Make RandomNumberGenerator compatible with <random>

[serge-sans-paille]

Summary

As described in https://llvm.org/bugs/show_bug.cgi?id=25105 LLVM's RandomNumberGenerator is not compatible with the random distribution from <random>.

This microscopic patch updates the API accordingly.

[mehdi_amini]

Can you add a unittest for this? (That'd show that PR25105 is fixed)

include/llvm/Support/RandomNumberGenerator.h

41

Isn't this comment more on point if you place it before the using generator_type = std::mt19937_64; line above?

[serge-sans-paille]

New diff that takes into account Mehdi's comments.

A few unittest that test both C++11 compatibility and reproducibility.
More comment locality ;-)

[mehdi_amini]

LGTM.

[mehdi_amini]

r283854

include/llvm/Support/RandomNumberGenerator.h

 /// Module::createRNG to create a new RNG instance for use with that
 /// module.
 class RandomNumberGenerator {
+
+  using generator_type = std::mt19937_64;
+
 public:
+  // 64-bit Mersenne Twister by Matsumoto and Nishimura, 2000
+  // http://en.cppreference.com/w/cpp/numeric/random/mersenne_twister_engine
+  // This RNG is deterministically portable across C++11
+  // implementations.
+  using result_type = generator_type::result_type;
+
   /// Returns a random number in the range [0, Max).
-  uint_fast64_t operator()();
+  result_type operator()();
+  static constexpr result_type min() { return generator_type::min(); }
+  static constexpr result_type max() { return generator_type::max(); }
 
 private:
   /// Seeds and salts the underlying RNG engine.

   /// Module::createRNG to create a new RNG salted with the Module ID.
   RandomNumberGenerator(StringRef Salt);
 
-  // 64-bit Mersenne Twister by Matsumoto and Nishimura, 2000
-  // http://en.cppreference.com/w/cpp/numeric/random/mersenne_twister_engine
-  // This RNG is deterministically portable across C++11
-  // implementations.
-  std::mt19937_64 Generator;
+  generator_type Generator;
 
   // Noncopyable.
   RandomNumberGenerator(const RandomNumberGenerator &other) = delete;

lib/Support/RandomNumberGenerator.cpp

   Generator.seed(SeedSeq);
 }
 
-uint_fast64_t RandomNumberGenerator::operator()() {
+RandomNumberGenerator::result_type RandomNumberGenerator::operator()() {
   return Generator();
 }