llvm noalias data gets lost when passing large structs #131905
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Description
https://godbolt.org/z/b8as1TxdP
#[repr(C)]
pub struct Big<'a>(&'a mut u64, &'a mut u64, usize);
#[inline(never)]
pub unsafe fn bad(a: Big) -> u64 {
*a.0 = 0;
*a.1 = 1;
*a.0
}
#[repr(C)]
pub struct Good<'a>(&'a mut u64, &'a mut u64);
#[inline(never)]
pub unsafe fn good(a: Good) -> u64 {
*a.0 = 0;
*a.1 = 1;
*a.0
}example::bad::h15eeefb1851d2a2e:
mov rax, qword ptr [rdi]
mov qword ptr [rax], 0
mov rcx, qword ptr [rdi + 8]
mov qword ptr [rcx], 1
mov rax, qword ptr [rax]
ret
example::good::h034657cbabb42f00:
mov qword ptr [rdi], 0
mov qword ptr [rsi], 1
xor eax, eax
reti expected the functions to emit similar codegen, but the bad one doesn't get the noalias on the references themselves because llvm thinks they're passed by pointer since the struct is large.
maybe alias.scope can help with this?
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rustc --version --verbose:
rustc 1.84.0-nightly (3ed6e3cc6 2024-10-17)
binary: rustc
commit-hash: 3ed6e3cc69857129c1d314daec00119ff47986ed
commit-date: 2024-10-17
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1
Compiler returned: 0
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