From 822c1d21a3c710831af65a6e3bc83f558fb39044 Mon Sep 17 00:00:00 2001 From: Jonathan Wakely Date: Thu, 29 Oct 2020 14:47:18 +0000 Subject: [PATCH] libstdc++: Allow Lemire's algorithm to be used in more cases This extends the fast path to also work when the URBG's range of possible values is not the entire range of its result_type. Previously, the slow path would be used for engines with a uint_fast32_t result type if that type is actually a typedef for uint64_t rather than uint32_t. After this change, the generator's result_type is not important, only the range of possible value that generator can produce. If the generator's range is exactly UINT64_MAX then the calculation will be done using 128-bit and 64-bit integers, and if the range is UINT32_MAX it will be done using 64-bit and 32-bit integers. In practice, this benefits most of the engines and engine adaptors defined in [rand.predef] on x86_64-linux and other 64-bit targets. This is because std::minstd_rand0 and std::mt19937 and others use uint_fast32_t, which is a typedef for uint64_t. The code now makes use of the recently-clarified requirement that the generator's min() and max() functions are usable in constant expressions (see LWG 2154). libstdc++-v3/ChangeLog: * include/bits/uniform_int_dist.h (_Power_of_two): Add constexpr. (uniform_int_distribution::_S_nd): Add static_assert to ensure the wider type is twice as wide as the result type. (uniform_int_distribution::__generate_impl): Add static_assert and declare variables as constexpr where appropriate. (uniform_int_distribution:operator()): Likewise. Only consider the uniform random bit generator's range of possible results when deciding whether _S_nd can be used, not the __uctype type. --- libstdc++-v3/include/bits/uniform_int_dist.h | 52 +++++++++++++------- 1 file changed, 33 insertions(+), 19 deletions(-) diff --git a/libstdc++-v3/include/bits/uniform_int_dist.h b/libstdc++-v3/include/bits/uniform_int_dist.h index cf6ba35c675..524593bb984 100644 --- a/libstdc++-v3/include/bits/uniform_int_dist.h +++ b/libstdc++-v3/include/bits/uniform_int_dist.h @@ -58,7 +58,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { /* Determine whether number is a power of 2. */ template - inline bool + constexpr bool _Power_of_2(_Tp __x) { return ((__x - 1) & __x) == 0; @@ -242,9 +242,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION static _Up _S_nd(_Urbg& __g, _Up __range) { - using __gnu_cxx::__int_traits; - static_assert(!__int_traits<_Up>::__is_signed, "U must be unsigned"); - static_assert(!__int_traits<_Wp>::__is_signed, "W must be unsigned"); + using _Up_traits = __gnu_cxx::__int_traits<_Up>; + using _Wp_traits = __gnu_cxx::__int_traits<_Wp>; + static_assert(!_Up_traits::__is_signed, "U must be unsigned"); + static_assert(!_Wp_traits::__is_signed, "W must be unsigned"); + static_assert(_Wp_traits::__digits == (2 * _Up_traits::__digits), + "W must be twice as wide as U"); // reference: Fast Random Integer Generation in an Interval // ACM Transactions on Modeling and Computer Simulation 29 (1), 2019 @@ -260,7 +263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION __low = _Up(__product); } } - return __product >> __gnu_cxx::__int_traits<_Up>::__digits; + return __product >> _Up_traits::__digits; } }; @@ -275,9 +278,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION typedef typename make_unsigned::type __utype; typedef typename common_type<_Gresult_type, __utype>::type __uctype; - const __uctype __urngmin = __urng.min(); - const __uctype __urngmax = __urng.max(); - const __uctype __urngrange = __urngmax - __urngmin; + static_assert( __urng.min() < __urng.max(), + "Uniform random bit generator must define min() < max()"); + + constexpr __uctype __urngmin = __urng.min(); + constexpr __uctype __urngmax = __urng.max(); + constexpr __uctype __urngrange = __urngmax - __urngmin; const __uctype __urange = __uctype(__param.b()) - __uctype(__param.a()); @@ -288,21 +294,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION const __uctype __uerange = __urange + 1; // __urange can be zero - using __gnu_cxx::__int_traits; +#if defined __UINT64_TYPE__ && defined __UINT32_TYPE__ #if __SIZEOF_INT128__ - if (__int_traits<__uctype>::__digits == 64 - && __urngrange == __int_traits<__uctype>::__max) + if _GLIBCXX17_CONSTEXPR (__urngrange == __UINT64_MAX__) { - __ret = _S_nd(__urng, __uerange); + // __urng produces values that use exactly 64-bits, + // so use 128-bit integers to downscale to desired range. + __UINT64_TYPE__ __u64erange = __uerange; + __ret = _S_nd(__urng, __u64erange); } else #endif - if (__int_traits<__uctype>::__digits == 32 - && __urngrange == __int_traits<__uctype>::__max) + if _GLIBCXX17_CONSTEXPR (__urngrange == __UINT32_MAX__) { - __ret = _S_nd<__UINT64_TYPE__>(__urng, __uerange); + // __urng produces values that use exactly 32-bits, + // so use 64-bit integers to downscale to desired range. + __UINT32_TYPE__ __u32erange = __uerange; + __ret = _S_nd<__UINT64_TYPE__>(__urng, __u32erange); } else +#endif { // fallback case (2 divisions) const __uctype __scaling = __urngrange / __uerange; @@ -361,9 +372,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION typedef typename make_unsigned::type __utype; typedef typename common_type<_Gresult_type, __utype>::type __uctype; - const __uctype __urngmin = __urng.min(); - const __uctype __urngmax = __urng.max(); - const __uctype __urngrange = __urngmax - __urngmin; + static_assert( __urng.min() < __urng.max(), + "Uniform random bit generator must define min() < max()"); + + constexpr __uctype __urngmin = __urng.min(); + constexpr __uctype __urngmax = __urng.max(); + constexpr __uctype __urngrange = __urngmax - __urngmin; const __uctype __urange = __uctype(__param.b()) - __uctype(__param.a()); @@ -417,7 +431,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { do { - const __uctype __uerngrange = __urngrange + 1; + constexpr __uctype __uerngrange = __urngrange + 1; __tmp = (__uerngrange * operator() (__urng, param_type(0, __urange / __uerngrange))); __ret = __tmp + (__uctype(__urng()) - __urngmin); -- 2.30.2