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From: firth@sei.cmu.edu.UUCP
Newsgroups: comp.lang.c
Subject: Re: C and Floating Point (really parentheses)
Message-ID: <908@aw.sei.cmu.edu.sei.cmu.edu>
Date: Thu, 9-Apr-87 09:26:26 EST
Article-I.D.: aw.908
Posted: Thu Apr  9 09:26:26 1987
Date-Received: Sat, 18-Apr-87 02:37:03 EST
References: <kUSjqey00WABcJA0Nr@andrew.cmu.edu> <1094@ius2.cs.cmu.edu>
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In article <1094@ius2.cs.cmu.edu> edw@ius2.cs.cmu.edu (Eddie Wyatt) writes:

>#define X			0
>#define Y			1
>#define _EPSILON		.0002
>#define sqr(x)			(x*x)
>#define distance(x1,x2,y1,y2)	(sqrt((double (sqr(x1-x2) + sqr(y1-y2)))))
>#define approx_equal(x,y)	(fabs(double (x - y)) < _EPSILON)
>#define same_point(x1,x2,y1,y2) (approx_equal(distance(x1,x2,y1,y2),0))
>
>.
>.
>.
>
>if (same_point(p1[X],0.0,p1[Y],0.0)) ....
>
>  I trust that my C compiler is going to optimize the hell out of this
>expression.  Is this a good enough reason to allow reordering?

Maybe I'm missing something obvious, but, no, I DON'T see why this
is reason to allow reordering.  Doesn't it expand to:

	((fabs(double((sqrt(((p1[X]-0.0)*(p1[X]-0.0)) + \
	((p1[Y]-0.0)*(p1[Y]-0.0)))) - 0.0)) < 0.0002))

We can then elide "-0.0", replace "p1[X]*p1[X]" by "p1[X]*ibid",
and remove the redundant coercions and parentheses.  But I can't
see any scope for serious reordering.