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