Honor vnet_rewrite_data_t data_bytes == 0

Avoid the cache miss consequences of spraying [functionally harmless]
junk into un-prefetched rewrite space. As things stand, several tunnel
encap rewrites set rewrite data_bytes = 0, and take a performance hit
due to unwanted speculative copying.

Should be performance-neutral in speed-path cases, which won't execute
the added check.

Change-Id: Id83c0325e58c0f31631b4bae5a06457dfc7ed567
Signed-off-by: Dave Barach <dave@barachs.net>

diff --git a/src/vnet/adj/rewrite.h b/src/vnet/adj/rewrite.h
index 58d47e0..3278113 100644 (file)
--- a/src/vnet/adj/rewrite.h
+++ b/src/vnet/adj/rewrite.h
@@ -189,7 +189,13 @@ _vnet_rewrite_one_header (vnet_rewrite_header_t * h0,
       clib_memcpy (d, s, sizeof (eh_copy_t));
       return;
     }
-
+  /*
+   * Stop now if the data_bytes field is zero, to avoid the cache
+   * miss consequences of spraying [functionally harmless] junk into
+   * un-prefetched rewrite space.
+   */
+  if (PREDICT_FALSE (h0->data_bytes == 0))
+    return;
 
 #define _(i)                                                           \
   do {                                                                 \
@@ -244,6 +250,14 @@ _vnet_rewrite_two_headers (vnet_rewrite_header_t * h0,
       return;
     }
 
+  /*
+   * Stop now if both rewrite data_bytes fields are zero, to avoid the cache
+   * miss consequences of spraying [functionally harmless] junk into
+   * un-prefetched rewrite space.
+   */
+  if (PREDICT_FALSE (h0->data_bytes + h1->data_bytes == 0))
+    return;
+
 #define _(i)                                                           \
   do {                                                                 \
     if (most_likely_size > ((i)-1)*sizeof (vnet_rewrite_data_t))       \