allocate before calling T::default in <Box<T>>::default()

jwong101 · jwong101 · commit 669e25597c82 · 2024-10-10T09:49:24.000-04:00
The `Box&lt;T: Default&gt;` 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
`&lt;Box&lt;T&gt;&gt;::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.
diff --git a/alloc/src/boxed.rs b/alloc/src/boxed.rs
@@ -1688,7 +1688,7 @@ impl<T: Default> Default for Box<T> {
     /// Creates a `Box<T>`, with the `Default` value for T.
     #[inline]
     fn default() -> Self {
-        Box::new(T::default())
+        Box::write(Box::new_uninit(), T::default())
     }
 }
 

Original file line number	Diff line number	Diff line change
`@@ -1688,7 +1688,7 @@ impl<T: Default> Default for Box<T> {`
`1688`	`1688`	/// Creates a `Box<T>`, with the `Default` value for T.
`1689`	`1689`	`#[inline]`
`1690`	`1690`	`fn default() -> Self {`
`1691`		`- Box::new(T::default())`
	`1691`	`+ Box::write(Box::new_uninit(), T::default())`
`1692`	`1692`	`}`
`1693`	`1693`	`}`
`1694`	`1694`