/*

 * 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

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 */

#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