How can I add a new optimization rule?

Beginners
1

I’m trying to add a new optimization rule that checks the prefix of a string.

Context and Example

Here is the link to the compiler explorer.

Say, we had something like, this.

pub fn example_function(a: &str) -> i64 {
    match a {
        "asdf" => -1,
        "asdf1" => 1,
        "asdf2" => 2,
        "asdf3" => 3,
        "asdf4" => 4,
        "asdf10" => 10,
        "asdf11" => 11,
        "asdf12" => 12,
        "asdf13" => 13,
        "asdf14" => 14,
        "asdf100" => 100,
        "asdf101" => 101,
        "asdf102" => 102,
        "asdf103" => 103,
        "asdf104" => 104,
        _ => 0
    }
}

Compiler translates this into a large if-else ladder, checking the entire string over and over, like this.

.LBB0_4:
        mov     eax, 1717859169
        mov     ecx, dword ptr [rdi]
        xor     ecx, eax
        movzx   edx, byte ptr [rdi + 4]
        xor     edx, 50
        or      edx, ecx
        je      .LBB0_5
        xor     eax, dword ptr [rdi]
        movzx   ecx, byte ptr [rdi + 4]
        xor     ecx, 51
        or      ecx, eax
        je      .LBB0_7
        mov     eax, 1717859169
        mov     ecx, dword ptr [rdi]
        xor     ecx, eax
        movzx   edx, byte ptr [rdi + 4]
        xor     edx, 52
        or      edx, ecx

You can see that generated code is checking whether the first 4 letters of the input is “asdf” over and over, even though you already know this.

I considered adding a rule on the rust compiler’s side as well, but I suspecting it would be better off implementing them on the llvm’s side.
This should be especially beneficial for cases where the value doesn’t match with any of the match arm, as you can basically skip every single cases.

2

For this particular case, the easiest way would probably be writing a custom Pass.

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