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const eval Vec::new() the same as vec![] #54475
Description
For some reason, it seems that rustc can deduce .len() == 0 better for vec![] than for Vec::new().
pub fn a() -> usize {
let v: Vec<u8> = Vec::new();
let mut v2: Vec<u8> = Vec::new();
if v.len() > 1 { // false
v2 = vec![2];
} else {
v2 = Vec::new();
}
v.len() + v2.len() // 0
}example::a:
push rbx
sub rsp, 48
xorps xmm0, xmm0
movaps xmmword ptr [rsp + 32], xmm0
mov qword ptr [rsp + 8], 1
movups xmmword ptr [rsp + 16], xmm0
mov rsi, qword ptr [rsp + 16]
mov rbx, qword ptr [rsp + 24]
test rsi, rsi
je .LBB0_2
mov edi, 1
mov edx, 1
call __rust_dealloc@PLT
.LBB0_2:
mov rax, rbx
add rsp, 48
pop rbx
rethttps://rust.godbolt.org/z/xEhPhI
surprisingly, when I replaced one Vec::new() with vec![], rustc was able to const-eval the return value to 0.
pub fn a() -> usize {
let v: Vec<u8> = Vec::new();
let mut v2: Vec<u8> = Vec::new();
if v.len() > 1 { // false
v2 = vec![2];
} else {
v2 = vec![]; // I changed this
}
v.len() + v2.len() // 0
}example::a:
xor eax, eax
rethttps://rust.godbolt.org/z/cdbEGU
Apparently
pub fn a() -> usize {
let v: Vec<u8> = vec![];
v.len()
}and
pub fn a() -> usize {
let v: Vec<u8> = Vec::new();
v.len()
}generate the same code tough.
Activity
cynecx commentedon Sep 24, 2018
This may be a limitation with llvm's alias analysis (or other components). Does rust perhaps emit suboptimal llvm-ir?:
(reduced test-case)
https://godbolt.org/z/-IvKEW
matthiaskrgr commentedon Dec 12, 2019
Note that making
Vec::new()aconst fndid not fix the issue.tspiteri commentedon Mar 31, 2020
If you create a const
Vecand use it instead ofVec::new(), it does seem to work around the issue.https://rust.godbolt.org/z/ftbvHX
RalfJung commentedon Mar 31, 2020
And unsurprisingly so -- making a function
const fnhas exactly no effect at all on the generated LLVM IR.nikic commentedon Mar 13, 2021
The Vec::new() variant optimizes well on nightly, presumably as a result of the LLVM 12 upgrade. I'm not sure which upstream change is responsible in this case though.