From 64bd7db1d1b148594edfde112cdb6d6260e2cfc3 Mon Sep 17 00:00:00 2001
From: DaniPopes <57450786+DaniPopes@users.noreply.github.com>
Date: Sat, 26 Jul 2025 22:43:45 +0200
Subject: [PATCH] Re-implement likely/unlikely with `#[cold]`

Since https://github.com/rust-lang/rust/pull/120370 `#[cold]`
affects branch weights in the backend (again?). Re-implement
`likely` and `unlikely` in terms of a `#[cold]` function.

You can verify that the LLVM IR in [this godbolt link](https://godbolt.org/z/n3v7hsMqK)
contains a `branch_weights` metadata.

These were removed in https://github.com/rust-lang/hashbrown/commit/d677fd42df6978522045a9561242588ef970ed0c.
---
 src/util.rs | 32 ++++++++++++++++++++++++++++----
 1 file changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/util.rs b/src/util.rs
index c8a811732..90a8df311 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -1,10 +1,34 @@
-// FIXME: Branch prediction hint. This is currently only available on nightly
-// but it consistently improves performance by 10-15%.
-#[cfg(not(feature = "nightly"))]
-pub(crate) use core::convert::{identity as likely, identity as unlikely};
+// FIXME: Replace with `core::hint::{likely, unlikely}` once they are stable.
 #[cfg(feature = "nightly")]
 pub(crate) use core::intrinsics::{likely, unlikely};
 
+#[cfg(not(feature = "nightly"))]
+#[inline(always)]
+#[cold]
+fn cold_path() {}
+
+#[cfg(not(feature = "nightly"))]
+#[inline(always)]
+pub(crate) fn likely(b: bool) -> bool {
+    if b {
+        true
+    } else {
+        cold_path();
+        false
+    }
+}
+
+#[cfg(not(feature = "nightly"))]
+#[inline(always)]
+pub(crate) fn unlikely(b: bool) -> bool {
+    if b {
+        cold_path();
+        true
+    } else {
+        false
+    }
+}
+
 // FIXME: use strict provenance functions once they are stable.
 // Implement it with a transmute for now.
 #[inline(always)]