Codec that might be useful for `wasmi` register machine encoding of 64-bit values embedded in the instruction sequence consisting of 8-byte instruction words with 6-bytes parameter per instruction word optimized for WebAssembly execution.

bytes6_codec.rs

#[derive(Copy, Clone)]
pub struct Bytes2(u16);

#[derive(Copy, Clone)]
pub struct Bytes6(u16, u32);

pub fn encode_u64(value: u64) -> (Bytes2, Bytes6) {
    let lo_lo = (value >> 48 & 0xFFFF) as u16;
    let lo_hi = (value >> 32 & 0xFFFF) as u16;
    let hi = (value & 0xFFFF_FFFF) as u32;
    (Bytes2(lo_lo), Bytes6(lo_hi, hi))
}

pub fn decode_u64(bytes2: Bytes2, bytes6: Bytes6) -> u64 {
    ((bytes2.0 as u64) << 48) |
    ((bytes6.0 as u64) << 32) |
    (bytes6.1 as u64)
}

output.asm

example::encode_u64:
        mov     rax, rdi
        mov     rcx, rsi
        shr     rcx, 48
        mov     word ptr [rdi], cx
        mov     dword ptr [rdi + 4], esi
        shr     rsi, 32
        mov     word ptr [rdi + 8], si
        ret

example::decode_u64:
        shl     rdi, 48
        movzx   ecx, dx
        shl     rcx, 32
        or      rcx, rdi
        mov     eax, esi
        or      rax, rcx
        ret

output.wat

example::encode_u64:
        local.get       0
        local.get       1
        i64.store32     4
        local.get       0
        i32.const       8
        i32.add 
        local.get       1
        i64.const       32
        i64.shr_u
        i64.store16     0
        local.get       0
        local.get       1
        i64.const       48
        i64.shr_u
        i64.store16     0
        end_function

example::decode_u64:
        local.get       2
        i64.extend_i32_u
        i64.const       65535
        i64.and 
        i64.const       32
        i64.shl 
        local.get       0
        i64.extend_i32_u
        i64.const       48
        i64.shl 
        i64.or  
        local.get       1
        i64.extend_i32_u
        i64.or  
        end_function