div_pow2.cpp

/*
  `x / (1<<n)` with shifts and an addition.
  works with gcc 5.4.0

  http://joisino.hatenablog.com/entry/2017/09/14/210000
  
  Copyright (c) 2017 joisino
  Released under the MIT license
  http://opensource.org/licenses/mit-license.php
 */

#include <stdio.h>

int div_pow2( int x , int n ){
  return ( x + (int)((unsigned int)(x >> 31) >> (32-n)) ) >> n;
}

int div_pow2_if( int x , int n ){
  if( x < 0 ){
    x += (1 << n) - 1;
  }
  return x >> n;
}

int main(){
  for( int i = -8; i <= 8; i++ ){
    printf( "%d %d\n", i, div_pow2( i , 2 ) );
  }
}