Use trigrams to speed up SpecialCaseList.

Summary:
it's often the case when the rules in the SpecialCaseList
are of the form hel.o*bar. That gives us a chance to build
trigram index to quickly discard 99% of inputs without
running a full regex. A similar idea was used in Google Code Search
as described in the blog post:
https://swtch.com/~rsc/regexp/regexp4.html

The check is defeated, if there's at least one regex
more complicated than that. In this case, all inputs
will go through the regex. That said, the real-world
rules are often simple or can be simplied. That considerably
speeds up compiling Chromium with CFI and UBSan.

As measured on Chromium's content_message_generator.cc:

before, CFI: 44 s
after, CFI: 23 s
after, CFI, no blacklist: 23 s (~1% slower, but 3 runs were unable to show the difference)
after, regular compilation to bitcode: 23 s

Reviewers: pcc

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D27188

llvm-svn: 288303

llvm/include/llvm/Support/TrigramIndex.h

@@ -0,0 +1,70 @@
+//===-- TrigramIndex.h - a heuristic for SpecialCaseList --------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//===----------------------------------------------------------------------===//
+//
+// TrigramIndex implements a heuristic for SpecialCaseList that allows to
+// filter out ~99% incoming queries when all regular expressions in the
+// SpecialCaseList are simple wildcards with '*' and '.'. If rules are more
+// complicated, the check is defeated and it will always pass the queries to a
+// full regex.
+//
+// The basic idea is that in order for a wildcard to match a query, the query
+// needs to have all trigrams which occur in the wildcard. We create a trigram
+// index (trigram -> list of rules with it) and then count trigrams in the query
+// for each rule. If the count for one of the rules reaches the expected value,
+// the check passes the query to a regex. If none of the rules got enough
+// trigrams, the check tells that the query is definitely not matched by any
+// of the rules, and no regex matching is needed.
+// A similar idea was used in Google Code Search as described in the blog post:
+// https://swtch.com/~rsc/regexp/regexp4.html
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_TRIGRAMINDEX_H
+#define LLVM_SUPPORT_TRIGRAMINDEX_H
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringMap.h"
+
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+namespace llvm {
+class StringRef;
+
+class TrigramIndex {
+ public:
+  /// Inserts a new Regex into the index.
+  void insert(std::string Regex);
+
+  /// Returns true, if special case list definitely does not have a line
+  /// that matches the query. Returns false, if it's not sure.
+  bool isDefinitelyOut(StringRef Query) const;
+
+  /// Returned true, iff the heuristic is defeated and not useful.
+  /// In this case isDefinitelyOut always returns false.
+  bool isDefeated() { return Defeated; }
+ private:
+  // If true, the rules are too complicated for the check to work, and full
+  // regex matching is needed for every rule.
+  bool Defeated = false;
+  // The minimum number of trigrams which should match for a rule to have a
+  // chance to match the query. The number of elements equals the number of
+  // regex rules in the SpecialCaseList.
+  std::vector<unsigned> Counts;
+  // Index holds a list of rules indices for each trigram. The same indices
+  // are used in Counts to store per-rule limits.
+  // If a trigram is too common (>4 rules with it), we stop tracking it,
+  // which increases the probability for a need to match using regex, but
+  // decreases the costs in the regular case.
+  std::unordered_map<unsigned, SmallVector<size_t, 4>> Index{256};
+};
+
+}  // namespace llvm
+
+#endif  // LLVM_SUPPORT_TRIGRAMINDEX_H

llvm/lib/Support/CMakeLists.txt

@@ -94,6 +94,7 @@ add_llvm_library(LLVMSupport
   ThreadPool.cpp
   Timer.cpp
   ToolOutputFile.cpp
+  TrigramIndex.cpp
   Triple.cpp
   Twine.cpp
   Unicode.cpp

