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From: meissner@tiktok.dg.com (Michael Meissner)
Newsgroups: comp.lang.c
Subject: Re: Type-independent ABS
Keywords: abs labs
Message-ID: <1752@xyzzy.UUCP>
Date: 10 Oct 89 14:42:10 GMT
References: <1392@cipc1.Dayton.NCR.COM> <20042@mimsy.UUCP>
Sender: usenet@xyzzy.UUCP
Reply-To: meissner@tiktok.UUCP (Michael Meissner)
Organization: Data General (Languages @ Research Triangle Park, NC.)
Lines: 32

In article <20042@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
| In general, the latter is true.  The proposed ANSI C standard requires
| that side effects take place exactly once, which effectively prohibits
| a straightforward macro version of abs() and labs().  One can still
| write, e.g.,
| 
| 	int abs(int);
| 	#define	abs(x)	__abs(x)
| 	static __inline int __abs(int x) { return x < 0 ? -x : x; }
| 
| in GCC.  I have used this in <stdio.h> to make putc() readable (`#ifdef'ed
| on __GNUC__, of course).

I know the above example is to demostrate the use of __inline, but
with GNU C, it does support a builtin abs function (though for some
reason it's not documented).  You can simplify the above to:

	#ifdef __GNUC__
	extern int abs(int);
	#define abs(x) __builtin_abs(x)
	#endif

Some machines have fast ways of doing abs (as compared to the branch
method above).  Gnu C also has the following builtin functions related
to abs:

	__builtin_labs(long)
	__builtin_fabs(double)
--
Michael Meissner, Data General.
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