Arithmetic in compiling

Posted on | Edited on | In

simple calc

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int calc(int i, int j) {
  return ((i + j - 1) & ~(j - 1));
}
  • variable 0 is this -> aload_0

javap -c

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int calc(int, int);
    Code:
       0: iload_1
       1: iload_2
       2: iadd
       3: iconst_1
       4: isub
       5: iload_2
       6: iconst_1
       7: isub
       8: iconst_m1
       9: ixor
      10: iand
      11: ireturn
a b
[0] push i value into stack
[1] push j value into stack
[2] add; result push stack
[3] push constant 1 into stack
[4] subtract; push result
[5] j value push stack
[6] push constant 1 again
[7] subtract; push result
[8] push constant -1
[9] do XOR; push result
[10] do AND; push result
[11] return int
  • ~(j-1) transform into -1^(j-1)
    exclusive or, -1 bytes is all high bit.
    any int oxr with all high bit, the high bit will be low, low will be high.