Date: Fri, 1 Nov 2013 08:38:32 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Steve Kargl <sgk@troutmask.apl.washington.edu> Cc: freebsd-numerics@FreeBSD.org Subject: Re: Representation of 128 bit floating point numbers in FreeBSD amd64 and Clang Message-ID: <20131101075253.D1002@besplex.bde.org> In-Reply-To: <20131031143832.GA60432@troutmask.apl.washington.edu> References: <CAOgwaMs=NKfDpjOY9=5%2BKYFUdbzeMpxMF_nxZP21y8RTb_cR9w@mail.gmail.com> <20131031143832.GA60432@troutmask.apl.washington.edu>
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On Thu, 31 Oct 2013, Steve Kargl wrote: > On Thu, Oct 31, 2013 at 09:27:34AM -0400, Mehmet Erol Sanliturk wrote: >> >> In FreeBSD amd64 and Clang , >> how can I represent 128 bits ( 34 digits ) variables ? With difficulty, since it is not supported. > Not sure it can be done with clang, but GCC supports > a __float128 type. GCC refers to this as its TCmode. > gfortran, the Fortran compiler that supports REAL(16), > uses __float128 internally. I've never directly used > __float128, so can't help beyond this. > > If you need 128-bits in C on ia32 or x86_64 hardware, > you should probably look into using mpfr and mpc. Even gcc-4.2.1 in FreeBSD generates code to use __float128, but the support for it isn't compiled into libgcc for some reason. Why would anyone want to use 128-bit FP on x86? It is emulated similarly to on sparc64. On sparc64, emulated 128-bit FP is about 100 times slower than hardware 64-bit FP. The emulation is not very good, but 128-bit FP is part of the ABI on sparc64 so I would expect the emulation to give an even larger slowdown factor in x86. With 80-bit FP, you can't quite exactly count the number of atoms in the universe, but you can count the world's GNP in cents for a thousand years or so. Extra accuracy can reduce problems from numerica instability and rounding bugs, but a slowdown factor of 100 times is a large price to pay for that. Bruce
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