boost :: integer_mask gcc编译器错误

Question

I'm unable to compile the following code (using boost 1.61 ).

#include <boost/integer/integer_mask.hpp>
#include <iostream>

int main() {
    uint a =  boost::low_bits_mask_t<1>::sig_bits;
    std::cout << "bitmask " << a << std::endl;
}

On clang it compiles fine.
With g++ (version 6.2.1) I get

 file included from /usr/include/boost/config.hpp:61:0,
                 from /usr/include/boost/integer_fwd.hpp:15,
                 from /usr/include/boost/integer/integer_mask.hpp:13,
                 from boostbug.cpp:1:
/usr/include/boost/integer/integer_mask.hpp: In instantiation of ‘const least boost::low_bits_mask_t<1ul>::sig_bits’:
boostbug.cpp:7:42:   required from here
/usr/include/boost/integer/integer_mask.hpp:66:5: error: left operand of shift expression ‘(-1 << 1ul)’ is negative [-fpermissive]
     BOOST_STATIC_CONSTANT( least, sig_bits = (~( ~(least( 0u )) << Bits )) );
     ^

What am I doing wrong?
Is this a gcc or a boost bug?

Answer 1

Let's analyze the ~(least( 0u )) << Bits expression. Why does this become -1? First, the least type will be the smallest type that can hold the number of bits you want (1). This will be unsigned char , so the expression now looks like ~(unsigned char( 0u )) << Bits . The ~ operator will perform integral promotions. This results in the unsigned char being promoted to int (signed), because that type is the first in the list of types that an unsigned char can be promoted to and it will hold all the possible values. The ones' complement of 0, when interpreted as a signed integer in a system that uses a twos complement is -1. The shift left of a negative integer is undefined behavior, which is why you get the error you see.

sig_bits_fast has this same problem, so it does not provide a solution.

This is a boost bug, since the boost header code relies on undefined behavior (left shift of a signed quantity).

Answer 2

I do not think that you do anything wrong. It is clearly compiler bug. I compiled your code with online http://cpp.sh/ compiler.

I could also check it with VS2015 and boost 1.58.

#include <boost/integer/integer_mask.hpp>
#include <iostream>
int main()
{
  uint a0 =  boost::low_bits_mask_t<0>::sig_bits;
  std::cout << "bitmask <0> = " << a0 << std::endl;   
  uint a =  boost::low_bits_mask_t<1>::sig_bits;
  std::cout << "bitmask <1> = " << a << std::endl;    
  uint b =  boost::low_bits_mask_t<2>::sig_bits;    
  std::cout << "bitmask <2> = " << b << std::endl;   
  uint c =  boost::low_bits_mask_t<3>::sig_bits;    
  std::cout << "bitmask <3> = " << c << std::endl;   
  uint d =  boost::low_bits_mask_t<4>::sig_bits;    
  std::cout << "bitmask <4> = " << d << std::endl;
}

Result:

bitmask <0> = 0
bitmask <1> = 1
bitmask <2> = 3
bitmask <3> = 7
bitmask <4> = 15