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From: djfiander@watnot.UUCP
Newsgroups: comp.lang.c
Subject: Re: C and floating point (optimizing compilers)
Message-ID: <12840@watnot.UUCP>
Date: Fri, 10-Apr-87 12:50:53 EST
Article-I.D.: watnot.12840
Posted: Fri Apr 10 12:50:53 1987
Date-Received: Sat, 11-Apr-87 14:46:11 EST
References: <742@instable.UUCP>
Reply-To: djfiander@watnot.UUCP (David Fiander)
Organization: U. of Waterloo, Ontario
Lines: 23
Keywords: parentheses in expressions, optimizers

In article <742@instable.UUCP> chaim@instable.UUCP (Chaim Bendelac) writes:
>First, for us, the next two sequences are of course THE SAME: 
>
>	(1)	x = (a+b)+c;
>
>	(2)	temp = a+b;
>		x = temp+c;
>
>The optimization technique which reduces (2) into (1) is called copy (or value)
>propagation. The value of 'temp' is propagated. So, if you are worried about
>the correct evaluation of (1), then writing it as (2) will not save you.

Bad news:  K&R state that if the user is really worried about order
of evaluation, then she should use something like code fragment (2).
This, rather obviously, blows any compiler which does what you describe
out of the water.

-- 
"Are you police officers?"
	"No ma'am, we're musicians."

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