mirror of
https://github.com/rust-lang/rust.git
synced 2025-11-18 17:37:00 +00:00
8d52aae968
Reduce `Box::default` stack copies in debug mode The `Box::new(T::default())` implementation of `Box::default` only had two stack copies in debug mode, compared to the current version, which has four. By avoiding creating any `MaybeUninit<T>`'s and just writing `T` directly to the `Box` pointer, the stack usage in debug mode remains the same as the old version. Another option would be to mark `Box::write` as `#[inline(always)]`, and change it's implementation to to avoid calling `MaybeUninit::write` (which creates a `MaybeUninit<T>` on the stack) and to use `ptr::write` instead. Fixes: #136043
The files here use the LLVM FileCheck framework, documented at https://llvm.org/docs/CommandGuide/FileCheck.html.
One extension worth noting is the use of revisions as custom prefixes for FileCheck. If your codegen test has different behavior based on the chosen target or different compiler flags that you want to exercise, you can use a revisions annotation, like so:
// revisions: aaa bbb
// [bbb] compile-flags: --flags-for-bbb
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so:
// CHECK: expected code
// aaa-SAME: emitted-only-for-aaa
// aaa-NOT: emitted-only-for-bbb
// bbb-NOT: emitted-only-for-aaa
// bbb-SAME: emitted-only-for-bbb