bjoernager/rust
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Optimize Rc::<str>::default() implementation

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This PR lets `impl Default for Rc<str>` re-use the implementation
for `Rc::<[u8]>::default()`. The previous version only calculted the
memory layout at runtime, even though it should be known at compile
time, resulting in an additional function call.

The same optimization is done for `Rc<CStr>`.

Generated byte code: <https://godbolt.org/z/dfq73jsoP>.

Resolves <https://github.com/rust-lang/rust/issues/135784>.
This commit is contained in:
René Kijewski 2025-01-26 19:12:54 +01:00
parent 15c6f7e1a3
commit e090db8d22
2 changed files with 6 additions and 3 deletions
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5
library/alloc/src/ffi/c_str.rs
View file

@ -965,8 +965,9 @@ impl Default for Rc<CStr> {
/// This may or may not share an allocation with other Rcs on the same thread.
#[inline]
fn default() -> Self {
let c_str: &CStr = Default::default();
Rc::from(c_str)
let rc = Rc::<[u8]>::from(*b"\0");
// `[u8]` has the same layout as `CStr`, and it is `NUL` terminated.
unsafe { Rc::from_raw(Rc::into_raw(rc) as *const CStr) }
}
}

4
library/alloc/src/rc.rs
View file

@ -2369,7 +2369,9 @@ impl Default for Rc<str> {
/// This may or may not share an allocation with other Rcs on the same thread.
#[inline]
fn default() -> Self {
Rc::from("")
let rc = Rc::<[u8]>::default();
// `[u8]` has the same layout as `str`.
unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
}
}