Introduce cold function attribute

Attribute to deemphasize certain code paths.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

include/compiler.h

@@ -285,6 +285,22 @@ size_t strnlen(const char *s, size_t maxlen);
 # define no_return void
 #endif
 
+/*
+ * How to tell the compiler that a function is unlikely to be executed.
+ * This differs from unlikely() in that it is applied to a function call,
+ * not a boolean condition.
+ */
+#ifndef HAVE_FUNC_ATTRIBUTE_COLD
+# define unlikely_func __attribute__((cold))
+#else
+# define unlikely_func
+#endif
+
+/*
+ * A fatal function is both unlikely and no_return
+ */
+#define fatal_func no_return unlikely_func
+
 /*
  * How to tell the compiler that a function takes a printf-like string
  */