If a block loads a value into the index register with a value different

from what it had on entry, we can't eliminate it, even if we don't use
the index register value.

diff --git a/optimize.c b/optimize.c
index da7a1e4b910c974a4548a69ae02828d1a318b9fd..ae2f9d6530ebfa94cfb18a51acba1aac676cf562 100644 (file)
--- a/optimize.c
+++ b/optimize.c
@@ -22,7 +22,7 @@
  */
 #ifndef lint
 static const char rcsid[] _U_ =
-    "@(#) $Header: /tcpdump/master/libpcap/optimize.c,v 1.77 2003-11-15 23:24:00 guy Exp $ (LBL)";
+    "@(#) $Header: /tcpdump/master/libpcap/optimize.c,v 1.78 2004-11-07 22:43:01 guy Exp $ (LBL)";
 #endif
 
 #ifdef HAVE_CONFIG_H
@@ -1101,7 +1101,7 @@ opt_blk(b, do_stmts)
        struct slist *s;
        struct edge *p;
        int i;
-       bpf_int32 aval;
+       bpf_int32 aval, xval;
 
 #if 0
        for (s = b->stmts; s && s->next; s = s->next)
@@ -1130,17 +1130,36 @@ opt_blk(b, do_stmts)
                }
        }
        aval = b->val[A_ATOM];
+       xval = b->val[X_ATOM];
        for (s = b->stmts; s; s = s->next)
                opt_stmt(&s->s, b->val, do_stmts);
 
        /*
         * This is a special case: if we don't use anything from this
-        * block, and we load the accumulator with value that is
-        * already there, or if this block is a return,
+        * block, and we load the accumulator or index register with a
+        * value that is already there, or if this block is a return,
         * eliminate all the statements.
+        *
+        * XXX - what if it does a store?
+        *
+        * XXX - why does it matter whether we use anything from this
+        * block?  If the accumulator or index register doesn't change
+        * its value, isn't that OK even if we use that value?
+        *
+        * XXX - if we load the accumulator with a different value,
+        * and the block ends with a conditional branch, we obviously
+        * can't eliminate it, as the branch depends on that value.
+        * For the index register, the conditional branch only depends
+        * on the index register value if the test is against the index
+        * register value rather than a constant; if nothing uses the
+        * value we put into the index register, and we're not testing
+        * against the index register's value, and there aren't any
+        * other problems that would keep us from eliminating this
+        * block, can we eliminate it?
         */
        if (do_stmts &&
-           ((b->out_use == 0 && aval != 0 &&b->val[A_ATOM] == aval) ||
+           ((b->out_use == 0 && aval != 0 && b->val[A_ATOM] == aval &&
+             xval != 0 && b->val[X_ATOM] == xval) ||
             BPF_CLASS(b->s.code) == BPF_RET)) {
                if (b->stmts != 0) {
                        b->stmts = 0;