diff --git a/library/core/src/hint.rs b/library/core/src/hint.rs index b7f78288b2b6..dccac26e07e6 100644 --- a/library/core/src/hint.rs +++ b/library/core/src/hint.rs @@ -658,7 +658,7 @@ pub const fn must_use(value: T) -> T { /// } /// } /// ``` -#[unstable(feature = "likely_unlikely", issue = "136873")] +#[unstable(feature = "likely_unlikely", issue = "151619")] #[inline(always)] pub const fn likely(b: bool) -> bool { crate::intrinsics::likely(b) @@ -708,7 +708,7 @@ pub const fn likely(b: bool) -> bool { /// } /// } /// ``` -#[unstable(feature = "likely_unlikely", issue = "136873")] +#[unstable(feature = "likely_unlikely", issue = "151619")] #[inline(always)] pub const fn unlikely(b: bool) -> bool { crate::intrinsics::unlikely(b) @@ -717,6 +717,10 @@ pub const fn unlikely(b: bool) -> bool { /// Hints to the compiler that given path is cold, i.e., unlikely to be taken. The compiler may /// choose to optimize paths that are not cold at the expense of paths that are cold. /// +/// Note that like all hints, the exact effect to codegen is not guaranteed. Using `cold_path` +/// can actually *decrease* performance if the branch is called more than expected. It is advisable +/// to perform benchmarks to tell if this function is useful. +/// /// # Examples /// /// ``` @@ -741,6 +745,38 @@ pub const fn unlikely(b: bool) -> bool { /// } /// } /// ``` +/// +/// This can also be used to implement `likely` and `unlikely` helpers to hint the condition rather +/// than the branch: +/// +/// ``` +/// #![feature(cold_path)] +/// use core::hint::cold_path; +/// +/// #[inline(always)] +/// pub const fn likely(b: bool) -> bool { +/// if !b { +/// cold_path(); +/// } +/// b +/// } +/// +/// #[inline(always)] +/// pub const fn unlikely(b: bool) -> bool { +/// if b { +/// cold_path(); +/// } +/// b +/// } +/// +/// fn foo(x: i32) { +/// if likely(x > 0) { +/// println!("this branch is likely to be taken"); +/// } else { +/// println!("this branch is unlikely to be taken"); +/// } +/// } +/// ``` #[unstable(feature = "cold_path", issue = "136873")] #[inline(always)] pub const fn cold_path() {