Xref:	news2.mv.net comp.os.msdos.djgpp:4892
From:	lehmann AT mathematik DOT th-darmstadt DOT de (Alexander Lehmann)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Speed optimization: memcpy() or for loop ??
Date:	12 Jun 1996 16:48:30 GMT
Organization:	Technische Hochschule Darmstadt
Lines:	38
Message-ID:	<4pmscu$nrt@rs18.hrz.th-darmstadt.de>
References:	<4pmlrp$p7u AT crc-news DOT doc DOT ca>
NNTP-Posting-Host:	fb0408.mathematik.th-darmstadt.de
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

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."
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