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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: MSC C arithmetic
Message-ID: <7976@mimsy.UUCP>
Date: Fri, 14-Aug-87 01:49:53 EDT
Article-I.D.: mimsy.7976
Posted: Fri Aug 14 01:49:53 1987
Date-Received: Sat, 15-Aug-87 14:29:40 EDT
References: <8792@brl-adm.ARPA>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 39

In article <8792@brl-adm.ARPA> jdickson@zook.Jpl.Nasa.GOV (Jeff
Dickson) writes:
>	unsigned int repeat_cnt;
>	unsigned int units;
>	unsigned long n;

>	for (n = repeat_cnt * units; n > 0; n--)

[and]

>	double total_bits;
>	unsigned int repeat_cnt;
>	unsigned int units;
>	unsigned int blksiz;

>	total_bits = repeat_cnt * units * blksiz * 16.0

[produce unexpected results].

Microsoft C is compiling the first example correctly.  The extension
rules require only that `repeat_cnt * units' be done in unsigned
arithmetic; at least one of the two multiplicands must be of type
long or unsigned long for the operation to be done in long arithmetic.

The second result is not so defensible; the 16.0 should modify the
entire operation so that it is done in double.  This follows from
the type matching rules: typeof(repeat_cnt * (expr0)) is
typemax(typeof(repeat_cnt), typeof(expr0)); typeof(expr0) is
typemax(typeof(units), typeof(expr1)); typeof(expr1) is
typemax(typeof(blksize), typeof(16.0)) which is typemax(type_u_int,
type_double) which is just type_double; this propagates all the
way back to the top.

The first example works on a Sun because there sizeof(u_int) ==
sizeof(u_long), so unsigned int arithmetic is the same as
unsigned long arithmetic.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	seismo!mimsy!chris