llvm/lib/Support/SpecialCaseList.cpp

@@ -15,6 +15,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/SpecialCaseList.h"
+#include "llvm/Support/TrigramIndex.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSet.h"
@@ -33,10 +34,15 @@ namespace llvm {
 /// literal strings than Regex.
 struct SpecialCaseList::Entry {
   StringSet<> Strings;
+  TrigramIndex Trigrams;
   std::unique_ptr<Regex> RegEx;
 
   bool match(StringRef Query) const {
-    return Strings.count(Query) || (RegEx && RegEx->match(Query));
+    if (Strings.count(Query))
+      return true;
+    if (Trigrams.isDefinitelyOut(Query))
+      return false;
+    return RegEx && RegEx->match(Query);
   }
 };
 
@@ -104,10 +110,12 @@ bool SpecialCaseList::parse(const MemoryBuffer *MB, std::string &Error) {
     StringRef Category = SplitRegexp.second;
 
     // See if we can store Regexp in Strings.
+    auto &Entry = Entries[Prefix][Category];
     if (Regex::isLiteralERE(Regexp)) {
-      Entries[Prefix][Category].Strings.insert(Regexp);
+      Entry.Strings.insert(Regexp);
       continue;
     }
+    Entry.Trigrams.insert(Regexp);
 
     // Replace * with .*
     for (size_t pos = 0; (pos = Regexp.find('*', pos)) != std::string::npos;

llvm/lib/Support/TrigramIndex.cpp

@@ -0,0 +1,98 @@
+//===-- TrigramIndex.cpp - a heuristic for SpecialCaseList ----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// TrigramIndex implements a heuristic for SpecialCaseList that allows to
+// filter out ~99% incoming queries when all regular expressions in the
+// SpecialCaseList are simple wildcards with '*' and '.'. If rules are more
+// complicated, the check is defeated and it will always pass the queries to a
+// full regex.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/TrigramIndex.h"
+#include "llvm/ADT/SmallVector.h"
+
+#include <unordered_map>
+#include <set>
+#include <string>
+
+using namespace llvm;
+
+static const char RegexAdvancedMetachars[] = "()^$|+?[]\\{}";
+
+static bool isSimpleWildcard(StringRef Str) {
+  // Check for regex metacharacters other than '*' and '.'.
+  return Str.find_first_of(RegexAdvancedMetachars) == StringRef::npos;
+}
+
+void TrigramIndex::insert(std::string Regex) {
+  if (Defeated) return;
+  if (!isSimpleWildcard(Regex)) {
+    Defeated = true;
+    return;
+  }
+
+  std::set<unsigned> Was;
+  unsigned Cnt = 0;
+  unsigned Tri = 0;
+  unsigned Len = 0;
+  for (unsigned Char : Regex) {
+    if (Char == '.' || Char == '*') {
+      Tri = 0;
+      Len = 0;
+      continue;
+    }
+    Tri = ((Tri << 8) + Char) & 0xFFFFFF;
+    Len++;
+    if (Len < 3)
+      continue;
+    // We don't want the index to grow too much for the popular trigrams,
+    // as they are weak signals. It's ok to still require them for the
+    // rules we have already processed. It's just a small additional
+    // computational cost.
+    if (Index[Tri].size() >= 4)
+      continue;
+    Cnt++;
+    if (!Was.count(Tri)) {
+      // Adding the current rule to the index.
+      Index[Tri].push_back(Counts.size());
+      Was.insert(Tri);
+    }
+  }
+  if (!Cnt) {
+    // This rule does not have remarkable trigrams to rely on.
+    // We have to always call the full regex chain.
+    Defeated = true;
+    return;
+  }
+  Counts.push_back(Cnt);
+}
+
+bool TrigramIndex::isDefinitelyOut(StringRef Query) const {
+  if (Defeated)
+    return false;
+  std::vector<unsigned> CurCounts(Counts.size());
+  unsigned Tri = 0;
+  for (size_t I = 0; I < Query.size(); I++) {
+    Tri = ((Tri << 8) + Query[I]) & 0xFFFFFF;
+    if (I < 2)
+      continue;
+    const auto &II = Index.find(Tri);
+    if (II == Index.end())
+      continue;
+    for (size_t J : II->second) {
+      CurCounts[J]++;
+      // If we have reached a desired limit, we have to look at the query
+      // more closely by running a full regex.
+      if (CurCounts[J] >= Counts[J])
+        return false;
+    }
+  }
+  return true;
+}

