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### 1235. Issue with C++0x random number proposal

Section: 26.6.3.6 [rand.req.dist] Status: NAD Submitter: Matthias Troyer Opened: 2009-10-12 Last modified: 2019-02-26

Priority: Not Prioritized

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Discussion:

There exist optimized, vectorized vendor libraries for the creation of random number generators, such as Intel's MKL [1] and AMD's ACML [2]. In timing tests we have seen a performance gain of a factor of up to 80 (eighty) compared to a pure C++ implementation (in Boost.Random) when using these generator to generate a sequence of normally distributed random numbers. In codes dominated by the generation of random numbers (we have application codes where random number generation is more than 50% of the CPU time) this factor 80 is very significant.

To make use of these vectorized generators, we use a C++ class modeling the `RandomNumberEngine` concept and forwarding the generation of random numbers to those optimized generators. For example:

>     namespace mkl {
>      class mt19937 {.... };
>     }

For the generation of random variates we also want to dispatch to optimized vectorized functions in the MKL or ACML libraries. See this example:

>     mkl::mt19937 eng;
>     std::normal_distribution<double> dist;

>     double n = dist(eng);

Since the variate generation is done through the `operator()` of the distribution there is no customization point to dispatch to Intel's or AMD's optimized functions to generate normally distributed numbers based on the `mt19937` generator. Hence, the performance gain of 80 cannot be achieved.

Contrast this with TR1:

>     mkl::mt19937 eng;
>     std::tr1::normal_distribution<double> dist;
>     std::tr1::variate_generator<mkl::mt19937,std::tr1::normal_distribution<double> > rng(eng,dist);
>     double n = rng();

This - admittedly much uglier from an aestethic point of view - design allowed optimization by specializing the `variate_generator` template for `mkl::mt19937`:

>     namespace std { namespace tr1 {

>     template<>
>     class variate_generator<mkl::mt19937,std::tr1::normal_distribution<double> > { .... };

>     } }

A similar customization point is missing in the C++0x design and prevents the optimized vectorized version to be used.

Suggested resolution:

Add a customization point to the distribution concept. Instead of the `variate_generator` template this can be done through a call to a free function `generate_variate` found by ADL instead of `operator()` of the distribution:

>     template <RandomNumberDistribution, class RandomNumberEngine>
>     typename RandomNumberDistribution ::result_type
>     generate_variate(RandomNumberDistribution const& dist, RandomNumberEngine& eng);

This function can be overloaded for optimized enginges like `mkl::mt19937`.

[ 2009-10 Santa Cruz: ]

> NAD Future. No time to add this feature for C++0X.

[LEWG Kona 2017]

Recommend NAD: The standard has changed enough that the issue doesn't make sense anymore. Write a paper proposing a way to get this performance as changes to the current library.

[Kona 2019: Jonathan notes:]

Libstdc++ has the following non-standard extensions for more efficient generation of large numbers of random numbers:

>     template<typename ForwardIterator, typename UniformRandomNumberGenerator>
>     void __generate(ForwardIterator, ForwardIterator,
>                     UniformRandomNumberGenerator&);

>     template<typename ForwardIterator, typename UniformRandomNumberGenerator>
>     void __generate(ForwardIterator, ForwardIterator,
>                     UniformRandomNumberGenerator&, const param_type&);

>     template<typename UniformRandomNumberGenerator>
>     void __generate(result_type*, result_type*,
>                     UniformRandomNumberGenerator&, const param_type&);

Proposed resolution: