From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 9496 invoked by alias); 5 Jan 2015 13:11:59 -0000 Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-bugs-owner@gcc.gnu.org Received: (qmail 9452 invoked by uid 55); 5 Jan 2015 13:11:50 -0000 From: "walter.mascarenhas at gmail dot com" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/64497] std::scalbln does not round correctly for long doubles Date: Mon, 05 Jan 2015 13:11:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c++ X-Bugzilla-Version: 4.8.2 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: walter.mascarenhas at gmail dot com X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-SW-Source: 2015-01/txt/msg00227.txt.bz2 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497 --- Comment #2 from Walter Mascarenhas --- What if there is a difference in the expected behavior for this function in C and C++11? Is it not up to g++ for implementing what is mandated in C++11? (This is not a rhetorical question, I really do not know the answer.) In http://man7.org/linux/man-pages/man3/scalbn.3.html it is written that scalbln should return 0 in case of underflow: "If the result underflows, a range error occurs, and the functions return zero, with a sign the same as *x*." On the other hand, http://en.cppreference.com/w/cpp/numeric/math/scalbn states that "If a range error due to underflow occurs, the correct result (after rounding) is returned." I looked at the standard (N3797.pdf) but did not find anything specific about std::scalbln. If there is indeed a discrepancy in the definitions of scalbln in C and C++11 then there may be no bug in libm, and my vendor will not change it. I do not have a copy of the ISO 60599 standard, and I do not know whether the content of the pages http://man7.org/linux/man-pages/man3/scalbn.3.html and http://en.cppreference.com/w/cpp/numeric/math/scalbn are compatible with any standards. Therefore I am in no position to argue, but maybe you could think a bit longer about this.. On Mon, Jan 5, 2015 at 10:29 AM, redi at gcc dot gnu.org < gcc-bugzilla@gcc.gnu.org> wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497 > > --- Comment #1 from Jonathan Wakely --- > GCC just calls the scalnlnl() function in libm, so it's not a GCC bug, and > is > not specific to C++ either. I suggest you report it to your libc vendor. > > Complete testcase in C: > > #include > #include > #include > > int main() > { > long double di = scalbnl(1.1L, -16446); > assert( di != 0.0L ); > long double dl = scalblnl(1.1L, -16446L); > assert( dl != 0.0L ); > } > > -- > You are receiving this mail because: > You are on the CC list for the bug. > You reported the bug. >