diff --git a/share/man/man9/style.9 b/share/man/man9/style.9
--- a/share/man/man9/style.9
+++ b/share/man/man9/style.9
@@ -902,6 +902,19 @@
 .Fn _Static_assert
 instead of the older
 .Fn CTASSERT .
+.Pp
+.Fn __predict_true
+and
+.Fn __predict_false
+should only be used in performance critical code.
+Consistently use
+.Fn __predict_false
+for error conditions that are not typical, and document the exceptions.
+Use
+.Fn __predict_true
+for operations expected to almost always succeed.
+Do not add these annotations without empirical evidence of the likelihood of the
+branch.
 .Sh FILES
 .Bl -tag -width indent
 .It Pa /usr/src/tools/build/checkstyle9.pl
diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -321,6 +321,13 @@
 #define	__restrict	restrict
 #endif
 
+/*
+ * All modern compilers have explicit branch prediction so that the CPU back-end
+ * can hint to the processor and also so that code blocks can be reordered such
+ * that the predicted path sees a more linear flow, thus improving cache
+ * behavior, etc. Use sparingly, except in performance critical code where
+ * they make things faster.
+ */
 #define	__predict_true(exp)     __builtin_expect((exp), 1)
 #define	__predict_false(exp)    __builtin_expect((exp), 0)