Subject: Re: [Boost-bugs] [Boost C++ Libraries] #12527: cpp_bin_float: Anal fixation. Part 3. Double rounding when result of convert_to<double>() is a subnormal
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-11-16 18:58:59
#12527: cpp_bin_float: Anal fixation. Part 3. Double rounding when result of
convert_to<double>() is a subnormal
-------------------------------+----------------------------
Reporter: Michael Shatz | Owner: johnmaddock
Type: Bugs | Status: reopened
Milestone: To Be Determined | Component: multiprecision
Version: Boost 1.62.0 | Severity: Problem
Resolution: | Keywords:
-------------------------------+----------------------------
Comment (by johnmaddock):
I've just pushed some changes to develop which address this, using the
attached test program and comparing to your routine in comment 14 here are
the results I see:
MSVC:
{{{
Testing times for type: cpp_bin_float_double
Reference time: 0.519 seconds (total sum = 0.000)
Boost time: 0.345 seconds (total sum = 383586723786.860)
Testing times for type: cpp_bin_float_quad
Reference time: 1.450 seconds (total sum = 383870655523.786)
Boost time: 0.506 seconds (total sum = 383870655523.786)
Testing times for type: cpp_bin_float_50
Reference time: 1.261 seconds (total sum = 383509869056.583)
Boost time: 0.495 seconds (total sum = 383509869056.583)
Testing times for type: cpp_bin_float_100
Reference time: 1.762 seconds (total sum = 383488757195.157)
Boost time: 0.508 seconds (total sum = 383488757195.157)
}}}
GCC Mingw64:
{{{
Testing times for type: cpp_bin_float_double
Reference time: 0.292 seconds (total sum = 0.000)
Boost time: 0.121 seconds (total sum = 383586723786.860)
Testing times for type: cpp_bin_float_quad
Reference time: 0.417 seconds (total sum = 383870655523.786)
Boost time: 0.271 seconds (total sum = 383870655523.786)
Testing times for type: cpp_bin_float_50
Reference time: 2.416 seconds (total sum = 383509869056.583)
Boost time: 0.303 seconds (total sum = 383509869056.583)
Testing times for type: cpp_bin_float_100
Reference time: 1.669 seconds (total sum = 383488757195.157)
Boost time: 0.307 seconds (total sum = 383488757195.157)
}}}
GCC Ubuntu times are similar.
So the new Boost code is nearly twice as fast for you preferred 128-bit
float, and much more than that for 100 decimal digit precision.
The remaining issue was that the bit-scanning/testing code wasn't properly
optimised for {{{__int128}}}, fixing that and handling ldexp better led to
the speedup. I've attached the test program I used to this bug case, can
you test at your end as well? BTW you will need all the develop branch of
Multiprecision, not just cpp_bin_float.hpp as it's some of the core
integer ops that needed work.
-- Ticket URL: <https://svn.boost.org/trac/boost/ticket/12527#comment:18> Boost C++ Libraries <http://www.boost.org/> Boost provides free peer-reviewed portable C++ source libraries.
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