Pattern matching on enum creates unecessary jump table

[Kmeakin]

I tried this code:
Say we define an AST type for expressions, each carrying a span to give its position in the original source code:

pub type ByteSpan = (u32, u32);

pub enum Expr<'core> {
    Error(ByteSpan),
    Int(ByteSpan, u64),
    Array(ByteSpan, &'core [Self]),
    Call(ByteSpan, &'core Self, &'core [Self]),
}

pub fn span(expr: &Expr) -> ByteSpan {
    match expr {
        Expr::Error(span, ..)
        | Expr::Int(span, ..)
        | Expr::Array(span, ..)
        | Expr::Call(span, ..) => *span,
    }
}

I expected to see this happen:
The ByteSpan is placed at the same offset for every variant of the enum, so the final assembly for span should be:

example::span:
        mov     eax, dword ptr [rdi + 4]
        mov     edx, dword ptr [rdi + 8]
        ret

Instead, this happened:
A jump table is generated, even though every entry in the jump table is identical (godbolt):

example::span:
        mov     eax, dword ptr [rdi]
        lea     rcx, [rip + .LJTI0_0]
        movsxd  rax, dword ptr [rcx + 4*rax]
        add     rax, rcx
        jmp     rax
.LBB0_1:
        lea     rcx, [rdi + 4]
        mov     eax, dword ptr [rdi + 4]
        mov     edx, dword ptr [rcx + 4]
        ret
.LJTI0_0:
        .long   .LBB0_1-.LJTI0_0
        .long   .LBB0_1-.LJTI0_0
        .long   .LBB0_1-.LJTI0_0
        .long   .LBB0_1-.LJTI0_0

If the last 2 variants are removed, the optimal code is produced. If only the last variant is removed, the code is better, but still suboptimal:

example::span:
        mov     eax, dword ptr [rdi]
        test    rax, rax
        je      .LBB0_2
        cmp     eax, 1
.LBB0_2:
        lea     rcx, [rdi + 4]
        mov     eax, dword ptr [rdi + 4]
        mov     edx, dword ptr [rcx + 4]
        ret

Meta

rustc --version --verbose:

rustc 1.74.0-nightly (1e746d774 2023-09-07)
binary: rustc
commit-hash: 1e746d7741d44551e9378daf13b8797322aa0b74
commit-date: 2023-09-07
host: x86_64-unknown-linux-gnu
release: 1.74.0-nightly
LLVM version: 17.0.0
Compiler returned: 0

[Kmeakin]

@rustbot label I-slow

[dianqk]

Looking at the IR form https://godbolt.org/z/G79jaedqa, I believe this is a duplicate issue of #113506.

@rustbot claim

[the8472]

The ByteSpan is placed at the same offset for every variant of the enum, so the final assembly for span should be

The ordering of the variant payloads is not guaranteed to be in source order for repr(Rust) enums

[Kmeakin]

The ByteSpan is placed at the same offset for every variant of the enum, so the final assembly for span should be

The ordering of the variant payloads is not guaranteed to be in source order for repr(Rust) enums

But in this case rustc did decide to put it at the same offset for each variant, so no jump table should be needed

[nikic]

Not fixed by the LLVM 18 upgrade. Keeping the label though, as LLVM 19 should fix this thanks to getelementptr i8 canonicalization.

[erikdesjardins]

Fixed in nightly by #121665: https://godbolt.org/z/rPhrxTq61

I don't think this needs a test, since tests/codegen/issues/issue-121719-common-field-offset.rs added in that PR is basically the same.

[dianqk]

Fixed in nightly by #121665: https://godbolt.org/z/rPhrxTq61

I don't think this needs a test, since tests/codegen/issues/issue-121719-common-field-offset.rs added in that PR is basically the same.

Yes. InstCombine now do this transformation. It's different from #113506.

[WaffleLapkin]

Closing as per #115742 (comment)