compiler-rt/lib/builtins/popcountti2.c

//===-- popcountti2.c - Implement __popcountti2
//----------------------------===//
//
// 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 __popcountti2 for the compiler_rt library.
//
//===----------------------------------------------------------------------===//

#include "int_lib.h"

#ifdef CRT_HAS_128BIT

// Returns: count of 1 bits

COMPILER_RT_ABI int __popcountti2(ti_int a) {
  tu_int x3 = (tu_int)a;
  x3 = x3 - ((x3 >> 1) &
             (((tu_int)0x5555555555555555uLL << 64) | 0x5555555555555555uLL));
  // Every 2 bits holds the sum of every pair of bits (64)
  x3 = ((x3 >> 2) &
        (((tu_int)0x3333333333333333uLL << 64) | 0x3333333333333333uLL)) +
       (x3 & (((tu_int)0x3333333333333333uLL << 64) | 0x3333333333333333uLL));
  // Every 4 bits holds the sum of every 4-set of bits (3 significant bits) (32)
  x3 = (x3 + (x3 >> 4)) &
       (((tu_int)0x0F0F0F0F0F0F0F0FuLL << 64) | 0x0F0F0F0F0F0F0F0FuLL);
  // Every 8 bits holds the sum of every 8-set of bits (4 significant bits) (16)
  du_int x2 = (du_int)(x3 + (x3 >> 64));
  // Every 8 bits holds the sum of every 8-set of bits (5 significant bits) (8)
  su_int x = (su_int)(x2 + (x2 >> 32));
  // Every 8 bits holds the sum of every 8-set of bits (6 significant bits) (4)
  x = x + (x >> 16);
  // Every 8 bits holds the sum of every 8-set of bits (7 significant bits) (2)
  //
  // Upper 16 bits are garbage
  return (x + (x >> 8)) & 0xFF; // (8 significant bits)
}

#endif // CRT_HAS_128BIT