Path: utzoo!attcan!uunet!mcvax!dik
From: dik@cwi.nl (Dik T. Winter)
Newsgroups: comp.lang.c
Subject: Re: iAPX86 code for ABS (using Microsof
Message-ID: <8208@boring.cwi.nl>
Date: 19 Jun 89 23:52:47 GMT
References: <1202@draken.nada.kth.se> <225800188@uxe.cso.uiuc.edu>
Organization: CWI, Amsterdam
Lines: 23

In article <225800188@uxe.cso.uiuc.edu> mcdonald@uxe.cso.uiuc.edu writes:
 > ; b = abs(a)
...
 > 	cwd	
 > 	xor	ax,dx
 > 	sub	ax,dx
...
 > So your review was correct!  I assume that they do this odd thing in
 > order to not empty instruction prefetch queues????????????????
 > 
No, it is faster; just count clock cycles and compare to:
	jns	lab
	neg	ax
lab:

On many machines jumps are not cheap, so it may be better to avoid them.
(There are machines where
	if(a >= 0 && b >= 0)
is cheaper if && is not short-circuited.)
-- 
dik t. winter, cwi, amsterdam, nederland
INTERNET   : dik@cwi.nl
BITNET/EARN: dik@mcvax