Using low bitsize integral types like `Int8` and what they are for

Question

Recently I've learned that every computation cycle performs on machine words which on most contemporary processors and OS'es are either 32-bit or 64-bit. So what are the benefits of using the smaller bit-size values like Int16, Int8, Word8? What are they exactly for? Is it storage reduction only?

I write a complex calculation program which consists of several modules but is interfaced by only a single function which returns a Word64 value, so the whole program results in Word64 value. I'm interested in the answer to this question because inside this program I found myself utilizing a lot of different Integral types like Word16 and Word8 to represent small entities, and seeing that they quite often got converted with fromIntegral got me thinking: was I making a mistake there and what was the exact benefit of those types which I not knowing about got blindly attracted by? Did it make sense at all to utilize other integral types and evetually convert them with fromIntegral or maybe I should have just used Word64 everywhere?

Answer 1

In GHC, the fixed-size integral types all take up a full machine word, so there's no space savings to be had. Using machine-word-sized types (i.e. Int and Word) will probably be faster than the fixed-size types in most cases, but using a fixed-size integral type will be faster than doing explicit wrap-around.

You should choose the appropriate type for the range of values you're using. maxBound :: Word8 is 255, 255 + 1 :: Word8 is 0 — and if you're dealing with octets, that's exactly what you want. (For instance, ByteStrings are defined as storing Word8s.)

If you just have some integers that don't need a specific number of bits, and the calculations you're doing aren't going to overflow, just use Int or Word (or even Integer). Fixed-size types are less common than the regular integral types because, most of the time, you don't need a specific size.

So, don't use them for performance; use them if you're looking for their specific semantics: fixed-size integral types with defined overflow behaviour.