1 files changed, 74 insertions, 0 deletions
diff --git a/libcxx/include/__random/log2.h b/libcxx/include/__random/log2.h
new file mode 100644
index 000000000000..3d9640c1f787
--- /dev/null
+++ b/libcxx/include/__random/log2.h
@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___RANDOM_LOG2_H
+#define _LIBCPP___RANDOM_LOG2_H
+
+#include <__config>
+#include <cstddef>
+#include <type_traits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _UIntType, _UIntType _Xp, size_t _Rp>
+struct __log2_imp;
+
+template <unsigned long long _Xp, size_t _Rp>
+struct __log2_imp<unsigned long long, _Xp, _Rp>
+{
+    static const size_t value = _Xp & ((unsigned long long)(1) << _Rp) ? _Rp
+                                           : __log2_imp<unsigned long long, _Xp, _Rp - 1>::value;
+};
+
+template <unsigned long long _Xp>
+struct __log2_imp<unsigned long long, _Xp, 0>
+{
+    static const size_t value = 0;
+};
+
+template <size_t _Rp>
+struct __log2_imp<unsigned long long, 0, _Rp>
+{
+    static const size_t value = _Rp + 1;
+};
+
+#ifndef _LIBCPP_HAS_NO_INT128
+
+template <__uint128_t _Xp, size_t _Rp>
+struct __log2_imp<__uint128_t, _Xp, _Rp>
+{
+    static const size_t value = (_Xp >> 64)
+        ? (64 + __log2_imp<unsigned long long, (_Xp >> 64), 63>::value)
+        : __log2_imp<unsigned long long, _Xp, 63>::value;
+};
+
+#endif // _LIBCPP_HAS_NO_INT128
+
+template <class _UIntType, _UIntType _Xp>
+struct __log2
+{
+    static const size_t value = __log2_imp<
+#ifndef _LIBCPP_HAS_NO_INT128
+        typename conditional<
+                sizeof(_UIntType) <= sizeof(unsigned long long),
+                    unsigned long long,
+                    __uint128_t
+            >::type,
+#else
+        unsigned long long,
+#endif // _LIBCPP_HAS_NO_INT128
+        _Xp, sizeof(_UIntType) * __CHAR_BIT__ - 1>::value;
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___RANDOM_LOG2_H