Path: utzoo!mnetor!tmsoft!torsqnt!jarvis.csri.toronto.edu!rutgers!iuvax!watmath!rbutterworth
From: rbutterworth@watmath.waterloo.edu (Ray Butterworth)
Newsgroups: comp.lang.c
Subject: Re: sizeof(int) on 16-bit Atari ST.
Message-ID: <31784@watmath.waterloo.edu>
Date: 21 Nov 89 19:25:33 GMT
References: <31505@watmath.waterloo.edu>
Reply-To: rbutterworth@watmath.waterloo.edu (Ray Butterworth)
Distribution: comp
Organization: U. of Waterloo, Ontario
Lines: 25

In article <31505@watmath.waterloo.edu> rbutterworth@watmath.waterloo.edu (Ray Butterworth) writes:
>e.g. #define SIGNBIT(x) (0x8000 & (x))
>makes a big assumption about int being 16 bits.
>But  #define SIGNBIT(x) ( (~(( (unsigned int)(~0) )>>1)) & (x) )
>will work fine regardless of the size of int, and will generate
>the same machine code as the first macro when int is 16 bits.

Thanks to Niels J|rgen Kruse (njk@diku.dk), who had to tell me twice
before I'd believe the obvious, and Karl Heuer (karl@haddock.isc.com),
who inform me that the above isn't quite as portable as I'd thought.

All of the operators are logical-bit-operators *except* for the cast.
Casting to (unsigned int) is an arithmetic operator and as such it
might change the bit pattern, in particular on 1's-complement and
sign-magnitude architectures.

The standard (3.1.2.5) requires that "(unsigned int)n" have the
same bit pattern as "n" if n is a non-negative int, so the cast
should of course be done to the 0, not to the ~0.  i.e.
#define SIGNBIT(x) ( (~(( ~(unsigned int)0 )>>1)) & (x) )

>Coding for portabability may require a little extra effort,
>but it doesn't mean the result has to be any lesss efficient.

Maybe that "little" is an understatement.