From 60d9f254876a00260992b2f37639ef4d82d9db8f Mon Sep 17 00:00:00 2001 From: Jonathan Wakely Date: Sat, 31 Oct 2020 07:16:47 +0000 Subject: [PATCH] libstdc++: Prefer double to long double in std::shuffle_order_engine The transition algorithm for std::shuffle_order_engine uses long double to ensure that the value (max() - min() + 1) can be accurately represented, to avoid bias in the shuffling. However, when the base engine's range is small enough we can avoid slower long double arithmetic by using double. For example, long double is unnecessary for any base engine returning 32-bit values. This makes std::knuth_b::operator() about 15% faster on x86_64, and probably even more on targets where long double uses soft-float. libstdc++-v3/ChangeLog: * include/bits/random.h (independent_bit_engine): Fix typo in comment. (shuffle_order_engine): Fix incorrect description in comment. * include/bits/random.tcc (__representable_as_double (__p1_representable_as_double): New helper functions. (shuffle_order_engine::operator()): Use double for calculation if (max() - min() + 1) is representable as double. * testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error line number. --- libstdc++-v3/include/bits/random.h | 10 +++-- libstdc++-v3/include/bits/random.tcc | 38 ++++++++++++++++++- .../26_numerics/random/pr60037-neg.cc | 2 +- 3 files changed, 43 insertions(+), 7 deletions(-) diff --git a/libstdc++-v3/include/bits/random.h b/libstdc++-v3/include/bits/random.h index 0be1191e07d..32537f80df7 100644 --- a/libstdc++-v3/include/bits/random.h +++ b/libstdc++-v3/include/bits/random.h @@ -1099,7 +1099,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION /** * Produces random numbers by combining random numbers from some base - * engine to produce random numbers with a specifies number of bits @p __w. + * engine to produce random numbers with a specified number of bits @p __w. */ template class independent_bits_engine @@ -1316,9 +1316,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION /** - * @brief Produces random numbers by combining random numbers from some - * base engine to produce random numbers with a specifies number of bits - * @p __k. + * @brief Produces random numbers by reordering random numbers from some + * base engine. + * + * The values from the base engine are stored in a sequence of size @p __k + * and shuffled by an algorithm that depends on those values. */ template class shuffle_order_engine diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc index bf39a51559b..3205442f2f6 100644 --- a/libstdc++-v3/include/bits/random.tcc +++ b/libstdc++-v3/include/bits/random.tcc @@ -804,13 +804,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION constexpr size_t shuffle_order_engine<_RandomNumberEngine, __k>::table_size; + namespace __detail + { + // Determine whether an integer is representable as double. + template + constexpr bool + __representable_as_double(_Tp __x) noexcept + { + static_assert(numeric_limits<_Tp>::is_integer); + static_assert(!numeric_limits<_Tp>::is_signed); + // All integers <= 2^53 are representable. + return (__x <= (1ull << __DBL_MANT_DIG__)) + // Between 2^53 and 2^54 only even numbers are representable. + || (!(__x & 1) && __detail::__representable_as_double(__x >> 1)); + } + + // Determine whether x+1 is representable as double. + template + constexpr bool + __p1_representable_as_double(_Tp __x) noexcept + { + static_assert(numeric_limits<_Tp>::is_integer); + static_assert(!numeric_limits<_Tp>::is_signed); + return numeric_limits<_Tp>::digits < __DBL_MANT_DIG__ + || (bool(__x + 1u) // return false if x+1 wraps around to zero + && __detail::__representable_as_double(__x + 1u)); + } + } + template typename shuffle_order_engine<_RandomNumberEngine, __k>::result_type shuffle_order_engine<_RandomNumberEngine, __k>:: operator()() { - size_t __j = __k * ((_M_y - _M_b.min()) - / (_M_b.max() - _M_b.min() + 1.0L)); + constexpr result_type __range = max() - min(); + size_t __j = __k; + const result_type __y = _M_y - min(); + // Avoid using slower long double arithmetic if possible. + if _GLIBCXX17_CONSTEXPR (__detail::__p1_representable_as_double(__range)) + __j *= __y / (__range + 1.0); + else + __j *= __y / (__range + 1.0L); _M_y = _M_v[__j]; _M_v[__j] = _M_b(); diff --git a/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc b/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc index 0b5f597040b..ba252ef34fe 100644 --- a/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc +++ b/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc @@ -12,4 +12,4 @@ auto x = std::generate_canonical