compiler-rt/lib/builtins/popcountsi2.c

   1 //===-- popcountsi2.c - Implement __popcountsi2 ---------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements __popcountsi2 for the compiler_rt library.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "int_lib.h"
  14
  15 // Returns: count of 1 bits
  16
  17 COMPILER_RT_ABI int __popcountsi2(si_int a) {
  18   su_int x = (su_int)a;
  19   x = x - ((x >> 1) & 0x55555555);
  20   // Every 2 bits holds the sum of every pair of bits
  21   x = ((x >> 2) & 0x33333333) + (x & 0x33333333);
  22   // Every 4 bits holds the sum of every 4-set of bits (3 significant bits)
  23   x = (x + (x >> 4)) & 0x0F0F0F0F;
  24   // Every 8 bits holds the sum of every 8-set of bits (4 significant bits)
  25   x = (x + (x >> 16));
  26   // The lower 16 bits hold two 8 bit sums (5 significant bits).
  27   //    Upper 16 bits are garbage
  28   return (x + (x >> 8)) & 0x0000003F; // (6 significant bits)
  29 }