Xref: news2.mv.net comp.os.msdos.djgpp:5129
From: devitto AT senna DOT ferndown DOT ate DOT slb DOT com (Dom De Vitto)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Speed optimization: memcpy() or for loop ??
Date: 18 Jun 1996 12:26:34 GMT
Organization: Schlumber Technologies (Board Systems)
Lines: 51
Message-ID: <4q679q$4op@snlsu1>
References: <4pmlrp$p7u AT crc-news DOT doc DOT ca> <31C2577D DOT 22FD99BD AT alcyone DOT com>
NNTP-Posting-Host: senna.ferndown.ate.slb.com
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

On Fri, 14 Jun 1996 23:26:05 -0700, Erik Max Francis (max AT alcyone DOT com) wrote:
: Richard Young 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.

If typeof(x) is a POD type (plain old data) then is it likely that the
system-supplied version is (hand) written in assembler, and so could be
much faster - but no slower (ignoring a single function call overhead).

: Use the standard library function.

Good advice, for POD types.

: The whole idea behind having such a function is that it is supposed to be a
: good implementation of the operation you're interested in.  And, furthermore,
: it knows which platform it's running under, so to speak, and so it can do the
: right thing.

Except that memcpy() is a C function, nothing to do with C++ - it doesn't
know exactly how to copy objects is - so can't "do the right thing".

If the array is non-POD, using memcpy is just plain wrong, and the loop will
be right - but slow, maybe much slower, because it's actually doing more.

If the array is a POD type, then you really should use memcpy as it's 
a standard C function.

Incedentally,

memcpy(x,y,sizeof(y)) will be faster if you know that 'n' is constant.

: > With C++ is it better code practice to use b) over a)?
: Same thing.

Different thing.

Dom
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