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[libc++] Fix broken precondition of __bit_log2 (#155476)

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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`.
This commit is contained in:
Louis Dionne 2025-08-28 18:07:59 -04:00 committed by GitHub
parent 6b4506009a
commit 2ae4b92a1c
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3 changed files with 8 additions and 0 deletions
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3
libcxx/include/__algorithm/sort.h
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@ -860,6 +860,9 @@ __sort<__less<long double>&, long double*>(long double*, long double*, __less<lo
template <class _AlgPolicy, class _RandomAccessIterator, class _Comp>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
__sort_dispatch(_RandomAccessIterator __first, _RandomAccessIterator __last, _Comp& __comp) {
if (__first == __last) // log(0) is undefined, so don't try computing the depth
return;
typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
difference_type __depth_limit = 2 * std::__bit_log2(std::__to_unsigned_like(__last - __first));

2
libcxx/include/__bit/bit_log2.h
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@ -9,6 +9,7 @@
#ifndef _LIBCPP___BIT_BIT_LOG2_H
#define _LIBCPP___BIT_BIT_LOG2_H
#include <__assert>
#include <__bit/countl.h>
#include <__config>
#include <__type_traits/integer_traits.h>
@ -23,6 +24,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
template <class _Tp>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __bit_log2(_Tp __t) _NOEXCEPT {
static_assert(__is_unsigned_integer_v<_Tp>, "__bit_log2 requires an unsigned integer type");
_LIBCPP_ASSERT_INTERNAL(__t != 0, "logarithm of 0 is undefined");
return numeric_limits<_Tp>::digits - 1 - std::__countl_zero(__t);
}

3
libcxx/src/algorithm.cpp
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@ -13,6 +13,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
template <class Comp, class RandomAccessIterator>
void __sort(RandomAccessIterator first, RandomAccessIterator last, Comp comp) {
if (first == last) // log(0) is undefined, so don't try computing the depth
return;
auto depth_limit = 2 * std::__bit_log2(static_cast<size_t>(last - first));
// Only use bitset partitioning for arithmetic types. We should also check