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2ae4b92a1c
In #135303, we started using `__bit_log2` instead of `__log2i` inside `std::sort`. However, `__bit_log2` has a precondition that `__log2i` didn't have, which is that the input is non-zero. While it technically makes no sense to request the logarithm of 0, `__log2i` handled that case and returned 0 without issues. After switching to `__bit_log2`, passing 0 as an input results in an unsigned integer overflow which can trigger `-fsanitize=unsigned-integer-overflow`. While not technically UB in itself, it's clearly not intended either. To fix this, we add an internal assertion to `__bit_log2` which catches the issue in our test suite, and we make sure not to violate `__bit_log2`'s preconditions before we call it from `std::sort`.
34 lines
1.1 KiB
C++
34 lines
1.1 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___BIT_BIT_LOG2_H
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#define _LIBCPP___BIT_BIT_LOG2_H
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#include <__assert>
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#include <__bit/countl.h>
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#include <__config>
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#include <__type_traits/integer_traits.h>
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#include <limits>
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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 _Tp>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __bit_log2(_Tp __t) _NOEXCEPT {
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static_assert(__is_unsigned_integer_v<_Tp>, "__bit_log2 requires an unsigned integer type");
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_LIBCPP_ASSERT_INTERNAL(__t != 0, "logarithm of 0 is undefined");
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return numeric_limits<_Tp>::digits - 1 - std::__countl_zero(__t);
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}
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___BIT_BIT_LOG2_H
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