Re: [boost] [math] Comeau can't match math::tools::fmod_workaround

Subject: Re: [boost] [math] Comeau can't match math::tools::fmod_workaround
From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2009-11-26 14:48:27

Mateusz Loskot wrote:
> Mateusz Loskot wrote:
>> Hi,
>>
>> I'm not sure if I should submit this issue directly to Trac, so I 
>> decided to post it here first. Compiling Boost.Math using Comeau +
>> GCC on Linux throws error that suggests mismatch of dobule and long
>> double where single type T is expected:
> 
> I've just found very similar issue in other place. It is std::pow
> specialisations seem ambiguous for Comeau C/C++ compiler.
> 
> "../../../boost/math/special_functions/spherical_harmonic.hpp", line 39:
> error:
>           more than one instance of overloaded function "pow" matches the
>           argument list:
>             function "std::pow(const std::complex<long double> &, const long
>                       double &)"
>             function "std::pow(const double &, const
> std::complex<double> &)"
>             function "std::pow(long double, long double)"
>             function "std::pow(double, int)"
>             function "std::pow(double, double)"
>             argument types are: (double, long double)
>      T leg = detail::legendre_p_imp(n, m, x, pow(fabs(sin_theta), T(m)),
> pol);
>                                              ^
>           detected during:
>             instantiation of "T
> 
> boost::math::detail::spherical_harmonic_prefix(unsigned
>                       int, unsigned int, T, const Policy &) [with T=long
>                       double, Policy=c_policies::c_policy]" at line 71
>             instantiation of "T
>                       boost::math::detail::spherical_harmonic_r(unsigned
> int,
>                       int, T, T, const Policy &) [with T=long double,
>                       Policy=c_policies::c_policy]" at line 172

Similarly to original issue with math::tools::fmod_workaround, caching
intermediate values solves the problem.
Replacing line 39 in boost/math/special_functions/spherical_harmonic.hpp
with cached fabs and pow results:

   T f = fabs(sin_theta);
   T pfm = pow(f, T(m));
   T leg = detail::legendre_p_imp(n, m, x, pfm, pol);

By the way, I'm wondering, is it intentional, in Boost.Math, to omit
const qualifier for intermediate results? I know modern compilers can
optimise perfectly well even if const is not present, but *perhaps* it
would help some compilers to in optimisation.

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
Charter Member of OSGeo, http://osgeo.org