Date: Tue, 11 Apr 2006 19:35:13 -0400 From: Nicolas Blais <nb_root@videotron.ca> To: freebsd-questions@freebsd.org Cc: Andy Reitz <reitz@eecs.cwru.edu>, Jonathan Herriott <herriojr@gmail.com>, Colin Percival <cperciva@freebsd.org> Subject: Re: pow function working unexpectedly Message-ID: <200604111935.14023.nb_root@videotron.ca> In-Reply-To: <443C3AA7.4020500@freebsd.org> References: <6a56d69c0604111554o587ce2c5ha1ff4ea20bbab0a4@mail.gmail.com> <Pine.SOL.4.53.0604111911260.3684@bender> <443C3AA7.4020500@freebsd.org>
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[-- Attachment #1 --] On Tuesday 11 April 2006 19:24, Colin Percival wrote: > Andy Reitz wrote: > > So, clearly, something is optimizing the pow() function away when the > > arguments are hard-coded lvalues, instead of varibles. > > > > Now, what that thing *is*, I don't know. > > The C compiler precomputes constant expressions; your "pow(2,3)" is > being rewritten to "8" by the compiler. Similarly, if you write > "1 + 2 / 3 + 4 * 5 - 6", the C compiler will turn this into "15" > rather than producing a series of instructions which computes the > expression. > > When you reference variables, this optimization isn't possible, since > those variables might be modified before you reach the line where > they are used. (Obviously this doesn't happen in your program, but > the compiler isn't smart enough to figure that out.) > > Colin Percival +1. The compiler sees constants differently from variables (which reside in a memory space). With the constant, the optimizer can rewrite the equation with its answer. Anyhow, it can't be seen as a bug because you should include the math library whenever you're using a function from it (see man pow). Therefore, if you would have included the library in the first place, you would have never found this 'oddity'. Nicolas. -- FreeBSD 7.0-CURRENT #1: Sat Apr 8 21:33:25 EDT 2006 root@clk01a:/usr/obj/usr/src/sys/CLK01A PGP? : http://www.clkroot.net/security/nb_root.asc [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (FreeBSD) iD8DBQBEPD0y4wTBlvcsbJURAnPgAJ9t6qoGsuSZQrmtVquKvBUf0C8t1QCgoc5x PSSmS21cCmq02IVCjLdnRJM= =/Zz/ -----END PGP SIGNATURE-----
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