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From: drh@romeo.cs.duke.edu (D. Richard Hipp)
Newsgroups: comp.lang.c
Subject: Re: () ignored in some expressions
Message-ID: <18833@duke.cs.duke.edu>
Date: 10 Apr 90 13:03:30 GMT
References: <48079@lanl.gov> <101287@convex.convex.com>
Sender: news@duke.cs.duke.edu
Reply-To: drh@duke.cs.edu
Organization: Duke University CS Dept.; Durham, NC
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>In MOST cases this [Ignoring ()]
>is not a problem with INTEGER math, but it is a BIG problem with
>floating point numbers.

I often what to do things like increase an integer by 50%.  This can
be done as follows:    i = (i*3)/2;   If an optimizing compiler can
ignore the parenthesis, then it will probably notice that it can evaluate
3/2 at compile time to 1.  It will then notice that a multiplication by
1 can be ignored.  The end result is that my optimizing compiler will
decide not to generate any code at all for the above statement.

According to K&R, the only way to avoid this disaster is to code the
operation in two steps:   i *= 3; i /= 2;