vppinfra: add CLIB_ASSUME() macro

This macro privides a way to tell compiler that it is safe to assume
that specified expression is true so it can optimize code accordingly.

i.e.

    CLIB_ASSUME (n < 3);

    while (n)
    {
      /* ... */
    }

Will tell compiler that n is never going to be >= 3 so instead of
creating loop it will just unroll loop 2 times.

Type: improvement
Change-Id: I718a9b95ff7980d7ac68eb9a88357a4ab6eee74a
Signed-off-by: Damjan Marion <damarion@cisco.com>

diff --git a/src/vppinfra/clib.h b/src/vppinfra/clib.h
index 81b59e2..52e7217 100644 (file)
--- a/src/vppinfra/clib.h
+++ b/src/vppinfra/clib.h
@@ -124,6 +124,13 @@
 #define PREDICT_FALSE(x) __builtin_expect((x),0)
 #define PREDICT_TRUE(x) __builtin_expect((x),1)
 #define COMPILE_TIME_CONST(x) __builtin_constant_p (x)
+#define CLIB_ASSUME(x)                                                        \
+  do                                                                          \
+    {                                                                         \
+      if (!(x))                                                               \
+       __builtin_unreachable ();                                             \
+    }                                                                         \
+  while (0)
 
 /*
  * Compiler barrier