72 lines
3.0 KiB
C++
72 lines
3.0 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 "metaprogramming/conditional.hpp"
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#include "metaprogramming/enableIf.hpp"
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#include "metaprogramming/isIntegral.hpp"
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#include "metaprogramming/isSigned.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 and the text that
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// follows.
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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. For example, with current build configurations, __builtin_popcount(x)
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// generate a call to a similar but slower 64-bit version when calling with
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// a 32-bit integer type.
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template <typename T>
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inline unsigned population_count(T x) {
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STATIC_ASSERT(BitsPerWord <= 128);
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STATIC_ASSERT(BitsPerByte == 8);
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STATIC_ASSERT(IsIntegral<T>::value);
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STATIC_ASSERT(!IsSigned<T>::value);
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// We need to take care with implicit integer promotion when dealing with
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// integers < 32-bit. We chose to do this by explicitly widening constants
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// to unsigned
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typedef typename Conditional<(sizeof(T) < sizeof(unsigned)), unsigned, T>::type P;
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const T all = ~T(0); // 0xFF..FF
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const P fives = all/3; // 0x55..55
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const P threes = (all/15) * 3; // 0x33..33
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const P z_ones = all/255; // 0x0101..01
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const P z_effs = z_ones * 15; // 0x0F0F..0F
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P r = x;
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r -= ((r >> 1) & fives);
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r = (r & threes) + ((r >> 2) & threes);
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r = ((r + (r >> 4)) & z_effs) * z_ones;
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// The preceeding multiply by z_ones is the only place where the intermediate
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// calculations can exceed the range of T. We need to discard any such excess
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// before the right-shift, hence the conversion back to T.
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return static_cast<T>(r) >> (((sizeof(T) - 1) * BitsPerByte));
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}
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#endif // SHARE_UTILITIES_POPULATION_COUNT_HPP
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