**Section:** 29.6.1.6 [rand.req.dist] **Status:** LEWG
**Submitter:** Matthias Troyer **Opened:** 2009-10-12 **Last modified:** 2017-04-24

**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.

**Proposed resolution:**