[Boost-bugs] [Boost C++ Libraries] #13264: Floating point math going wrong using cpp_bin_float with 64 bit exponent

Subject: [Boost-bugs] [Boost C++ Libraries] #13264: Floating point math going wrong using cpp_bin_float with 64 bit exponent
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2017-10-18 20:06:33

#13264: Floating point math going wrong using cpp_bin_float with 64 bit exponent
--------------------------+----------------------------
 Reporter:  private@…     |      Owner:  John Maddock
     Type:  Bugs          |     Status:  new
Milestone:                |  Component:  multiprecision
  Version:  Boost 1.65.0  |   Severity:  Problem
 Keywords:                |
--------------------------+----------------------------
 I tried the following:

 {{{
 #include <cmath>
 #include <iostream>
 #include <boost/multiprecision/cpp_bin_float.hpp>

 using float256 =
 boost::multiprecision::number<boost::multiprecision::backends::cpp_bin_float<192,
                   boost::multiprecision::backends::digit_base_2, void,
 long long int>, boost::multiprecision::et_off>;

 void Test()
 {
    double a = exp(-0.5);
    double b =
 boost::multiprecision::exp(float256(-0.5)).convert_to<double>();
    cout << a << endl << b << endl;
 }
 }}}

 Unfortunately this doesn't work. I am getting 0.606531 for a but 1 for b.
 Using an int as exponent fixes the problem.

 I'm running: boost 1.65.1, Linux, gcc 5.4.0.

-- 
Ticket URL: <https://svn.boost.org/trac10/boost/ticket/13264>
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