Date: Tue, 11 Apr 2006 23:36:59 +0000 From: "Jonathan Herriott" <herriojr@gmail.com> To: "Nicolas Blais" <nb_root@videotron.ca> Cc: Andy Reitz <reitz@eecs.cwru.edu>, freebsd-questions@freebsd.org Subject: Re: pow function working unexpectedly Message-ID: <6a56d69c0604111636m5d302f8doe62061e856831d4d@mail.gmail.com> In-Reply-To: <200604111935.14023.nb_root@videotron.ca> References: <6a56d69c0604111554o587ce2c5ha1ff4ea20bbab0a4@mail.gmail.com> <Pine.SOL.4.53.0604111911260.3684@bender> <443C3AA7.4020500@freebsd.org> <200604111935.14023.nb_root@videotron.ca>
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Yes, thank you. I wasn't going to report it as a bug after you pointed out my blatant mistake. Thanks again for the info. Jon On 4/11/06, Nicolas Blais <nb_root@videotron.ca> wrote: > 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 > >
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