barsonme on The random number generator of DOOM

Comment on The random number generator of DOOM parent

It's not a pointer, it's a bitwise AND.

It's essentially the same[0] as `prndindex = (prndinex+1) % 256`, except bitwise AND takes less effort to compute.

Modulo is essentially[1]: `mod = a - b * (a / b)` unless the compiler can manage to use bitwise shifting/masks instead.

Back when Doom was popular, the bitmask would have been much more efficient.

[0] http://stackoverflow.com/q/3072665/2967113 [1] I'm not 100% positive that's how it's implemented with an IDIV instruction.

derf_11y

Any reasonable compiler will implement x%256 using bitwise arithmetic, but C semantics require that x == d*(x/d) + (x%d), which means that if x is negative (x%256) must be negative, while (x&0xff) will be positive.

This difference means that the AND is still faster than the MOD when used on signed integers, if the compiler is not smart enough to prove that the input will never be negative.

For example, with gcc 4.7.3 the resulting assembly is

  movl %edi, %edx
  sarl $31, %edx
  shrl $24, %edx
  leal (%rdi, %rdx), %eax
  andl $255, %eax
  subl %edx, %eax

I.e., six instructions instead of one. This is still faster than using a divide.

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