Merge pull request #631 from DaniPopes/readd-likely

Re-implement likely/unlikely with `#[cold]`

Author: Amanieu

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)]