3 files changed, 109 insertions, 2 deletions

diff --git a/llvm/lib/Support/CMakeLists.txt b/llvm/lib/Support/CMakeLists.txt
index 33ab04df5b5..d6cbfbda2c4 100644
--- a/llvm/lib/Support/CMakeLists.txt
+++ b/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
diff --git a/llvm/lib/Support/SpecialCaseList.cpp b/llvm/lib/Support/SpecialCaseList.cpp
index 62dcafefd88..df524b35235 100644
--- a/llvm/lib/Support/SpecialCaseList.cpp
+++ b/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;
diff --git a/llvm/lib/Support/TrigramIndex.cpp b/llvm/lib/Support/TrigramIndex.cpp
new file mode 100644
index 00000000000..bba996e58ec
--- /dev/null
+++ b/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;
+}