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From: wcs@ho95e.UUCP
Newsgroups: comp.lang.c
Subject: Re: fabs(x) vs. (x) < 0 ? -(x) : (x)
Message-ID: <1315@ho95e.ATT.COM>
Date: Thu, 5-Feb-87 21:50:25 EST
Article-I.D.: ho95e.1315
Posted: Thu Feb  5 21:50:25 1987
Date-Received: Sat, 7-Feb-87 18:26:45 EST
References: <4943@mimsy.UUCP> <2550005@hpisod2.HP> <756@unc.unc.UUCP> <2181@batcomputer.tn.cornell.edu>
Reply-To: wcs@ho95e.UUCP (46133-#Bill.Stewart,2G202,x0705,)
Distribution: na
Organization: AT&T Bell Labs 46133, Holmdel, NJ
Lines: 24

In article <2181@batcomputer.tn.cornell.edu> braner@batcomputer.UUCP (braner) writes:
>Somebody already said this a LONG time ago:  Any macro definition of
>fabs() requires floating-point arithmetic (e.g. comparision with 0).
>That is A LOT slower than a dedicated fabs() function, written in
>assembly language of course, that simply clears the sign bit.

	#define	fabs(x) ( (x) & 0x7FFF )

is probably a *lot* faster than your assembly language function, since
it doesn't need a function call.  You'll have to #ifdef it to get the right bit
pattern for your machine, and worry about details for double and double-extended
versions, but for no-more-bits-than-a-long floating point  it's ok.

>[various complaints about most C compilers' FP code quality]
Yeah, a lot of it's not exciting.  The standard complaint is that everything is
really done in double precision, when you can often get by with single.  I
suspect the 80*8* world is heading back in the PDP-11 direction though, since
the 8087 gives you fairly wide arithmetic.

>THE COMPLETE FP LIBRARY MUST BE WRITTEN IN HAND-OPTIMIZED ASSEMBLER LANGUAGE!!!
Only if your compilers are inadequate for your machines, though some
hand-tuning helps.
-- 
# Bill Stewart, AT&T Bell Labs 2G-202, Holmdel NJ 1-201-949-0705 ihnp4!ho95c!wcs