Compiler fails to remove jumps using jump table if every label in it is same #110797
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I tried this code:
pub enum K{
A(Box<[i32]>),
B(Box<[u8]>),
C(Box<[String]>),
D(Box<[u16]>),
}
pub fn get_len(arg: &K)->usize{
match arg {
K::A(ref lst)=>lst.len(),
K::B(ref lst)=>lst.len(),
K::C(ref lst)=>lst.len(),
K::D(ref lst)=>lst.len(),
}
}I expected to see this happen:
Since layout of each enum variant is same, I expected to get machine code without any jumps, e.g.
example::get_len:
mov rax, qword ptr [rdi + 16]
retInstead, this happened:
Compiler generates jump table, every branch of it points to the same code, and code jumps using it despite being it redundant.
example::get_len:
mov rax, qword ptr [rdi]
lea rcx, [rip + .LJTI0_0]
movsxd rax, dword ptr [rcx + 4*rax]
add rax, rcx
jmp rax
.LBB0_1:
mov rax, qword ptr [rdi + 16]
ret
.LJTI0_0: ; Note that each of it jumps to same label at the end.
.long .LBB0_1-.LJTI0_0
.long .LBB0_1-.LJTI0_0
.long .LBB0_1-.LJTI0_0
.long .LBB0_1-.LJTI0_0Meta
rustc --version --verbose:
rustc 1.68.0 (2c8cc3432 2023-03-06)
binary: rustc
commit-hash: 2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74
commit-date: 2023-03-06
host: x86_64-unknown-linux-gnu
release: 1.68.0
LLVM version: 15.0.6
Compiler returned: 0
Also, godbolt link.
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Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.Category: This is a bug.Call for participation: An issue has been fixed and does not reproduce, but no test has been added.Issue: Problems and improvements with respect to performance of generated code.Medium priorityRelevant to the compiler team, which will review and decide on the PR/issue.Performance or correctness regression from one stable version to another.
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