libcxx: backport llvm/llvm-project#155476

https: //github.com/llvm/llvm-project/pull/155476
(cherry picked from commit 820dc9d)
Change-Id: I01c204c23df89dec6f7be874a079ddc54e376b36

Author: alexrp

lib/libcxx/include/__algorithm/sort.h

@@ -879,6 +879,9 @@ __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::__log2i(__last - __first);
 

lib/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/is_unsigned_integer.h>
@@ -25,6 +26,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr _Tp __bit_log2(_Tp __t) noexcept {
   static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__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);
 }
 

lib/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) // 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