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