Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!purdue!bu-cs!dartvax!eleazar.dartmouth.edu!earleh
From: earleh@eleazar.dartmouth.edu (Earle R. Horton)
Newsgroups: comp.lang.c
Subject: Re: C optimizer
Keywords: C Ultrix
Message-ID: <12197@dartvax.Dartmouth.EDU>
Date: 13 Feb 89 18:06:07 GMT
References: <515@larry.UUCP>
Sender: news@dartvax.Dartmouth.EDU
Reply-To: earleh@eleazar.dartmouth.edu (Earle R. Horton)
Organization: Thayer School of Engineering
Lines: 19

In article <515@larry.UUCP> jwp@larry.UUCP (Jeffrey W Percival) writes:
>I have a question about how much optimizing I should worry about when
>writing programs in C.  Suppose I have this code fragment:
>
>	x = (1 + cos(r)) / (cos(r) * sin(r));
>	y = (cos(r) - sin(r)) / (1 + sin(r));
>
>I made this up, but the point is the re-use of the sin() and cos()
>calls.  Now, can I expect the compiler to form only one call to sin and
>cos?

     No.  The compiler has no way of knowing what sin() and cos() do,
besides obviously returning a result.  Either or both might have side
effects, and optimizing out any of the calls prevents the side effects
from happening.

Earle R. Horton. 23 Fletcher Circle, Hanover, NH 03755--Graduate student.
He who puts his hand to the plow and looks back is not fit for the kingdom
of winners.  In any case, 'BACK' doesn't work.