Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

(Possible) cpp_bin_float_quad to checked_int128_t conversion bug #110

esakaen opened this issue Feb 3, 2019 · 3 comments


None yet
2 participants
Copy link

commented Feb 3, 2019

Whilst the above conversion worked for some test cases, the following error was thrown for the conversion of a cpp_bin_float_quad with a value of -1.75329768098667154050329834e+26: "Bitwise operations on negative values results in undefined behavior." It occurred with Visual Studio 2017 as well as with Xcode 10.1 (in both cases using the C++17 flag). Interestingly, it works, i.e., gives the correct result, if the value is converted into an unchecked int128_t. Provided that I understood the documentation correctly, an inadmissible conversion should result in a compiler error, which his not the case here.

For convenience, I attached a test program as well as the Visual Studio output and call stack to is_valid_bitwise_op below.

#include <iostream>
#include <exception>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_bin_float.hpp>

int main( int argc, char* argv[] )
  using namespace boost::multiprecision;

  std::cout.precision( 39 );
  std::cout.setf( std::ios::scientific );

  cpp_bin_float_quad qF( "-175329768098667154050329834" );

  std::cout << qF << std::endl;

  int128_t qI( qF );

  std::cout << qI << std::endl;

    checked_int128_t qCI( qF );

    std::cout << qCI << std::endl;
  catch ( const std::exception& e )
    std::cout << e.what() << std::endl;

  return 0;
Bitwise operations on negative values results in undefined behavior.
>   LongTypeTest2.exe!boost::multiprecision::backends::is_valid_bitwise_op<128,128,1,void>(const boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void> & result, const boost::mpl::int_<1> & __formal) Line 37 C++
    LongTypeTest2.exe!boost::multiprecision::backends::eval_left_shift<128,128,1,1,void>(boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void> & result, unsigned __int64 s) Line 435  C++
    LongTypeTest2.exe!boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void>,0>::operator<<=<int>(int val) Line 667   C++
    LongTypeTest2.exe!boost::multiprecision::detail::generic_interconvert_float2int<boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void>,boost::multiprecision::backends::cpp_bin_float<113,2,void,short,-16382,16383> >(boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void> & to, const boost::multiprecision::backends::cpp_bin_float<113,2,void,short,-16382,16383> & from, const boost::mpl::int_<2> & __formal) Line 492  C++
    LongTypeTest2.exe!boost::multiprecision::detail::generic_interconvert<boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void>,boost::multiprecision::backends::cpp_bin_float<113,2,void,short,-16382,16383> >(boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void> & to, const boost::multiprecision::backends::cpp_bin_float<113,2,void,short,-16382,16383> & from, const boost::mpl::int_<0> & __formal, const boost::mpl::int_<1> & __formal) Line 526  C++
    LongTypeTest2.exe!boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void>,0>::number<boost::multiprecision::backends::cpp_int_backend<128,128,1,1,void>,0><boost::multiprecision::backends::cpp_bin_float<113,2,void,short,-16382,16383>,0>(const boost::multiprecision::number<boost::multiprecision::backends::cpp_bin_float<113,2,void,short,-16382,16383>,0> & val, void * __formal) Line 136  C++
    LongTypeTest2.exe!main(int argc, char * * argv) Line 25 C++

This comment has been minimized.

Copy link

commented Feb 3, 2019

That's a weird one. Smells like a bug, will investigate.


This comment has been minimized.

Copy link

commented Feb 13, 2019

It's a bug, see: #115


This comment has been minimized.

Copy link

commented Mar 4, 2019

Fix wll be in the next release.

@jzmaddock jzmaddock closed this Mar 4, 2019

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.