compiler-rt/lib/popcountdi2.c

//===-- popcountdi2.c - Implement __popcountdi2 ----------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements __popcountdi2 for the compiler_rt library.
//
//===----------------------------------------------------------------------===//

#include "int_lib.h"

// Returns: count of 1 bits

si_int
__popcountdi2(di_int a)
{
    du_int x2 = (du_int)a;
    x2 = x2 - ((x2 >> 1) & 0x5555555555555555uLL);
    // Every 2 bits holds the sum of every pair of bits (32)
    x2 = ((x2 >> 2) & 0x3333333333333333uLL) + (x2 & 0x3333333333333333uLL);
    // Every 4 bits holds the sum of every 4-set of bits (3 significant bits) (16)
    x2 = (x2 + (x2 >> 4)) & 0x0F0F0F0F0F0F0F0FuLL;
    // Every 8 bits holds the sum of every 8-set of bits (4 significant bits) (8)
    su_int x = (su_int)(x2 + (x2 >> 32));
    // The lower 32 bits hold four 16 bit sums (5 significant bits).
    //   Upper 32 bits are garbage
    x = x + (x >> 16);
    // The lower 16 bits hold two 32 bit sums (6 significant bits).
    //   Upper 16 bits are garbage
    return (x + (x >> 8)) & 0x0000007F;  // (7 significant bits)
}