[src/trunk]: src/sys/net Enable ABC optimization when one branch returns 0.

[alnsn <alnsn%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/767f18e0cc2e
branches:  trunk
changeset: 796192:767f18e0cc2e
user:      alnsn <alnsn%NetBSD.org@localhost>
date:      Fri May 23 22:04:09 2014 +0000

description:
Enable ABC optimization when one branch returns 0.

diffstat:

 sys/net/bpfjit.c |  28 ++++++++++++++++++++++++----
 1 files changed, 24 insertions(+), 4 deletions(-)

diffs (50 lines):

diff -r b17df0e3d432 -r 767f18e0cc2e sys/net/bpfjit.c
--- a/sys/net/bpfjit.c  Fri May 23 20:50:16 2014 +0000
+++ b/sys/net/bpfjit.c  Fri May 23 22:04:09 2014 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: bpfjit.c,v 1.10 2014/05/23 19:51:16 alnsn Exp $        */
+/*     $NetBSD: bpfjit.c,v 1.11 2014/05/23 22:04:09 alnsn Exp $        */
 
 /*-
  * Copyright (c) 2011-2014 Alexander Nasonov.
@@ -31,9 +31,9 @@
 
 #include <sys/cdefs.h>
 #ifdef _KERNEL
-__KERNEL_RCSID(0, "$NetBSD: bpfjit.c,v 1.10 2014/05/23 19:51:16 alnsn Exp $");
+__KERNEL_RCSID(0, "$NetBSD: bpfjit.c,v 1.11 2014/05/23 22:04:09 alnsn Exp $");
 #else
-__RCSID("$NetBSD: bpfjit.c,v 1.10 2014/05/23 19:51:16 alnsn Exp $");
+__RCSID("$NetBSD: bpfjit.c,v 1.11 2014/05/23 22:04:09 alnsn Exp $");
 #endif
 
 #include <sys/types.h>
@@ -1146,7 +1146,27 @@
 
                switch (BPF_CLASS(pc->code)) {
                case BPF_RET:
-                       abc_length = 0;
+                       /*
+                        * It's quite common for bpf programs to
+                        * check packet bytes in increasing order
+                        * and return zero if bytes don't match
+                        * specified critetion. Such programs disable
+                        * ABC optimization completely because for
+                        * every jump there is a branch with no read
+                        * instruction.
+                        * With no side effects, BPF_RET+BPF_K 0 is
+                        * indistinguishable from out-of-bound load.
+                        * Therefore, abc_length can be set to
+                        * MAX_ABC_LENGTH and enable ABC for many
+                        * bpf programs.
+                        * If this optimization pass encounters any
+                        * instruction with a side effect, it will
+                        * reset abc_length.
+                        */
+                       if (BPF_RVAL(pc->code) == BPF_K && pc->k == 0)
+                               abc_length = MAX_ABC_LENGTH;
+                       else
+                               abc_length = 0;
                        break;
 
                case BPF_JMP: