Codegen Optimization for Non-Consecutive Enum Match Arms

[ray90514]

I tried this code:

enum Foo {
    Field1 = 10,
    Field2 = 11,
    Field3 = 12,
    Field4 = 20,
}


#[no_mangle]
fn f(value: u32) -> Foo {
    match value {
        10 => Foo::Field1,
        11 => Foo::Field2,
        12 => Foo::Field3,
        20 => Foo::Field4,
        _ => unreachable!(),
    }
}

It was compiled as jump table.

f:
        add     edi, -10
        cmp     edi, 10
        ja      .LBB0_3
        lea     rax, [rip + .LJTI0_0]
        movsxd  rcx, dword ptr [rax + 4*rdi]
        add     rcx, rax
        jmp     rcx
.LBB0_2:
        mov     al, 10
        ret
.LBB0_5:
        mov     al, 12
        ret
.LBB0_6:
        mov     al, 20
        ret
.LBB0_4:
        mov     al, 11
        ret

For the case of smaller range, the compiler used another optimization.

#[no_mangle]
fn f(value: u32) -> Foo {
    match value {
        10 => Foo::Field1,
        11 => Foo::Field2,
        12 => Foo::Field3,
        14 => Foo::Field4,
        _ => unreachable!(),
    }
}

f:
        add     edi, -10
        cmp     edi, 5
        jae     .LBB0_3
        mov     eax, 23
        bt      eax, edi
        jae     .LBB0_3
        shl     edi, 3
        movabs  rax, 86067907338
        mov     ecx, edi
        shr     rax, cl
        ret

For the case taht matching arms are more than 4, the compiler used lookup table.

enum Foo {
    Field1 = 10,
    Field2 = 11,
    Field3 = 12,
    Field4 = 13,
    Field5 = 20,
}


#[no_mangle]
fn f(value: u32) -> Foo {
    match value {
        10 => Foo::Field1,
        11 => Foo::Field2,
        12 => Foo::Field3,
        13 => Foo::Field4,
        20 => Foo::Field5,
        _ => unreachable!(),
    }
}

f:
        add     edi, -10
        cmp     edi, 11
        jae     .LBB0_3
        mov     eax, 1039
        bt      eax, edi
        jae     .LBB0_3
        mov     eax, edi
        lea     rcx, [rip + .Lswitch.table.f]
        movzx   eax, byte ptr [rax + rcx]
        ret

I expected this, which can be attained only when the arm values are consecutive.

enum Foo {
    Field1 = 10,
    Field2 = 11,
    Field3 = 12,
    Field4 = 13,
    Field5 = 14,
}


#[no_mangle]
fn f(value: u32) -> Foo {
    match value {
        10 => Foo::Field1,
        11 => Foo::Field2,
        12 => Foo::Field3,
        13 => Foo::Field4,
        14 => Foo::Field5,
        _ => unreachable!(),
    }
}

f:
        add     edi, -10
        cmp     edi, 5
        jae     .LBB0_2
        add     dil, 10
        mov     eax, edi
        ret

Meta

rustc --version --verbose:

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7
Internal compiler ID: r1860

Compiler options:

-Copt-level=3