I am learning rust and in the official tutorial, the author assigned the value 5 to a variable like so:
let x: i32 = 5;
I thought this was weird as one could use u8 as the type and the program would run fine. This got me thinking, are there any advantages to using a lower bit number? Is it faster?
The main advantage is that they use less memory. A vector<i32> with 1 billion elements will use 4GB, while a vector<u8> will use 1GB. This can be a significant advantage regardless of speed.
Arithmetic on smaller integer types on modern CPUs is not faster in general. There are some issues with using only part of a register but optimizers will almost certainly resolve these performance problems for you.
When you have a lot of integers and the optimizer can make use of vectorization (for example adding your 1 billion integers in the vector) then smaller types will typically yield better performance, because more of them fit in a SIMD register.
If you use them just as one scalar stack variable like in your example, I highly doubt there will be a difference in 99% of cases. Here other considerations are more important:
A bigger type will make overflows less likely, maybe you did calculate your maximal possible value wrong.
For public interfaces bigger types are more future proof.
Its better to cast from i8 to i32 than the other way round.