llvm/unittests/Support/CMakeLists.txt

@@ -48,6 +48,7 @@ add_llvm_unittest(SupportTests
   TimerTest.cpp
   TypeNameTest.cpp
   TrailingObjectsTest.cpp
+  TrigramIndexTest.cpp
   UnicodeTest.cpp
   YAMLIOTest.cpp
   YAMLParserTest.cpp

llvm/unittests/Support/SpecialCaseListTest.cpp

@@ -134,4 +134,48 @@ TEST_F(SpecialCaseListTest, MultipleBlacklists) {
     sys::fs::remove(Path);
 }
 
+TEST_F(SpecialCaseListTest, NoTrigramsInRules) {
+  std::unique_ptr<SpecialCaseList> SCL = makeSpecialCaseList("fun:b.r\n"
+                                                             "fun:za*az\n");
+  EXPECT_TRUE(SCL->inSection("fun", "bar"));
+  EXPECT_FALSE(SCL->inSection("fun", "baz"));
+  EXPECT_TRUE(SCL->inSection("fun", "zakaz"));
+  EXPECT_FALSE(SCL->inSection("fun", "zaraza"));
+}
+
+TEST_F(SpecialCaseListTest, NoTrigramsInARule) {
+  std::unique_ptr<SpecialCaseList> SCL = makeSpecialCaseList("fun:*bar*\n"
+                                                             "fun:za*az\n");
+  EXPECT_TRUE(SCL->inSection("fun", "abara"));
+  EXPECT_FALSE(SCL->inSection("fun", "bor"));
+  EXPECT_TRUE(SCL->inSection("fun", "zakaz"));
+  EXPECT_FALSE(SCL->inSection("fun", "zaraza"));
+}
+
+TEST_F(SpecialCaseListTest, RepetitiveRule) {
+  std::unique_ptr<SpecialCaseList> SCL = makeSpecialCaseList("fun:*bar*bar*bar*bar*\n"
+                                                             "fun:bar*\n");
+  EXPECT_TRUE(SCL->inSection("fun", "bara"));
+  EXPECT_FALSE(SCL->inSection("fun", "abara"));
+  EXPECT_TRUE(SCL->inSection("fun", "barbarbarbar"));
+  EXPECT_TRUE(SCL->inSection("fun", "abarbarbarbar"));
+  EXPECT_FALSE(SCL->inSection("fun", "abarbarbar"));
+}
+
+TEST_F(SpecialCaseListTest, SpecialSymbolRule) {
+  std::unique_ptr<SpecialCaseList> SCL = makeSpecialCaseList("src:*c\\+\\+abi*\n");
+  EXPECT_TRUE(SCL->inSection("src", "c++abi"));
+  EXPECT_FALSE(SCL->inSection("src", "c\\+\\+abi"));
+}
+
+TEST_F(SpecialCaseListTest, PopularTrigram) {
+  std::unique_ptr<SpecialCaseList> SCL = makeSpecialCaseList("fun:*aaaaaa*\n"
+                                                             "fun:*aaaaa*\n"
+                                                             "fun:*aaaa*\n"
+                                                             "fun:*aaa*\n");
+  EXPECT_TRUE(SCL->inSection("fun", "aaa"));
+  EXPECT_TRUE(SCL->inSection("fun", "aaaa"));
+  EXPECT_TRUE(SCL->inSection("fun", "aaaabbbaaa"));
+}
+
 }

