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libstdc++: Allow Lemire's algorithm to be used in more cases

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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.
This commit is contained in:
Jonathan Wakely 2020-10-29 14:47:18 +00:00
parent d067bd7293
commit 822c1d21a3
1 changed files with 33 additions and 19 deletions
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52
libstdc++-v3/include/bits/uniform_int_dist.h
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@ -58,7 +58,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
/* Determine whether number is a power of 2. */
template<typename _Tp>
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<result_type>::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<unsigned __int128>(__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<unsigned __int128>(__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<result_type>::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);