Path: utzoo!attcan!uunet!mcvax!kth!sunic!chalmers!cs.chalmers.se!augustss
From: augustss@cs.chalmers.se (Lennart Augustsson)
Newsgroups: comp.unix.wizards
Subject: Re: C code optimization for a = (b < c);
Message-ID: <3102@fnatte.cs.chalmers.se>
Date: 28 May 89 21:49:41 GMT
References: <19787@adm.BRL.MIL>
Sender: news@cs.chalmers.se
Organization: Chalmers University of Technology, Sweden
Lines: 22

In article <19787@adm.BRL.MIL> WILCOX@nosc-tecr.arpa writes:
>
> ...
>     int a;
>     int b, c;
>     a = (b < c);
>
> [ should be replaced by ]
>
>a = (unsigned)((b - c) >> (WIDTH_OF_INT_IN_BITS - 1));
>
Good idea, but it doesn't work.  Assuming 16 bit integers, take
  b = -32768;	(0x8000)
  c = 32767;    (0x7fff)
now clearly b < c will evaluate to 1, but (unsigned)((b - c) >> 15 will
evaluate to 0, since b-c evaluates to 1 (0x0001).

	-- Lennart Augustsson


	Lennart Augustsson
Email:	augustss@cs.chalmers.se or augustss@chalmers.csnet