == c) return p + 16;
	if (*(p + 17) == c) return p + 17;
	if (*(p + 18) == c) return p + 18;
	if (*(p + 19) == c) return p + 19;
	if (*(p + 20) == c) return p + 20;
	if (*(p + 21) == c) return p + 21;
	if (*(p + 22) == c) return p + 22;
	if (*(p + 23) == c) return p + 23;
	if (*(p + 24) == c) return p + 24;
	if (*(p + 25) == c) return p + 25;
	if (*(p + 26) == c) return p + 26;
	if (*(p + 27) == c) return p + 27;
	if (*(p + 28) == c) return p + 28;
	if (*(p + 29) == c) return p + 29;
	if (*(p + 30) == c) return p + 30;
	if (*(p + 31) == c) return p + 31;
	p += 32;
     }
   
   n2 = n % 32;
   
   while (n2--)
     {
	if (*p == c) return p;
	p++;
     }
   return(NULL);
}


-- 
John E. Davis                   Center for Space Research/AXAF Science Center
617-258-8119                    MIT 37-662c, Cambridge, MA 02139
http://space.mit.edu/~davis