c# - .NET Integer vs Int16?

Question

I have a questionable coding practice.

When I need to iterate through a small list of items whose count limit is under 32000, I use Int16 for my i variable type instead of Integer. I do this because I assume using the Int16 is more efficient than a full blown Integer.

Am I wrong? Is there no effective performance difference between using an Int16 vs an Integer? Should I stop using Int16 and just stick with Integer for all my counting/iteration needs?

Answer 1

You should almost always use Int32 or Int64 (and, no, you do not get credit by using UInt32 or UInt64) when looping over an array or collection by index.

The most obvious reason that it's less efficient is that all array and collection indexes found in the BCL take Int32s, so an implicit cast is always going to happen in code that tries to use Int16s as an index.

The less-obvious reason (and the reason that arrays take Int32 as an index) is that the CIL specification says that all operation-stack values are either Int32 or Int64. Every time you either load or store a value to any other integer type (Byte, SByte, UInt16, Int16, UInt32, or UInt64), there is an implicit conversion operation involved. Unsigned types have no penalty for loading, but for storing the value, this amounts to a truncation and a possible overflow check. For the signed types every load sign-extends, and every store sign-collapses (and has a possible overflow check).

The place that this is going to hurt you most is the loop itself, not the array accesses. For example take this innocent-looking loop:

for (short i = 0; i < 32000; i++) {
    ...
}

Looks good, right? Nope! You can basically ignore the initialization (short i = 0) since it only happens once, but the comparison (i<32000) and incrementing (i++) parts happen 32000 times. Here's some pesudo-code for what this thing looks like at the machine level:

  Int16 i = 0;
LOOP:
  Int32 temp0 = Convert_I16_To_I32(i); // !!!
  if (temp0 >= 32000) goto END;
  ...
  Int32 temp1 = Convert_I16_To_I32(i); // !!!
  Int32 temp2 = temp1 + 1;
  i = Convert_I32_To_I16(temp2); // !!!
  goto LOOP;
END:

There are 3 conversions in there that are run 32000 times. And they could have been completely avoided by just using an Int32 or Int64.

Update: As I said in the comment, I have now, in fact written a blog post on this topic, .NET Integral Data Types And You