48 lines
2.0 KiB
C++
48 lines
2.0 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_POPULATION_COUNT_HPP
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#define SHARE_UTILITIES_POPULATION_COUNT_HPP
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#include "utilities/debug.hpp"
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#include "utilities/globalDefinitions.hpp"
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// Returns the population count of x, i.e., the number of bits set in x.
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//
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// Adapted from Hacker's Delight, 2nd Edition, Figure 5-2.
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//
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// Ideally this should be dispatched per platform to use optimized
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// instructions when available, such as POPCNT on modern x86/AMD. Our builds
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// still target and support older architectures that might lack support for
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// these, however. For example, with current build configurations,
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// __builtin_popcount(x) would generate a call to a similar but slower 64-bit
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// version of this 32-bit implementation.
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static uint32_t population_count(uint32_t x) {
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x -= ((x >> 1) & 0x55555555);
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x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
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return (((x + (x >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
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}
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#endif // SHARE_UTILITIES_POPULATION_COUNT_HPP
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