Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!sharkey!atanasoff!hascall
From: hascall@atanasoff.cs.iastate.edu (John Hascall)
Newsgroups: comp.lang.c
Subject: Re: C optimizer
Keywords: C Ultrix
Message-ID: <795@atanasoff.cs.iastate.edu>
Date: 13 Feb 89 18:32:03 GMT
References: <515@larry.UUCP>
Reply-To: hascall@atanasoff.cs.iastate.edu (John Hascall)
Organization: Iowa State U. Computer Science Department, Ames, IA
Lines: 24

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?  ...

   I think it will/should call the function each time it appears as
   the compiler has no way of knowing the "side-effects" of a
   function call (i.e., file operations, accessing global variables, etc).

   Imagine:

	   x = (1 + printf(s)) / printf(s);
	   y = printf(s) / (1 - printf(s));

   (admittedly a hokey example--but you get the point...)

   John Hascall
   ISU Comp Center