[Boost-bugs] [Boost C++ Libraries] #10173: Epsilon value returned by std::numeric_limits is too small on PowerPC, causing tests to fail

Subject: [Boost-bugs] [Boost C++ Libraries] #10173: Epsilon value returned by std::numeric_limits is too small on PowerPC, causing tests to fail
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-07-02 16:27:31

#10173: Epsilon value returned by std::numeric_limits is too small on PowerPC,
causing tests to fail
-------------------------------------------------+-------------------------
 Reporter:  Harris Munir <hamunir@…>             |      Owner:  apolukhin
     Type:  Bugs                                 |     Status:  new
Milestone:  To Be Determined                     |  Component:
  Version:  Boost Release Branch                 |  lexical_cast
 Keywords:  lexical_cast_float_types_test,       |   Severity:  Problem
  epsilon                                        |
-------------------------------------------------+-------------------------
 In {{{lexical_cast_float_types_test}}}, {{{std::numeric_limits<long
 double>::epsilon()}}} is used as a tolerance value when comparing long
 double values to check the accuracy of lexical_cast.

 On PowerPC, this epsilon value is extremely small for the long double
 type. While the value is technically correct, since adding 1.0L to it does
 in fact result in a distinct value, it is not suitable for use as a
 tolerance value in this type of comparison. Instead, we need a value such
 that:

 {{{x + x * eps != x for all x}}}

 This value can be found by calculating 2^(1-D), where D is the number of
 bits of precision in the type.

 We found this same issue in many parts of the Boost math library, and
 resolved it by adding a special case of
 {{{boost::math::tools::epsilon<long double>}}} for platforms that use the
 strange "double-double" implementation of long double, such as PowerPC.
 This change is now in the master branch and can be seen here:

 https://github.com/boostorg/math/blob/7cb3316d0616db01f6c4a56126d4bf8c73e7b50d/include/boost/math/tools/precision.hpp#L160-178


 I have created a pull request which replaces all instances of
 {{{std::numeric_limits<T>::epsilon()}}} with
 {{{boost::math::tools::epsilon<T>}}} in
 {{{lexical_cast_float_types_test}}}. This change fixes 5222 of the 5255
 failures that currently occur within the test on a PowerPC machine with
 long double support enabled. The pull request can be found here:

 https://github.com/boostorg/lexical_cast/pull/3

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/10173>
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