X-Spam-Check-By: sourceware.org From: "Dave Korn" To: , References: <20070829153331.GA95558@tetsuo.karasik.eu.org> <46D5A79E.C09EF7CE@dessent.net> Subject: RE: 1.5.24: sin() bug Date: Wed, 29 Aug 2007 18:12:40 +0100 Message-ID: <003201c7ea5f$d01db8f0$2e08a8c0@CAM.ARTIMI.COM> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit X-Mailer: Microsoft Office Outlook 11 In-Reply-To: <46D5A79E.C09EF7CE@dessent.net> Mailing-List: contact cygwin-help@cygwin.com; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: cygwin-owner@cygwin.com Mail-Followup-To: cygwin@cygwin.com Delivered-To: mailing list cygwin@cygwin.com On 29 August 2007 18:07, Brian Dessent wrote: > Dmitry Karasik wrote: > >> #include >> #include >> int main( int argc, char ** argv) >> { >> double g = (double) 3.1415926535897900074; >> printf("sin(%.10g)=%.10g\n", g, sin(g)); >> } >> >> output is : >> >> sin(3.141592654)=3.231089149e-15 >> >> whereas all other sin() implementation I could find ( freebsd, linux, >> msvc) report this: >> >> sin(3.141592654)=3.231085104e-015 >> >> the difference is in 7th digit, and is significant for double precision. > > This appears to be due to an approximation in __kernel_sin in which if >> x| < 2**-27 the approximation sin(x) ~ x is used. Since when was pi less than 2^-27? > In any case this is more of a newlib issue than a Cygwin issue as we are > just a consumer of this code, so followups should go to the newlib list. I'd just like to point out that I got the exact same result as cygwin supplies on a recent linux box: On cygwin, I get: /tmp $ ./sin.exe sin(3.141592654)=3.231089149e-15 ... whereas on linux (x86_64), I get ... [dk@ucho gxlinux-ent]$ ./sin sin(3.141592654)=3.231089149e-15 ... and with mingw: $ ./sin.exe sin(3.141592654)=3.231085104e-015 I think it may be an artefact of FP precision and/or rounding mode, but I'd need to do more experiments to make sure. OTOH it could be an artifact of the printf %g specfier. Like I said, more experimentation is needed! cheers, DaveK -- Can't think of a witty .sigline today.... -- Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple Problem reports: http://cygwin.com/problems.html Documentation: http://cygwin.com/docs.html FAQ: http://cygwin.com/faq/