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Cow<[T]> layout forces unnecessary branching #117763

@Jules-Bertholet

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@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)

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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

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    A-layoutArea: Memory layout of typesC-bugCategory: This is a bug.E-needs-testCall for participation: An issue has been fixed and does not reproduce, but no test has been added.I-heavyIssue: Problems and improvements with respect to binary size of generated code.I-slowIssue: Problems and improvements with respect to performance of generated code.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.T-libsRelevant to the library team, which will review and decide on the PR/issue.

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