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: 2016-11-07 00:46:18

#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:  closed
 Milestone:  To Be Determined  |  Component:  multiprecision
   Version:  Boost 1.62.0      |   Severity:  Problem
Resolution:  fixed             |   Keywords:
-------------------------------+----------------------------

Comment (by Michael Shatz):

 Replying to [comment:5 johnmaddock]:
 > I was about to say that it's a non-issue because things that return zero
 are handled separately.  However, there was a bug in that (separate) case,
 see
 https://github.com/boostorg/multiprecision/commit/51686cae8739741f808f58be4cdc2059ad1f2502


 I think, you didn't understand my case.

 If I correctly understood the voodoo of numeric_limits, your condition
 if(e < min_exp_limit) is the same as if (x.backend().exponent() < -1077).

 But the case that I am talking about happens when (x.backend().exponent()
 == -1075) for the smallest absolute value in this range.
 The rest of the range is rounded away from zero, i.e. to smallest negative
 number == -(2!**-1074), but this single point on the edge of the range, is
 a tie, and as a good tie it has to be rounded to nearest even, i.e. to
 negative zero.
 However my original workaround rounds this point to positive zero. It
 happens due to same IEEE rule that we discussed few weeks ago - difference
 of equals numbers is always a positive zero.

 That's why a new workaround is better than the original and why any change
 of min_exp_limit would not help this particular unique point.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12527#comment:6>
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