llvm/unittests/Support/TrigramIndexTest.cpp

@@ -0,0 +1,112 @@
+//===- TrigramIndexTest.cpp - Unit tests for TrigramIndex -----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/TrigramIndex.h"
+#include "gtest/gtest.h"
+
+#include <string>
+#include <vector>
+
+using namespace llvm;
+
+namespace {
+
+class TrigramIndexTest : public ::testing::Test {
+protected:
+  std::unique_ptr<TrigramIndex> makeTrigramIndex(
+      std::vector<std::string> Rules) {
+    std::unique_ptr<TrigramIndex> TI =
+        make_unique<TrigramIndex>();
+    for (auto &Rule : Rules)
+      TI->insert(Rule);
+    return TI;
+  }
+};
+
+TEST_F(TrigramIndexTest, Empty) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({});
+  EXPECT_FALSE(TI->isDefeated());
+  EXPECT_TRUE(TI->isDefinitelyOut("foo"));
+}
+
+TEST_F(TrigramIndexTest, Basic) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"*hello*", "*wor.d*"});
+  EXPECT_FALSE(TI->isDefeated());
+  EXPECT_TRUE(TI->isDefinitelyOut("foo"));
+}
+
+TEST_F(TrigramIndexTest, NoTrigramsInRules) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"b.r", "za*az"});
+  EXPECT_TRUE(TI->isDefeated());
+  EXPECT_FALSE(TI->isDefinitelyOut("foo"));
+  EXPECT_FALSE(TI->isDefinitelyOut("bar"));
+  EXPECT_FALSE(TI->isDefinitelyOut("zakaz"));
+}
+
+TEST_F(TrigramIndexTest, NoTrigramsInARule) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"*hello*", "*wo.ld*"});
+  EXPECT_TRUE(TI->isDefeated());
+  EXPECT_FALSE(TI->isDefinitelyOut("foo"));
+}
+
+TEST_F(TrigramIndexTest, RepetitiveRule) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"*bar*bar*bar*bar*bar", "bar*bar"});
+  EXPECT_FALSE(TI->isDefeated());
+  EXPECT_TRUE(TI->isDefinitelyOut("foo"));
+  EXPECT_TRUE(TI->isDefinitelyOut("bar"));
+  EXPECT_FALSE(TI->isDefinitelyOut("barbara"));
+  EXPECT_FALSE(TI->isDefinitelyOut("bar+bar"));
+}
+
+TEST_F(TrigramIndexTest, PopularTrigram) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"*aaa*", "*aaaa*", "*aaaaa*", "*aaaaa*", "*aaaaaa*"});
+  EXPECT_TRUE(TI->isDefeated());
+}
+
+TEST_F(TrigramIndexTest, PopularTrigram2) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"class1.h", "class2.h", "class3.h", "class4.h", "class.h"});
+  EXPECT_TRUE(TI->isDefeated());
+}
+
+TEST_F(TrigramIndexTest, TooComplicatedRegex) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"[0-9]+"});
+  EXPECT_TRUE(TI->isDefeated());
+}
+
+TEST_F(TrigramIndexTest, TooComplicatedRegex2) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"foo|bar"});
+  EXPECT_TRUE(TI->isDefeated());
+}
+
+TEST_F(TrigramIndexTest, SpecialSymbol) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"*c\\+\\+*"});
+  EXPECT_TRUE(TI->isDefeated());
+}
+
+TEST_F(TrigramIndexTest, Sequence) {
+  std::unique_ptr<TrigramIndex> TI =
+      makeTrigramIndex({"class1.h", "class2.h", "class3.h", "class4.h"});
+  EXPECT_FALSE(TI->isDefeated());
+  EXPECT_FALSE(TI->isDefinitelyOut("class1"));
+  EXPECT_TRUE(TI->isDefinitelyOut("class.h"));
+  EXPECT_TRUE(TI->isDefinitelyOut("class"));
+}
+
+}  // namespace