--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/share/utilities/count_leading_zeros.hpp	Mon Jan 28 23:00:31 2019 +0100
@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_UTILITIES_COUNT_LEADING_ZEROS_HPP
+#define SHARE_UTILITIES_COUNT_LEADING_ZEROS_HPP
+
+#include "utilities/debug.hpp"
+#include "utilities/globalDefinitions.hpp"
+#include "utilities/count_trailing_zeros.hpp"
+
+#if defined(TARGET_COMPILER_visCPP)
+#include <intrin.h>
+#pragma intrinsic(_BitScanReverse)
+#elif defined(TARGET_COMPILER_xlc)
+#include <builtins.h>
+#endif
+
+// uint32_t count_leading_zeros(uint32_t x)
+// Return the number of leading zeros in x, e.g. the zero-based index
+// of the most significant set bit in x.  Undefined for 0.
+inline uint32_t count_leading_zeros(uint32_t x) {
+  assert(x != 0, "precondition");
+#if defined(TARGET_COMPILER_gcc)
+  return __builtin_clz(x);
+#elif defined(TARGET_COMPILER_visCPP)
+  unsigned long index;
+  _BitScanReverse(&index, x);
+  return index ^ 31u;
+#elif defined(TARGET_COMPILER_xlc)
+  return __cntlz4(x);
+#else
+  // Efficient and portable fallback implementation:
+  // http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
+  // - with positions xor'd by 31 to get number of leading zeros
+  // rather than position of highest bit.
+  static const int MultiplyDeBruijnBitPosition[32] = {
+      31, 22, 30, 21, 18, 10, 29,  2, 20, 17, 15, 13, 9,  6, 28, 1,
+      23, 19, 11,  3, 16, 14,  7, 24, 12,  4,  8, 25, 5, 26, 27, 0
+  };
+
+  x |= x >> 1; // first round down to one less than a power of 2
+  x |= x >> 2;
+  x |= x >> 4;
+  x |= x >> 8;
+  x |= x >> 16;
+  return MultiplyDeBruijnBitPosition[(uint32_t)( x * 0x07c4acddu ) >> 27];
+#endif
+}
+
+#endif // SHARE_UTILITIES_COUNT_LEADING_ZEROS_HPP