https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85439
--- Comment #5 from W E Brown <webrown.cpp at gmail dot com> --- (In reply to Marc "Foddex" Oude Kotte from comment #4) > The reason I was expecting the same result everywhere is because of this > statement on cppreference.com: > > > "Notes > The 10000th consecutive invocation of a default-contructed std::mt19937 is > required to produce the value 4123659995. > > The 10000th consecutive invocation of a default-contructed std::mt19937_64 > is required to produce the value 9981545732273789042" > > > Source: > http://en.cppreference.com/w/cpp/numeric/random/mersenne_twister_engine > > Obviously I'm not testing the 10000th consecutive invocation but the 1st, > but I still expected things to be similar (and they are, in 7/8 test cases). The cited text is correct, but the attached program doesn't test it. In particular, the attached program tests nothing about calling any Mersenne twister object; rather, it looks at the results of calling uniform_int_distribution objects. As specified in C++, distribution objects are not required to produce identical results across platforms, even if given identical inputs. Any expectation to the contrary is not supported by the C++ standard. ISTM that testing the behavior of std::mt19937 and std::mt19937_64 -- or of any engine, for that matter -- ought not involve any distribution object at all. Instead, such a test program should IMO be looking at the result(s) of directly invoking the engine. Comment #2 was exactly right. I recommend this report be closed as invalid.
