third_party/llvm-project/compiler-rt/lib/builtins/popcountsi2.c

//===-- popcountsi2.c - Implement __popcountsi2 ---------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements __popcountsi2 for the compiler_rt library.
//
//===----------------------------------------------------------------------===//

#include "int_lib.h"

// Returns: count of 1 bits

COMPILER_RT_ABI int __popcountsi2(si_int a) {
  su_int x = (su_int)a;
  x = x - ((x >> 1) & 0x55555555);
  // Every 2 bits holds the sum of every pair of bits
  x = ((x >> 2) & 0x33333333) + (x & 0x33333333);
  // Every 4 bits holds the sum of every 4-set of bits (3 significant bits)
  x = (x + (x >> 4)) & 0x0F0F0F0F;
  // Every 8 bits holds the sum of every 8-set of bits (4 significant bits)
  x = (x + (x >> 16));
  // The lower 16 bits hold two 8 bit sums (5 significant bits).
  //    Upper 16 bits are garbage
  return (x + (x >> 8)) & 0x0000003F; // (6 significant bits)
}