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

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: 2016-11-08 21:47:03

#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 Michael Shatz):

 Unfortunately, you are right.
 Still, I can think about another workaround to keep my workaround alive:
 1. Convert to cpp_bin_float<N+52>
 2. add numeric_limits::min(). Now addition is exact!
 3. convert to double
 4. subtract numeric_limits::min().

 Practically it's probably faster to convert to cpp_bin_float<N+64>

 BTW, special-casing of (x.backend().exponent() < -1023 - 52) in my code is
 a performance optimization. This step is not necessary for correction.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12527#comment:9>
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