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From: msb@sq.uucp (Mark Brader)
Newsgroups: comp.lang.c
Subject: Re: MSC C arithmetic
Message-ID: <1987Aug17.185845.20211@sq.uucp>
Date: Mon, 17-Aug-87 18:58:45 EDT
Article-I.D.: sq.1987Aug17.185845.20211
Posted: Mon Aug 17 18:58:45 1987
Date-Received: Tue, 18-Aug-87 05:43:49 EDT
References: <8792@brl-adm.ARPA> <7976@mimsy.UUCP>
Reply-To: msb@sq.UUCP (Mark Brader)
Organization: SoftQuad Inc., Toronto
Lines: 52
Checksum: 10487


To the original question about:

> >	unsigned int repeat_cnt;
> >	unsigned int units;
> >	unsigned long n;
> >	for (n = repeat_cnt * units; n > 0; n--) ...

> >	double total_bits;
> >	unsigned int repeat_cnt;
> >	unsigned int units;
> >	unsigned int blksiz;
> >	total_bits = repeat_cnt * units * blksiz * 16.0

Chris Torek writes:

> 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.

which is true, but he continues:

> 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 ...

which I think is wrong.  I quote K&R appendix A section 7.3: "The
multiplicative operators *, /, and % group left-to-right."  In the
ANSI draft, the same statement is concealed behind the grammar
in 3.3.5 that defines a multiplicative expression as being,
among other things, "multiplicative-expression * cast-expression".

It seems clear to me that both sources are saying that the assignment
must be considered as:

	total_bits = ((repeat_cnt * units) * blksiz) * 16.0

whereupon the two left-hand multiplies are done, as the MS C compiler
compiled, in unsigned ints.

I must admit that it is NOT clear to me whether the regrouping rules
allow the compiler to alternatively compile this as:

	total_bits = repeat_cnt * (units * (blksiz * 16.0))

whereupon the arithmetic would indeed be done in double.  I think they do.
Perhaps Doug or someone who has studied this closely could comment on it.
That is, if the removal (to which I probably object) of the regrouping
rules has not already happened.

Mark Brader			"The language should match the users,
utzoo!sq!msb			 not vice versa"  -- Brian W. Kernighan