From 669e25597c8244ce4991414a8dfb47869854fda3 Mon Sep 17 00:00:00 2001 From: Joshua Wong Date: Wed, 9 Oct 2024 14:24:05 -0400 Subject: [PATCH] allocate before calling T::default in >::default() The `Box` 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 `>::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. --- alloc/src/boxed.rs | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/alloc/src/boxed.rs b/alloc/src/boxed.rs index 5f20729568352..3e791416820ef 100644 --- a/alloc/src/boxed.rs +++ b/alloc/src/boxed.rs @@ -1688,7 +1688,7 @@ impl Default for Box { /// Creates a `Box`, with the `Default` value for T. #[inline] fn default() -> Self { - Box::new(T::default()) + Box::write(Box::new_uninit(), T::default()) } }