From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 3594 invoked by alias); 2 Nov 2006 14:41:45 -0000 Received: (qmail 3531 invoked by uid 48); 2 Nov 2006 14:41:31 -0000 Date: Thu, 02 Nov 2006 14:41:00 -0000 Message-ID: <20061102144131.3530.qmail@sourceware.org> X-Bugzilla-Reason: CC References: Subject: [Bug middle-end/29335] transcendental functions with constant arguments should be resolved at compile-time In-Reply-To: Reply-To: gcc-bugzilla@gcc.gnu.org To: gcc-bugs@gcc.gnu.org From: "ghazi at gcc dot gnu dot org" Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Archive: List-Post: List-Help: Sender: gcc-bugs-owner@gcc.gnu.org X-SW-Source: 2006-11/txt/msg00144.txt.bz2 List-Id: ------- Comment #30 from ghazi at gcc dot gnu dot org 2006-11-02 14:41 ------- (In reply to comment #28) > (In reply to comment #27) > > It's likely that I'll end up doing it, so would you please tell me how? > According to the C rationale (I haven't checked), the sign of gamma(x) is -1 if > [iff] x < 0 && remainder(floor(x), 2) != 0. But if x is a non-positive integer, > the sign of gamma(x) isn't defined. Handle these cases first. > The test x < 0 is easy to do. In MPFR, you can compute floor(x) (or trunc(x)) > with the precision min(PREC(x),max(EXP(x),MPFR_PREC_MIN)), but then, there's no > direct function to decide whether the result is even or odd (I thought we added > this, but this isn't the case). The solution can be to divide x by 2 (this is > exact, except in case of underflow) and call mpfr_frac directly. If the result > is between -0.5 and 0, then gamma(x) is negative. If the result is between -1 > and -0.5, then gamma(x) is positive. So, a 2-bit precision for mpfr_frac should > be sufficient (as -0.5 is representable in this precision), but choose a > directed rounding (not GMP_RNDN) for that. Then you can just do a comparison > with -0.5; the case of equality with -0.5 depends on the chosen rounding (if > you obtain -0.5, then it is an inexact result since x is not an integer). For > instance, if you choose GMP_RNDZ, then a result > -0.5 means that gamma(x) is > negative, and a result <= -0.5 means that gamma(x) is positive. Vincent, thank you for the detailed instructions. I also read your two possible solutions posted here: http://sympa.loria.fr/wwsympa/arc/mpfr/2006-10/msg00036.html I could be satisfied with either solution from that message. However in the case of choice 1, I feel the calculation of signgam should be provided from a function call in the library rather than forcing each user to write a routine to calculate it. IMHO, I'd rather leave the math to the mathematicians. :-) E.g. you could add a function mpfr_signgam() that figures out the value for the user and thereby leave the interface for mpfr_lngamma() unchanged. Choice 2 also solves the issue by providing the int* parameter. Thanks. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29335