src/hotspot/share/utilities/population_count.hpp

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 *
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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 * 2 along with this work; if not, write to the Free Software Foundation,
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#ifndef SHARE_UTILITIES_POPULATION_COUNT_HPP
#define SHARE_UTILITIES_POPULATION_COUNT_HPP

#include "metaprogramming/enableIf.hpp"
#include "utilities/debug.hpp"
#include "utilities/globalDefinitions.hpp"

#include <type_traits>

// Returns the population count of x, i.e., the number of bits set in x.
//
// Adapted from Hacker's Delight, 2nd Edition, Figure 5-2 and the text that
// follows.
//
// Ideally this should be dispatched per platform to use optimized
// instructions when available, such as POPCNT on modern x86/AMD. Our builds
// still target and support older architectures that might lack support for
// these. For example, with current build configurations, __builtin_popcount(x)
// generate a call to a similar but slower 64-bit version when calling with
// a 32-bit integer type.
template <typename T>
inline unsigned population_count(T x) {
  STATIC_ASSERT(BitsPerWord <= 128);
  STATIC_ASSERT(BitsPerByte == 8);
  STATIC_ASSERT(std::is_integral<T>::value);
  STATIC_ASSERT(!std::is_signed<T>::value);
  // We need to take care with implicit integer promotion when dealing with
  // integers < 32-bit. We chose to do this by explicitly widening constants
  // to unsigned
  using P = std::conditional_t<(sizeof(T) < sizeof(unsigned)), unsigned, T>;
  const T all = ~T(0);           // 0xFF..FF
  const P fives = all/3;         // 0x55..55
  const P threes = (all/15) * 3; // 0x33..33
  const P z_ones = all/255;      // 0x0101..01
  const P z_effs = z_ones * 15;  // 0x0F0F..0F
  P r = x;
  r -= ((r >> 1) & fives);
  r = (r & threes) + ((r >> 2) & threes);
  r = ((r + (r >> 4)) & z_effs) * z_ones;
  // The preceding multiply by z_ones is the only place where the intermediate
  // calculations can exceed the range of T. We need to discard any such excess
  // before the right-shift, hence the conversion back to T.
  return static_cast<T>(r) >> (((sizeof(T) - 1) * BitsPerByte));
}

#endif // SHARE_UTILITIES_POPULATION_COUNT_HPP