[ConceptJunkie]

> The same trick can also be used for the other direction to save a division:

> NewValue = OldValue >> 3; > This is basically the same as

> NewValue = OldValue / 8;

> RCT does this trick all the time, and even in its OpenRCT2 version, this syntax hasn’t been changed, since compilers won’t do this optimization for you.

The author loses a lot of credibility by suggesting the compiler won't replace multiplying or dividing by a factor of 2 with the equivalent bit shift. That's a trivial optimization that's always been done. I'm sure compilers were doing that in the 70s.

[msk-lywenn]

I wouldn't go that far, but yes, no compiler will leave that on the table today or even twenty years ago. They do even more impressive transformations than that on basic math.

[nearbuy]

The author is partly right here. If those values are ints, you'll get something like this:

  sar eax, 0x1f
  and eax, 7
  add eax, edx
  sar eax, 3

You get 4 instructions instead of one because value >> 3 rounds towards negative infinity and value / 8 rounds towards zero.

And while this wouldn't apply to C++, in languages with checked arithmetic, the left shift won't necessarily set the overflow flag, so the compiler often can't use it.