Which is better option to use for dividing an integer number by 2?

 

Which of the following techniques is the best option for dividing an integer by 2 and why?

Technique 1: 

x = x >> 1;

Technique 2: 

x = x / 2;

Here x is an integer.

Use the operation that best describes what you are trying to do.

  • If you are treating the number as a sequence of bits, use bitshift.
  • If you are treating it as a numerical value, use division.

    • Note that they are not exactly equivalent. They can give different results for negative integers. For example: 

    -5 / 2  = -2
-5 >> 1 = -3

    (ideone)

    Does the first one look like dividing? No. If you want to divide, use x / 2 . Compiler can optimise it to use bit-shift if possible (it's called strength reduction), which makes it a useless micro-optimisation if you do it on your own.

    To pile on: there are so many reasons to favor using x = x / 2; Here are some:

  • it expresses your intent more clearly (assuming you're not dealing with bit twiddling register bits or something)

  • the compiler will reduce this to a shift operation anyway

  • even if the compiler didn't reduce it and chose a slower operation than the shift, the likelihood that this ends up affecting your program's performance in a measurable way is itself vanishingly small (and if it does affect it measurably, then you have an actual reason to use a shift)

  • if the division is going to be part of a larger expression, you're more likely to get the precedence right if you use the division operator: 

    x = x / 2 + 5;
x = x >> 1 + 5;  // not the same as above

  • signed arithmetic might complicate things even more than the precedence problem mentioned above

  • to reiterate - the compiler will already do this for you anyway. In fact, it'll convert division by a constant to a series of shifts, adds, and multiplies for all sorts of numbers, not just powers of two. See this question for links to even more information about this.

    • In short, you buy nothing by coding a shift when you really mean to multiply or divide, except maybe an increased possibility of introducing a bug. It's been a lifetime since compilers weren't smart enough to optimize this kind of thing to a shift when appropriate.

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