Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!ames!ucsd!nosc!cod!mball
From: mball@cod.NOSC.MIL (Michael S. Ball)
Newsgroups: comp.lang.c
Subject: Re: C optimizer
Keywords: C pure function optimization
Message-ID: <1404@cod.NOSC.MIL>
Date: 14 Feb 89 05:58:00 GMT
References: <515@larry.UUCP> <9648@smoke.BRL.MIL>
Reply-To: mball@cod.nosc.mil.UUCP (Michael S. Ball)
Organization: TauMetric Corporation
Lines: 18

In article <515@larry.UUCP> jwp@larry.UUCP (Jeffrey W Percival) writes:
>	x = (1 + cos(r)) / (cos(r) * sin(r));
>	y = (cos(r) - sin(r)) / (1 + sin(r));
>Now, can I expect the compiler to form only one call to sin and cos?

In K&R C this is impossible, as many have mentioned.  The draft ANSI standard
makes it possible, though not too many take advantage of it.  The Oregon
Software C & C++ compiler uses the trick proposed in the standard and uses
a macro to convert the math functions to built-in functions.  These are known
to be pure and are so optimized.  In fact, if the target machine has a
math chip the simple math functions are compiled inline as single instructions.

-Mike Ball-
TauMetric Corporation
1094 Cudahy Pl. Ste 302
San Diego, CA 92110
(619)275-6381
mball@cod.nosc.mil