1 /*
   2  * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #ifndef SHARE_UTILITIES_COUNT_LEADING_ZEROS_HPP
  26 #define SHARE_UTILITIES_COUNT_LEADING_ZEROS_HPP
  27 
  28 #include "utilities/debug.hpp"
  29 #include "utilities/globalDefinitions.hpp"
  30 #include "utilities/count_trailing_zeros.hpp"
  31 
  32 #if defined(TARGET_COMPILER_visCPP)
  33 #include <intrin.h>
  34 #pragma intrinsic(_BitScanReverse)
  35 #elif defined(TARGET_COMPILER_xlc)
  36 #include <builtins.h>
  37 #endif
  38 
  39 // uint32_t count_leading_zeros(uint32_t x)
  40 // Return the number of leading zeros in x, e.g. the zero-based index
  41 // of the most significant set bit in x.  Undefined for 0.
  42 inline uint32_t count_leading_zeros(uint32_t x) {
  43   assert(x != 0, "precondition");
  44 #if defined(TARGET_COMPILER_gcc)
  45   return __builtin_clz(x);
  46 #elif defined(TARGET_COMPILER_visCPP)
  47   unsigned long index;
  48   _BitScanReverse(&index, x);
  49   return index ^ 31u;
  50 #elif defined(TARGET_COMPILER_xlc)
  51   return __cntlz4(x);
  52 #else
  53   // Efficient and portable fallback implementation:
  54   // http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
  55   // - with positions xor'd by 31 to get number of leading zeros
  56   // rather than position of highest bit.
  57   static const int MultiplyDeBruijnBitPosition[32] = {
  58       31, 22, 30, 21, 18, 10, 29,  2, 20, 17, 15, 13, 9,  6, 28, 1,
  59       23, 19, 11,  3, 16, 14,  7, 24, 12,  4,  8, 25, 5, 26, 27, 0
  60   };
  61 
  62   x |= x >> 1; // first round down to one less than a power of 2
  63   x |= x >> 2;
  64   x |= x >> 4;
  65   x |= x >> 8;
  66   x |= x >> 16;
  67   return MultiplyDeBruijnBitPosition[(uint32_t)( x * 0x07c4acddu ) >> 27];
  68 #endif
  69 }
  70 
  71 #endif // SHARE_UTILITIES_COUNT_LEADING_ZEROS_HPP