b254c2e2c4
Fixes https://llvm.org/PR51520. The problem is that `uniform_int_distribution` currently uses an unsigned integer with at most 64 bits internally, which is then casted to the desired result type. If the result type is `int64_t`, this will produce a negative number if the most significant bit is set, but if the result type is `__int128_t`, the value remains non-negative and will be out of bounds for the example in PR#51520. (The reason why it also seems to work if the upper or lower bound is changed is because the branch at [1] will then no longer be taken, and proper rejection sampling takes place.) The bigger issue here is probably that `uniform_int_distribution` can be instantiated with `__int128_t` but will silently produce incorrect results (only the lowest 64 bits can ever be set). libstdc++ also supports `__int128_t` as a result type, so I have simply extended the maximum width of the internal intermediate result type. [1]: https://github.com/llvm/llvm-project/blob/6d28dffb6/libcxx/include/__random/uniform_int_distribution.h#L266-L267 Differential Revision: https://reviews.llvm.org/D114129
75 lines
2.0 KiB
C++
75 lines
2.0 KiB
C++
//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___RANDOM_LOG2_H
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#define _LIBCPP___RANDOM_LOG2_H
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#include <__config>
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#include <cstddef>
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#include <type_traits>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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#pragma GCC system_header
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#endif
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_LIBCPP_BEGIN_NAMESPACE_STD
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template <class _UIntType, _UIntType _Xp, size_t _Rp>
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struct __log2_imp;
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template <unsigned long long _Xp, size_t _Rp>
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struct __log2_imp<unsigned long long, _Xp, _Rp>
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{
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static const size_t value = _Xp & ((unsigned long long)(1) << _Rp) ? _Rp
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: __log2_imp<unsigned long long, _Xp, _Rp - 1>::value;
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};
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template <unsigned long long _Xp>
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struct __log2_imp<unsigned long long, _Xp, 0>
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{
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static const size_t value = 0;
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};
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template <size_t _Rp>
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struct __log2_imp<unsigned long long, 0, _Rp>
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{
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static const size_t value = _Rp + 1;
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};
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#ifndef _LIBCPP_HAS_NO_INT128
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template <__uint128_t _Xp, size_t _Rp>
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struct __log2_imp<__uint128_t, _Xp, _Rp>
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{
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static const size_t value = (_Xp >> 64)
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? (64 + __log2_imp<unsigned long long, (_Xp >> 64), 63>::value)
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: __log2_imp<unsigned long long, _Xp, 63>::value;
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};
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#endif // _LIBCPP_HAS_NO_INT128
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template <class _UIntType, _UIntType _Xp>
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struct __log2
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{
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static const size_t value = __log2_imp<
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#ifndef _LIBCPP_HAS_NO_INT128
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typename conditional<
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sizeof(_UIntType) <= sizeof(unsigned long long),
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unsigned long long,
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__uint128_t
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>::type,
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#else
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unsigned long long,
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#endif // _LIBCPP_HAS_NO_INT128
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_Xp, sizeof(_UIntType) * __CHAR_BIT__ - 1>::value;
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};
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___RANDOM_LOG2_H
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