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From: chris@mimsy.UUCP
Newsgroups: comp.lang.c
Subject: Re: Copying a constant number of bytes
Message-ID: <7044@mimsy.UUCP>
Date: Sun, 14-Jun-87 22:29:04 EDT
Article-I.D.: mimsy.7044
Posted: Sun Jun 14 22:29:04 1987
Date-Received: Tue, 16-Jun-87 01:06:47 EDT
References: <900@bloom-beacon.MIT.EDU>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 36

In article <900@bloom-beacon.MIT.EDU> newman@athena.mit.edu (Ron Newman)
writes:
>I want to copy a constant number of bytes NBYTES from address "a"
>to address "b", where "a" and "b" are pointers of unspecified type.
>Which is the more efficient method for most compilers and machines
>currently in use?
>1)	bcopy ((char *)a, (char *)b, NBYTES);
>2)	struct nbytes {char s[NBYTES];};
>	*(struct nbytes *)b = *(struct nbytes *)a;
>Method (1) has the disadvantage that it always involves a subroutine
>call and return.

Not necessarily: in the dpANS, some functions may be `built in' to
the compiler; in 4.3BSD, you can use the `inline' program, found in
/sys/vax/inline/inline, to expand certain calls (including bcopy)
in line.

>But can I count on a compiler generating efficient code for method (2)?

Obviously this is compiler dependent.  Some existing compilers do
not even support structure assignment.  I hope that any compiler
that does support it does the copy at least as fast as via a bcopy
subroutine call.

>Is the answer any different if I know that "a" and "b" are 32-bit
>aligned, or that NBYTES is a multiple of 4?

Not if you want real portability.

If this particular copy *must* be fast, I would suggest defining
a macro to do it, and using whichever method is fastest on your
machine.  The macro can be put in a file that is clearly marked
`system dependencies'.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	seismo!mimsy!chris