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

/* ===-- popcountsi2.c - Implement __popcountsi2 ---------------------------===
 *
 *                     The LLVM Compiler Infrastructure
 *
 * This file is dual licensed under the MIT and the University of Illinois Open
 * Source Licenses. See LICENSE.TXT for details.
 *
 * ===----------------------------------------------------------------------===
 *
 * This file implements __popcountsi2 for the compiler_rt library.
 *
 * ===----------------------------------------------------------------------===
 */

#include "int_lib.h"

/* Returns: count of 1 bits */

COMPILER_RT_ABI si_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) */
}