Re: [boost] [review] Multiprecision review (June 8th - 17th, 2012)

Subject: Re: [boost] [review] Multiprecision review (June 8th - 17th, 2012)
From: Sergey Mitsyn (svm_at_[hidden])
Date: 2012-07-29 12:13:54

Hello,

Not quite a review as it's way too late :), but some comments on 
probably interesting experience I've got while trying out multiprecision 
from sandbox together with Boost.UBLAS . Both libraries are generic in 
the way that one can be used instead of builtin float and double, and 
the other should support such types.

I've found out that they don't play well together, probably due to the 
fact that UBLAS uses its own expression templates. A simple example of 
code that fails to compile is below:

------------------

#include <boost/numeric/ublas/vector.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>

typedef boost::multiprecision::mp_number< 
boost::multiprecision::cpp_dec_float<50> , true > float_type;

int main(int argc, char* argv[])
{
     boost::numeric::ublas::c_vector<float_type, 3> v1,v2;
     inner_prod( v1, v2 );
}

-----------------

Fails with MSVC 9 and a little old clang from thunk (--version prints 
clang version 3.2 (trunk 156987)).

The easy fix is to turn off expression templates inside mp_number.

It also fails even with expression templates being turned off whenever 
it tries to compare mp_number with a convertible type:

-------------------------

#include <boost/multiprecision/cpp_dec_float.hpp>

typedef boost::multiprecision::mp_number< 
boost::multiprecision::cpp_dec_float<50> , false > float_type;

struct A
{
     operator float_type() const
     {
         return (float_type)1.0f;
     }
};

int main(int argc, char* argv[])
{
     float_type f = 123.0f;
     A a;

     bool r = (a < f);
}

-------------------------

Such constructions appear sometimes within UBLAS (e.g., lu_factorize 
with NDEBUG undefined). The fix is probably non-trivial. I've 
successfully overcome it by modifying is_valid_comparison_imp:

------------------------

template <class Exp1, class Exp2>
struct is_valid_comparison_imp
{
    <.......skipped.......>
       mpl::and_<
          is2,
          mpl::or_<
             is1,
             is_arithmetic<Exp1>
          >
       >,
+      mpl::and_<
+          is_convertible<Exp1,Exp2>,
+          is2
+      >,
+      mpl::and_<
+          is_convertible<Exp2,Exp1>,
+          is1
+      >
    >::type type;
};

----------------------------

Though I didn't run the tests - it might break something.

Also running the tests for UBLAS with mp_number put in place of 
floats/doubles would be interesting.

>
>      Do you think the library should be accepted as a Boost library?
>

Whenever issues with UBLAS interoperability are fixed (or some 
reasonable decision is made).

-- 
------------
Sergey Mitsyn.