Suboptimal codegen for match from enum to almost-same-value usize (unnecessary table lookups)

[oxalica]

I tried this code:

#[derive(Clone, Copy)]
#[repr(u8)]
pub enum Len {
    Zero = 0,
    One,
    Two,
    Three,
}

#[no_mangle]
pub fn convert_len(len: &Len) -> Option<usize> {
    match *len {
        // Len::Zero => Some(0), // This produces optimal code.
        Len::Zero => None, // This does not: two table lookups, one being identity table.
        Len::One => Some(1),
        Len::Two => Some(2),
        Len::Three => Some(3),
    }
}

// Manual optimal code.
#[no_mangle]
pub fn convert_len_optimal(len: &Len) -> Option<usize> {
    match *len {
        Len::Zero => None,
        len => Some(unsafe { std::mem::transmute::<Len, u8>(len) } as usize),
    }
}

It gives (Compiler explorer):

convert_len:
        movzx   eax, byte ptr [rdi]
        shl     eax, 3
        lea     rcx, [rip + .Lswitch.table.convert_len]
        mov     rdx, qword ptr [rax + rcx]
        lea     rcx, [rip + .Lswitch.table.convert_len.1]
        mov     rax, qword ptr [rax + rcx]
        ret

convert_len_optimal:
        movzx   edx, byte ptr [rdi]
        xor     eax, eax
        test    rdx, rdx
        setne   al
        ret

.Lswitch.table.convert_len:
        .zero   8
        .quad   1
        .quad   2
        .quad   3

.Lswitch.table.convert_len.1:
        .quad   0
        .quad   1
        .quad   1
        .quad   1

I expect convert_len to generate code like convert_len_optimal. No table lookup should be generated, especially not the first identity table. Though I don't expect performance to differ a lot when the cache is hot, but wasting cache on these trivial tables does not worth it.
I'm not sure if LLVM or rustc is to blame here.

Meta

Reproduced on compiler Explorer's 1.84, latest nightly, and local nightly (rustc 1.86.0-nightly (854f22563 2025-01-31))

[hanna-kruppe]

Tangential but can’t the transmute in the “optimal” variant be a safe as cast?

[oxalica]

Tangential but can’t the transmute in the “optimal” variant be a safe as cast?

Yes it is. I totally forget it exists 🤦 .