Use likely()/unlikely() macros to improve branch prediction.

src/compiler.h

@@ -0,0 +1,32 @@
+/*
+ * Compiler specific macros.
+ *
+ * Copyright (C) 2013  Simon Ruderich
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef COMPILER_H
+#define COMPILER_H 1
+
+/* Branch prediction information for the compiler. */
+#ifdef HAVE___BUILTIN_EXPECT
+# define likely(x)   __builtin_expect(!!(x), 1)
+# define unlikely(x) __builtin_expect(!!(x), 0)
+#else
+# define likely(x)   x
+# define unlikely(x) x
+#endif
+
+#endif