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From: ark@alice.UUCP
Newsgroups: comp.lang.c,comp.sys.atari.st,comp.sys.amiga
Subject: Re: 32bit = 16bit x 16bit
Message-ID: <7349@alice.UUCP>
Date: Mon, 12-Oct-87 21:35:50 EDT
Article-I.D.: alice.7349
Posted: Mon Oct 12 21:35:50 1987
Date-Received: Wed, 14-Oct-87 06:41:24 EDT
References: <141@kesmai.COM>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 29
Xref: mnetor comp.lang.c:4867 comp.sys.atari.st:5654 comp.sys.amiga:9342

In article <141@kesmai.COM>, dca@kesmai.UUCP writes:
> Does anyone know of a commercial test suite that checks to make sure that
> y = x * x where x is a 16 bit int and y is long (32 bits)  produces garbage
> instead of the proper result?

Garbage *IS* the proper result.

If x is a 16-bit number, x*x is also a 16-bit number.
The result of overflow is undefined.

If you want the 32-bit product of x and x on a machine with
32-bit longs, the way to write it is

	(long) x * (long) x

or
	x * (long) x

or

	(long) x * x

while is evaluated as

	((long) x) * x

You can then flame at your compiler vendor for not being smart
enough to generate a single instruction for the expression
(in fact, some compilers are smart enough).