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Datei: moveBits.hpp Sprache: C

/*
* Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/

#ifndef SHARE_UTILITIES_MOVEBITS_HPP
#define SHARE_UTILITIES_MOVEBITS_HPP

#include "metaprogramming/conditional.hpp"
#include "metaprogramming/enableIf.hpp"
#include "utilities/globalDefinitions.hpp"
#include <type_traits>

template <typename T>
class ReverseBitsImpl {
 static const size_t NB = sizeof(T) * BitsPerByte;

 static_assert((NB == 8) || (NB == 16) || (NB == 32) || (NB == 64),
 "unsupported size");

 // The unsigned integral type for calculations.
 using I = typename Conditional<NB <= 32, uint32_t, uint64_t>::type;

 static const I rep_5555 = static_cast(UCONST64(0x5555555555555555));
 static const I rep_3333 = static_cast(UCONST64(0x3333333333333333));
 static const I rep_0F0F = static_cast(UCONST64(0x0F0F0F0F0F0F0F0F));
 static const I rep_00FF = static_cast(UCONST64(0x00FF00FF00FF00FF));
 static const I rep_FFFF = static_cast(UCONST64(0x0000FFFF0000FFFF));

public:

 static constexpr T reverse_bits_in_bytes(T v) {
 // Based on Hacker's Delight Section 7-1
 auto x = static_cast(v);
 x = ((x & rep_5555) << 1) | ((x >> 1) & rep_5555);
 x = ((x & rep_3333) << 2) | ((x >> 2) & rep_3333);
 x = ((x & rep_0F0F) << 4) | ((x >> 4) & rep_0F0F);
 return static_cast<T>(x);
 }

 static constexpr T reverse_bytes(T v) {
 // Based on Hacker's Delight Section 7-1
 // NB: Compilers are good at recognizing byte-swap code and transforming
 // it into platform-specific instructions like x86 bswap.
 auto x = static_cast(v);
 switch (NB) {
 case 64:
 // The use of NB/2 rather than 32 avoids a warning in dead code when
 // I is uint32_t, because shifting a 32bit type by 32 is UB.
 x = (x << (NB/2)) | (x >> (NB/2));
 case 32: // fallthrough
 x = ((x & rep_FFFF) << 16) | ((x >> 16) & rep_FFFF);
 case 16: // fallthrough
 x = ((x & rep_00FF) << 8) | ((x >> 8) & rep_00FF);
 default: // fallthrough
 return static_cast<T>(x);
 }
 }
};

// Performs byte reversal of an integral type up to 64 bits.
template <typename T, ENABLE_IF(std::is_integral<T>::value)>
constexpr T reverse_bytes(T x) {
 return ReverseBitsImpl<T>::reverse_bytes(x);
}

// Performs bytewise bit reversal of each byte of an integral
// type up to 64 bits.
template <typename T, ENABLE_IF(std::is_integral<T>::value)>
constexpr T reverse_bits_in_bytes(T x) {
 return ReverseBitsImpl<T>::reverse_bits_in_bytes(x);
}

// Performs full bit reversal an integral type up to 64 bits.
template <typename T, ENABLE_IF(std::is_integral<T>::value)>
constexpr T reverse_bits(T x) {
 return reverse_bytes(reverse_bits_in_bytes(x));
}

#endif // SHARE_UTILITIES_MOVEBITS_HPP

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