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From: jjw@celerity.UUCP (Jim )
Newsgroups: comp.lang.c
Subject: Re: Disregarding parentheses in C
Message-ID: <73@celerity.UUCP>
Date: Fri, 24-Apr-87 16:28:08 EST
Article-I.D.: celerity.73
Posted: Fri Apr 24 16:28:08 1987
Date-Received: Sun, 26-Apr-87 00:38:48 EST
References: <200@m10ux.UUCP> <6378@mimsy.UUCP> <1038@wanginst.EDU>
Reply-To: jjw@celerity.UUCP (Jim (JJ) Whelan)
Organization: Celerity Computing, San Diego, Ca.
Lines: 39
Keywords: C parentheses evaluation order parsing

I hesitate to contradict what a standard says about itself but...

In article <1038@wanginst.EDU> mckeeman@wanginst.UUCP (William McKeeman) quotes
3j11 document July 9, 1986:

>Footnote 15 tells how to imply precedence from the syntax and order of
>presentation in the draft standard.
>
>[begin of quote of Rationale, dated 7 July 1986, section 2.1.2.3, Program
>execution.]
>
>   ... A question sometimes asked regarding optimization is, "Is the
>rearrangement still conforming if the pre-computed expression might raise a
>signal (such as division by zero)?"  Fortunately for optimizers, the answer
>is "Yes", because any evaluation that raises a computational signal has
>fallen into an _undefined_behavior_ (refer to section 3.3) for which any
>action is allowable.
>
>[end of quote]
>
>[begin comment by McKeeman]
>
>The only constraint on compiler rearrangements is that the TYPE of the
>operands and results not thereby be changed.  (emphasis mine)
>
>[end of comment]

The operative paragraph is:

>   An expression involving more than one occurrence of the same commutative
>and associate binary operator (*, +, &, ^, |) may be regrouped arbitrarily,
>even in the presence of parentheses, provided the types of the operands or of
>the results are not changed by this regrouping.

In my opinion, an exception resulting in an "undefined behavior" is a
different result from an un-regrouped expression that does not generate
an exception and therefore provides a "defined behavior"

					-- J. J. Whelan