Xref: news2.mv.net comp.os.msdos.djgpp:4941
From: richard DOT young AT crc DOT doc DOT ca (Richard Young)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Speed optimization: memcpy() or for loop ??
Date: 12 Jun 1996 19:56:41 GMT
Organization: Communications Research Centre
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Message-ID: <4pn7dp$2d2@crc-news.doc.ca>
References: <4pmlrp$p7u AT crc-news DOT doc DOT ca> <4pmscu$nrt AT rs18 DOT hrz DOT th-darmstadt DOT de>
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To: djgpp AT delorie DOT com
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In article <4pmscu$nrt AT rs18 DOT hrz DOT th-darmstadt DOT de>, 
lehmann AT mathematik DOT th-darmstadt DOT de says...
 >
 >Richard Young (richard DOT young AT crc DOT doc DOT ca) wrote:
 >: A question for the optimization experts:
 >
 >: For moving data, is it faster to use
 >
 >: a) memcpy(x,y,n*sizeof(x[0])) 
 >
 >: or 
 >
 >: b) for (i = 0; i < n; i++) x[i] = y[i];
 >
 >: or are they basically the same speed.
 >
 >: With C++ is it better code practice to use b) over a)?
 >
 >(a) uses the function dj_movedata, which will use the repeat
 >instruction to copy 4 byte values, which should be pretty fast.
 >
 >(b) requires a lots of address calculations, unless the compiler is
 >very smart (I don't think so), but it can be sped up a bit at least
 >(assuming that x and y are of type foo):
 >
 >foo *px,*py;
 >int i;
 >
 >for(i=n,px=x,py=y; i ; i--) *py++=*px++;
 >
 >You still don't not get a repeat instruction, but at least you save
 >the index calculations.
 >
 >
 >bye, Alexander
 >
 >--
 >Alexander Lehmann,                                  |  "On the Internet,
 >alex AT hal DOT rhein-main DOT de  (plain, MIME, NeXT)         |   nobody knows
 >lehmann AT mathematik DOT th-darmstadt DOT de (plain)          |   you're a dog."
 >!!CHANGED!! <URL:http://www.mathematik.th-darmstadt.de/~lehmann/>

With the powerful offset addressing modes of today's processors I find it hard 
to believe that 

	for(i=n,px=x,py=y; i ; i--) *py++=*px++;

is faster than 

	for (i = 0; i < n; i++) x[i] = y[i];

I would also expect that the code optimizers should be able to invoke repeat 
instructions for tight loops.  Just how good or bad are the optimizers?