Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Fortran computes cosine 300 times faster than C (on Sun3)
Message-ID: <1989Mar8.234929.9905@utzoo.uucp>
Organization: U of Toronto Zoology
References: <765@uceng.UC.EDU>
Date: Wed, 8 Mar 89 23:49:29 GMT

In article <765@uceng.UC.EDU> achhabra@uceng.UC.EDU (atul k chhabra) writes:
>I chanced upon a segment of code that runs approximately 300 times faster in
>FORTRAN than in C. I have tried the code on Sun3(OS3.5) and on Sun4(OS4.0)
>(of course, on Sun4 the -f68881 flag was not used.) The results are similar
>on both machines. Can anyone enlighten me on this bizzare result?

Two things.  First, you're asking for single-precision cosine in Fortran
and double-precision in C.  Second, Sun's Fortran optimizer is much
better than their C optimizer, and it has noticed that you're not *doing*
anything with those values and deleted the whole computation.  You're
timing the C code against an empty Fortran loop.
-- 
Welcome to Mars!  Your         |     Henry Spencer at U of Toronto Zoology
passport and visa, comrade?    | uunet!attcan!utzoo!henry henry@zoo.toronto.edu