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Optimize Box::default and Arc::default to construct more types in place #131460

Merged
merged 3 commits into from
Oct 16, 2024
Merged

Optimize Box::default and Arc::default to construct more types in place #131460

merged 3 commits into from
Oct 16, 2024

Commits on Oct 10, 2024

  1. add initial tests for placement new changes

    @jwong101
    jwong101 committed Oct 10, 2024
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  2. allocate before calling T::default in <Box<T>>::default() 

    The `Box<T: Default>` impl currently calls `T::default()` before allocating
the `Box`.

Most `Default` impls are trivial, which should in theory allow
LLVM to construct `T: Default` directly in the `Box` allocation when calling
`<Box<T>>::default()`.

However, the allocation may fail, which necessitates calling `T's` destructor if it has one.
If the destructor is non-trivial, then LLVM has a hard time proving that it's
sound to elide, which makes it construct `T` on the stack first, and then copy it into the allocation.

Create an uninit `Box` first, and then write `T::default` into it, so that LLVM now only needs to prove
that the `T::default` can't panic, which should be trivial for most `Default` impls.
    @jwong101
    jwong101 committed Oct 10, 2024
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  3. allocate before calling T::default in <Arc<T>>::default() 

    Same rationale as in the previous commit.
    @jwong101
    jwong101 committed Oct 10, 2024
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