[libc++] Fix broken precondition of __bit_log2

In llvm#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 logarihm 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`.

Author: ldionne

libcxx/include/__algorithm/sort.h

@@ -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)
+    return;
+
   typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
   difference_type __depth_limit = 2 * std::__bit_log2(std::__to_unsigned_like(__last - __first));
 

libcxx/include/__bit/bit_log2.h

@@ -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);
 }
 

libcxx/src/algorithm.cpp

@@ -13,6 +13,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class Comp, class RandomAccessIterator>
 void __sort(RandomAccessIterator first, RandomAccessIterator last, Comp comp) {
+  if (first == last)
+    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