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From: chris@mimsy.UUCP
Newsgroups: comp.sys.m68k
Subject: Incidentally ... (was Re: Recent Motorola ad)
Message-ID: <6134@mimsy.UUCP>
Date: Sat, 4-Apr-87 04:53:38 EST
Article-I.D.: mimsy.6134
Posted: Sat Apr  4 04:53:38 1987
Date-Received: Sun, 5-Apr-87 10:08:53 EST
References: <362@sbcs.UUCP> <1466@ncr-sd.SanDiego.NCR.COM> <580@plx.UUCP> <73@eps2.UUCP>
Distribution: comp
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 22

>	while (wy--)  {
>		j = wx;
>		while (j--)  {

(The above is an inner loop.)  Such loops should usually (always?)
be written as

		while (--j >= 0)

---assuming j is signed.  Why?  They both do the same thing, but
a dumb compiler will turn the former into `move j to tmp; decrement
j; test tmp; branch if zero', while the same dumb compiler will
turn the latter into `decrement j; branch if negative'.  Details
will vary depending on condition codes, but the former is often
four instructions, and the latter two.  On a Vax, the second version
is sometimes a single instruction.

Of course, a smart compiler will generate the same code for both.
That is wonderful---if you have a smart compiler.  Better check!
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu