Why the identity function can be used as unlikely function? about hashbrown HOT 3 CLOSED

The compiler can't distinguish them.

Without the nightly feature the likely,unlikely functions do nothing and are simply the identify function as you observed.

But with the nightly feature...

#[cfg(feature = "nightly")]
use core::intrinsics::{likely, unlikely};

the actual intrinsics are used, which are still nightly only: https://doc.rust-lang.org/stable/core/intrinsics/fn.likely.html

from hashbrown.

Ok. I am looking for the implementation of the likely and unlikely in stable. Some other links said that hashbrown has a stable implementation of these two functions. I noticed the implementation has changed recently.

So in the current stable rust, the old implementation does not work and we can not achieve it now? @exrok

from hashbrown.

Correct that is the my current understanding. I am not aware of a stable implementation of the likely and unlikely. You could try the olds impls from hashbrown (removed here d677fd4) the logic behind seems sounds it is just a question of if they help LLVM consistently.

Sometimes even the unstable intrinsics don't help consistently: rust-lang/rust#88767

Also note, the current likely/unlikely intrinsics are currently marked "perma-unstable", so don't expect them to stabilize soon: rust-lang/rust#26179 (comment)

In terms of stable solutions I think the closest would be PGO: https://doc.rust-lang.org/rustc/profile-guided-optimization.html
But that only helps if your the one building the application and complicates the build procedure.

from hashbrown.