[Boost-bugs] [Boost C++ Libraries] #8809: pow(cpp_dec_float_type(-1), cpp_dec_float_type(n)) != (+-)1 for n >= 0x8000000000000000u

Subject: [Boost-bugs] [Boost C++ Libraries] #8809: pow(cpp_dec_float_type(-1), cpp_dec_float_type(n)) != (+-)1 for n >= 0x8000000000000000u
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-07-10 12:05:08

#8809: pow(cpp_dec_float_type(-1), cpp_dec_float_type(n)) != (+-)1 for n >=
0x8000000000000000u
-------------------------------------+----------------------------
 Reporter:  Jan Bouwer <JBouwer@…>   |      Owner:  johnmaddock
     Type:  Bugs                     |     Status:  new
Milestone:  To Be Determined         |  Component:  multiprecision
  Version:  Boost Development Trunk  |   Severity:  Problem
 Keywords:                           |
-------------------------------------+----------------------------
 Function ''pow(const number-or-expression-template-type&, const number-or-
 expression-template-type&)'' for ''cpp_dec_float'' backend gives wrong
 answer when used with base -1 and a positive exponent equivalent to a type
 (unsigned 64bit on my platform) with the high bit set;
 i.e. an exponent >= 0x8000000000000000u.

 The answers suggest that the exponent may be interpreted as its 2's
 complement-negative value (not confirmed) - see sample output below.
 This may be related to #8711 & #8798.

 Output from attached code:
 {{{
 pow(-1, 9.22337e+18) = -0.00127263
 pow(-1, 9.22337e+18) = -0.00127263
 pow(-1, 1.84467e+19) = 0
 pow(-1, 1.84467e+19) = 0
 pow(-1, 9.22337e+18) = -0.00127263
 pow(-1, 9.22337e+18) = -0.00127263
 pow(-1, 1.84467e+19) = 0
 pow(-1, 1.84467e+19) = 0
 }}}

 Tested against trunk rev 8490 with clang (Apple LLVM version 4.2
 (clang-425.0.28) (based on LLVM 3.2svn)) target: x86_64-apple-darwin12.4.0
 with: -std=c++11 -stdlib=libc++.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/8809>
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