Cow<[T]> layout forces unnecessary branching

[Jules-Bertholet]

I tried this code (Godbolt):

pub fn test<'a>(cow: &'a std::borrow::Cow<'a, [u8]>) -> &'a [u8] {
    &*cow
}

I expected to see this happen: The generated assembly is branchless. The pointer and length are at the same offset in both variants of the Cow.

Instead, this happened: There is a branch in the generated assembly. The pointer to the slice is at a different offset in the Owned and Borrowed variants of the Cow.

---

The ideal layout for Cow<[T]> (and Cow<str>) would look like this:

If `T` is not a ZST:
┌──────────┬──────────┬────────────────────────────────────────────────────┐
│ pointer  │ length   │ capacity if `Owned`, or `usize::MAX` if `Borrowed` │
└──────────┴──────────┴────────────────────────────────────────────────────┘

If `T` is a ZST:
┌──────────┬──────────┬────────────────────────────────────────┐
│ pointer  │ length   │ boolean flag for `Owned` vs `Borrowed` │
└──────────┴──────────┴────────────────────────────────────────┘

The present layout looks like this:

`Owned` variant:
┌──────────┬──────────┬──────────┐
│ pointer  │ capacity │ length   │
└──────────┴──────────┴──────────┘

`Borrowed` variant:
┌──────────┬──────────┬──────────┐
│ 0x0      │ pointer  │ length   │
└──────────┴──────────┴──────────┘

Because of this non-optimal layout, every access to the contained pointer requires a branch on the enum variant.

(Fixing #45431 is a prerequisite to fixing this)

Meta

rustc --version --verbose:

rustc 1.75.0-nightly (189d6c71f 2023-11-06)
binary: rustc
commit-hash: 189d6c71f3bb6c52113b5639a80839791974fd22
commit-date: 2023-11-06
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: 17.0.4

@rustbot label A-layout I-heavy I-slow T-compiler T-libs

[the8472]

It wouldn't solve #45431, but #106790 would add a larger niche to RawVec which could be used for the flag. Though that might actually turn it into ((cap, ptr), len)... which should also work for this purpose because then the flag can be stored in the first field before the borrowed payload.

[DaniPopes]

Looks like this is fixed with #106790 in the latest nightly (Godbolt link in OP):

example::test:
        mov     rax, qword ptr [rdi + 8]
        mov     rdx, qword ptr [rdi + 16]
        ret