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From: flaps@utcsri.UUCP (Alan J Rosenthal)
Newsgroups: comp.lang.c
Subject: Re: order of evaluation
Message-ID: <4328@utcsri.UUCP>
Date: Sun, 8-Mar-87 02:11:06 EST
Article-I.D.: utcsri.4328
Posted: Sun Mar  8 02:11:06 1987
Date-Received: Mon, 9-Mar-87 01:56:45 EST
References: <616@viper.UUCP>
Reply-To: flaps@utcsri.UUCP (Alan J Rosenthal)
Organization: University of Toronto
Lines: 29
Summary: 

In article <616@viper.UUCP> john@viper.UUCP (John Stanley) writes:
>I still feel that the standard should have rules
>which will allow a programmer to -know- the exact order of evaluation for
>any equation.

It does.. use temporaries.  For example, if you want the sum a+b+c evaluated
in the order implied by (a+b)+c, simply write two statements, the first of
which is t=a+b; and the second of which contains the expression t+c.  (If you
have a REALLY good optimizer it's possible that this temporary variable will be
optimized out completely.  In any case, the redundant store/load will probably
be optimized out.)

>I know this is not likely for the sole reason that there
>are too many different compilers already existing that do the ordering in
>far too many different ways.

Also for the reason that there is quite likely a different best order for
different machines, so the compilers should be free to implement the best
order for their machine.

-- 

Alan J Rosenthal

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