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From: gwyn@brl-smoke.UUCP
Newsgroups: comp.lang.c
Subject: Re: C & fp (really parens)
Message-ID: <5739@brl-smoke.ARPA>
Date: Thu, 9-Apr-87 18:05:23 EST
Article-I.D.: brl-smok.5739
Posted: Thu Apr  9 18:05:23 1987
Date-Received: Sat, 11-Apr-87 11:53:31 EST
References: <6800@brl-adm.ARPA>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 48

In article <6800@brl-adm.ARPA> dsill@NSWC-OAS.arpa writes:
>Doug Gwyn wrote:
>>...  However, the dpANS for C has not changed
>>the significance of parentheses; C compilers have always been allowed to
>>reorder theoretically-commutative and -associative operations.
>>...
>
>From K&R, page 185:
>"Expressions involving a commutative and associative operator (*, +, &, |, ^)
> may be rearranged arbitrarily, even in the prescence of parentheses; to force
> a particular order of evaluation an explicit temporary must be used."
>
>This implies that when these operations aren't commutative AND associative,
>as in floating point, the expressions can't be reordered.

Wrong!  That is why I was careful to use the qualifier "theoretically-".
The abstract arithmetic definition of * and + for all numbers in C IS
associative and commutative.  It is not intended that machine quirks that
cause deviation from the theoretical model in practice somehow cancels the
model -- the programmer is not expected to have to take specific machine
properties into account in order to determine whether * and + are
considered associative and commutative; they are announced to be such for
purposes of operation reordering by K&R (and the dpANS).

>  If this is the
>case, what is the unary + needed for if it was added for the benefit of
>"numeric" programmers?

Unary plus was originally added for "symmetry" with unary minus.  It was
only later when the request for a way to force evaluation order in
expressions was considered that it was observed that unary plus happens
to have that nice effect.

>  Are there compilers that are reordering expressions
>with (*, +, &, |, ^) even when they aren't supposed to?

There is no such thing as a time when "they aren't supposed to" (if
sequence points, volatile data, or other such subtleties are not involved).
In fact there are many C compilers that rearrange such expressions, as
they have always been permitted to do.

>Given that a means for forcing the compiler to respect parentheses is needed,
>couldn't something less syntactically "tacky" than the unary + have been found?
>I don't think it's intuitive or obvious.

Many people would agree that it's a bit kludgy, but since the facility
was added anyway for other (not necessarily terrific) reasons and has the
desired effect, there is no need to invent anything more for this.