[Boost-bugs] [Boost C++ Libraries] #11999: int128_t from cpp_int gives unexpected results when bitshifting

Subject: [Boost-bugs] [Boost C++ Libraries] #11999: int128_t from cpp_int gives unexpected results when bitshifting
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-02-18 14:49:24

#11999: int128_t from cpp_int gives unexpected results when bitshifting
-----------------------------------------------+---------------------------
 Reporter:  Jordi Vermeulen <J.L.Vermeulen@…>  |      Owner:  johnmaddock
     Type:  Bugs                               |     Status:  new
Milestone:  To Be Determined                   |  Component:
  Version:  Boost 1.60.0                       |  multiprecision
 Keywords:                                     |   Severity:  Problem
-----------------------------------------------+---------------------------
 The 128-bit signed integer int128_t from cpp_int gives some unexpected
 results when using the bitshift operator. If I shift the value -1 left by
 31 bits, then shift it back right by 32 bits, I get the value -0 instead
 of -1. I do not encounter this problem with the signed integers of larger
 size (256, 512, 1024). The code below illustrates the problem. I'm using
 g++ 5.3.0 on Linux Mint (I can test it with Visual Studio 2015 on Windows
 7 later if desired).

 To be clear: this only seems to happen when shifting right further than
 left. If I shift right by 31 they all print -1. It's also not just a bug
 in the conversion to string: if I multiply by 10, int128_t prints 0, but
 the rest prints -10.

 {{{
 #include <iostream>
 #include <cstdint>
 #include <boost/multiprecision/cpp_int.hpp>

 using namespace boost::multiprecision;

 int main()
 {
     int64_t i0 = -1;
     i0 <<= 31;
     i0 >>= 32;
     std::cout << i0 << std::endl; // prints -1

     int128_t i1 = -1;
     i1 <<= 31;
     i1 >>= 32;
     std::cout << i1 << std::endl; // prints -0

     int256_t i2 = -1;
     i2 <<= 31;
     i2 >>= 32;
     std::cout << i2 << std::endl; // prints -1

     int512_t i3 = -1;
     i3 <<= 31;
     i3 >>= 32;
     std::cout << i3 << std::endl; // prints -1

     int1024_t i4 = -1;
     i4 <<= 31;
     i4 >>= 32;
     std::cout << i4 << std::endl; // prints -1

     return 0;
 }
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11999>
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