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 } 30