tag: Reintroduce tag library.

It is needed for the classifier partitioning optimization.  This commit
only reintroduces the parts that are actually needed.

Signed-off-by: Ben Pfaff <[email protected]>

lib/automake.mk

@@ -199,6 +199,8 @@ lib_libopenvswitch_a_SOURCES = \
 	lib/svec.h \
 	lib/table.c \
 	lib/table.h \
+	lib/tag.c \
+	lib/tag.h \
 	lib/timer.c \
 	lib/timer.h \
 	lib/timeval.c \

lib/tag.c

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2008, 2009, 2010, 2011, 2013 Nicira, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <config.h>
+#include "tag.h"
+#include <limits.h>
+
+#define N_TAG_BITS (CHAR_BIT * sizeof(tag_type))
+BUILD_ASSERT_DECL(IS_POW2(N_TAG_BITS));
+
+#define LOG2_N_TAG_BITS (N_TAG_BITS == 32 ? 5 : N_TAG_BITS == 64 ? 6 : 0)
+BUILD_ASSERT_DECL(LOG2_N_TAG_BITS > 0);
+
+/* Returns a tag deterministically generated from 'seed'.
+ *
+ * 'seed' should have data in all of its bits; if it has data only in its
+ * low-order bits then the resulting tags will be poorly distributed.  Use a
+ * hash function such as hash_bytes() to generate 'seed' if necessary. */
+tag_type
+tag_create_deterministic(uint32_t seed)
+{
+    int x = seed & (N_TAG_BITS - 1);
+    int y = (seed >> LOG2_N_TAG_BITS) % (N_TAG_BITS - 1);
+    y += y >= x;
+    return (1u << x) | (1u << y);
+}

lib/tag.h

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2008, 2011, 2012, 2013 Nicira, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef TAG_H
+#define TAG_H 1
+
+#include <stdbool.h>
+#include <stdint.h>
+#include "util.h"
+
+/*
+ * Tagging support.
+ *
+ * A 'tag' represents an arbitrary category.  Currently, tags are used to
+ * represent categories of flows and in particular the value of the 64-bit
+ * "metadata" field in the flow.  The universe of possible categories is very
+ * large (2**64).  The number of categories in use at a given time can also be
+ * large.  This means that keeping track of category membership via
+ * conventional means (lists, bitmaps, etc.) is likely to be expensive.
+ *
+ * Tags are actually implemented via a "superimposed coding", as discussed in
+ * Knuth TAOCP v.3 section 6.5 "Retrieval on Secondary Keys".  A tag is an
+ * unsigned integer in which exactly 2 bits are set to 1 and the rest set to 0.
+ * For 32-bit integers (as currently used) there are 32 * 31 / 2 = 496 unique
+ * tags; for 64-bit integers there are 64 * 63 / 2 = 2,016.
+ *
+ * Because there is a small finite number of unique tags, tags must collide
+ * after some number of them have been created.  In practice we generally
+ * create tags by choosing bits randomly or based on a hash function.
+ *
+ * The key property of tags is that we can combine them without increasing the
+ * amount of data required using bitwise-OR, since the result has the 1-bits
+ * from both tags set.  The necessary tradeoff is that the result is even more
+ * ambiguous: if combining two tags yields a value with 4 bits set to 1, then
+ * the result value will test as having 4 * 3 / 2 = 6 unique tags, not just the
+ * two tags that we combined.
+ *
+ * The upshot is this: a value that is the bitwise-OR combination of a number
+ * of tags will always include the tags that were combined, but it may contain
+ * any number of additional tags as well.  This is acceptable for our use,
+ * since we want to be sure that we check every classifier table that contains
+ * a rule with a given metadata value, but it is OK if we check a few extra
+ * tables as well.
+ *
+ * If we combine too many tags, then the result will have every bit set, so
+ * that it will test as including every tag.  This can happen, but we hope that
+ * this is not the common case.
+ */
+
+/* Represents a tag, or the combination of 0 or more tags. */
+typedef uint32_t tag_type;
+
+/* A 'tag_type' value that intersects every tag. */
+#define TAG_ALL UINT32_MAX
+
+/* An arbitrary tag. */
+#define TAG_ARBITRARY UINT32_C(3)
+
+tag_type tag_create_deterministic(uint32_t seed);
+static inline bool tag_intersects(tag_type, tag_type);
+
+/* Returns true if 'a' and 'b' have at least one tag in common,
+ * false if their set of tags is disjoint. */
+static inline bool
+tag_intersects(tag_type a, tag_type b)
+{
+    tag_type x = a & b;
+    return (x & (x - 1)) != 0;
+}
+
+#endif /* tag.h */