[boost] [multiprecision] Help needed with strange constexpr slowdown

Subject: [boost] [multiprecision] Help needed with strange constexpr slowdown
From: John Maddock (jz.maddock_at_[hidden])
Date: 2017-12-31 19:50:53

Folks I have a very strange issue that I need help with understanding, 
it comes from https://github.com/boostorg/multiprecision/issues/32 but 
in summary:

Given:

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

using  namespace  std;
using  namespace  boost::multiprecision;

int  main()
{
     __int128_t  i =0;
     __int128_t  j =0;

     int256_t  sum =0;

     for  (; i <1e9; i +=19, j +=13)
     {
         int256_t  diff = j;
         sum += i * diff;
     }

     std::cout <<"Result: "  << sum << std::endl;

     return  0;
}

There is a 20-30% slowdown when compiled in C++11 or later compared to 
C++03 with gcc 5.x or 6.x.

I've tracked this down to the constexpr default constructor for int256_t 
(which is called inside operator* to create a temporary for the result), 
if I make the default constructor non-constexpr by removing 
BOOST_CONSTEXPR from any of:

number.hpp:44 (number())

cpp_int.hpp:1077 (cpp_int_backend())

cpp_int.hpp:501 (cpp_int_base())

cpp_int.hpp: 435 (cpp_int_base::data_type())

Then the slowdown disappears.  All the line numbers refer to current 
develop BTW.

gcc-7 is better, but I notice it still generates different code if the 
constructor is constexpr - that greatly surprises me to be frank - is 
there something I'm missing about constexpr-ness that could cause this?

Confused yours, John.

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