I'm trying to implement an optimization rule that removes unnecessary memory allocation, and I wonder if I can implement this using typed-HIR.
Context
For some immutable variables declared with let keyword, you can reduce memory allocation by changing the keyword to const or static.
I thought I can use typed-HIR to implement an optimization pass which will change let declared variable to const, and use the inferred type.
I looked into MIR but it seems like it doesn't have any information for data types and how it was declared.
Example
You can see that there are 3 function, each declares an array within the scope of the function and it will return the integer at the location of the argument.
You can see that let_array comes with a lot of mov instruction that writes a lot of stuff on to the RAM (very slow!) while other function has only 1 mov instruction which is for reading a value from the memory.
#[no_mangle]
pub fn let_array(idx: u8) -> isize
{
let array = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 0, 0, 0, 0, 0, 0, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29];
return array[idx as usize];
}
#[no_mangle]
pub fn const_array(idx: u8) -> isize
{
const ARRAY: [isize; 42] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 0, 0, 0, 0, 0, 0, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29];
return ARRAY[idx as usize];
}
#[no_mangle]
pub fn static_array(idx: u8) -> isize
{
static ARRAY: [isize; 42] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 0, 0, 0, 0, 0, 0, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29];
return ARRAY[idx as usize];
}
