[libc++][NFC] Reuse __bit_log2 for sort (#135303)

The `__log2i` function template in `<algorithm/sort.h>` is basically
equivalent to `__bit_log2` in `<__bit/bit_log2.h>`. It seems better to
avoid duplication.

Author: frederick-vs-ja

libcxx/include/__algorithm/sort.h

@@ -17,6 +17,7 @@
 #include <__algorithm/partial_sort.h>
 #include <__algorithm/unwrap_iter.h>
 #include <__assert>
+#include <__bit/bit_log2.h>
 #include <__bit/blsr.h>
 #include <__bit/countl.h>
 #include <__bit/countr.h>
@@ -34,6 +35,7 @@
 #include <__type_traits/is_constant_evaluated.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_trivially_copyable.h>
+#include <__type_traits/make_unsigned.h>
 #include <__utility/move.h>
 #include <__utility/pair.h>
 #include <climits>
@@ -826,25 +828,6 @@ void __introsort(_RandomAccessIterator __first,
   }
 }
 
-template <typename _Number>
-inline _LIBCPP_HIDE_FROM_ABI _Number __log2i(_Number __n) {
-  if (__n == 0)
-    return 0;
-  if (sizeof(__n) <= sizeof(unsigned))
-    return sizeof(unsigned) * CHAR_BIT - 1 - __libcpp_clz(static_cast<unsigned>(__n));
-  if (sizeof(__n) <= sizeof(unsigned long))
-    return sizeof(unsigned long) * CHAR_BIT - 1 - __libcpp_clz(static_cast<unsigned long>(__n));
-  if (sizeof(__n) <= sizeof(unsigned long long))
-    return sizeof(unsigned long long) * CHAR_BIT - 1 - __libcpp_clz(static_cast<unsigned long long>(__n));
-
-  _Number __log2 = 0;
-  while (__n > 1) {
-    __log2++;
-    __n >>= 1;
-  }
-  return __log2;
-}
-
 template <class _Comp, class _RandomAccessIterator>
 void __sort(_RandomAccessIterator, _RandomAccessIterator, _Comp);
 
@@ -878,7 +861,7 @@ template <class _AlgPolicy, class _RandomAccessIterator, class _Comp>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 __sort_dispatch(_RandomAccessIterator __first, _RandomAccessIterator __last, _Comp& __comp) {
   typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
-  difference_type __depth_limit = 2 * std::__log2i(__last - __first);
+  difference_type __depth_limit = 2 * std::__bit_log2(std::__to_unsigned_like(__last - __first));
 
   // Only use bitset partitioning for arithmetic types.  We should also check
   // that the default comparator is in use so that we are sure that there are no

libcxx/include/__bit/bit_log2.h

@@ -20,16 +20,12 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 14
-
 template <class _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp __bit_log2(_Tp __t) noexcept {
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __bit_log2(_Tp __t) _NOEXCEPT {
   static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__bit_log2 requires an unsigned integer type");
   return numeric_limits<_Tp>::digits - 1 - std::__countl_zero(__t);
 }
 
-#endif // _LIBCPP_STD_VER >= 14
-
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___BIT_BIT_LOG2_H