Bug 1491151 - Part 1: Add MruCache. r=smaug,froydnj

This adds a most recently used cache that can be used as a quick lookup table
for frequently used entries.

--HG--
extra : rebase_source : 571c32f75e985e299113f73b959809d208aad5f3

xpcom/ds/MruCache.h

@@ -0,0 +1,183 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_MruCache_h
+#define mozilla_MruCache_h
+
+#include <cstdint>
+#include <type_traits>
+#include <utility>
+
+#include "mozilla/Attributes.h"
+#include "mozilla/HashFunctions.h"
+
+namespace mozilla {
+
+namespace detail {
+
+// Helper struct for checking if a value is empty.
+//
+// `IsNotEmpty` will return true if `Value` is not a pointer type or if the
+// pointer value is not null.
+template <typename Value, bool IsPtr = std::is_pointer<Value>::value>
+struct EmptyChecker
+{
+  static bool IsNotEmpty(const Value&) { return true; }
+};
+// Template specialization for the `IsPtr == true` case.
+template <typename Value>
+struct EmptyChecker<Value, true>
+{
+  static bool IsNotEmpty(const Value& aVal) { return aVal != nullptr; }
+};
+
+} // namespace detail
+
+// Provides a most recently used cache that can be used as a layer on top of
+// a larger container where lookups can be expensive. The default size is 31,
+// which as a prime number provides a better distrubution of cached entries.
+//
+// Users are expected to provide a `Cache` class that defines two required
+// methods:
+//   - A method for providing the hash of a key:
+//
+//     static HashNumber Hash(const KeyType& aKey)
+//
+//   - A method for matching a key to a value, for pointer types the value
+//     is guaranteed not to be null.
+//
+//     static bool Match(const KeyType& aKey, const ValueType& aVal)
+//
+// For example:
+//    class MruExample : public MruCache<void*, PtrInfo*, MruExample>
+//    {
+//      static HashNumber Hash(const KeyType& aKey)
+//      {
+//        return HashGeneric(aKey);
+//      }
+//      static Match(const KeyType& aKey, const ValueType& aVal)
+//      {
+//        return aVal->mPtr == aKey;
+//      }
+//    };
+template <class Key, class Value, class Cache, size_t Size=31>
+class MruCache
+{
+  // Best distribution is achieved with a prime number. Ideally the closest
+  // to a power of two will be the most efficient use of memory. This
+  // assertion is pretty weak, but should catch the common inclination to
+  // use a power-of-two.
+  static_assert(Size % 2 != 0, "Use a prime number");
+
+  // This is a stronger assertion but significantly limits the values to just
+  // those close to a power-of-two value.
+  //static_assert(Size == 7 || Size == 13 || Size == 31 || Size == 61 ||
+  //              Size == 127 || Size == 251 || Size == 509 || Size == 1021,
+  //              "Use a prime number less than 1024");
+
+public:
+  using KeyType = Key;
+  using ValueType = Value;
+
+  MruCache() = default;
+  MruCache(const MruCache&) = delete;
+  MruCache(const MruCache&&) = delete;
+
+  // Inserts the given value into the cache. Potentially overwrites an
+  // existing entry.
+  template <typename U>
+  void Put(const KeyType& aKey, U&& aVal)
+  {
+    *RawEntry(aKey) = std::forward<U>(aVal);
+  }
+
+  // Removes the given entry if it is in the cache.
+  void Remove(const KeyType& aKey)
+  {
+    Lookup(aKey).Remove();
+  }
+
+  // Clears all cached entries and resets them to a default value.
+  void Clear()
+  {
+    for (ValueType& val : mCache) {
+      val = ValueType{};
+    }
+  }
+
+  // Helper that holds an entry that matched a lookup key. Usage:
+  //
+  //    auto p = mCache.Lookup(aKey);
+  //    if (p) {
+  //      return p.Data();
+  //    }
+  //
+  //    auto foo = new Foo();
+  //    mTable.Insert(aKey, foo);
+  //    p.Set(foo);
+  //    return foo;
+  class Entry
+  {
+  public:
+    Entry(ValueType* aEntry, bool aMatch)
+        : mEntry(aEntry)
+        , mMatch(aMatch)
+    {
+      MOZ_ASSERT(mEntry);
+    }
+
+    explicit operator bool() const { return mMatch; }
+
+    ValueType& Data() const
+    {
+      MOZ_ASSERT(mMatch);
+      return *mEntry;
+    }
+
+    template<typename U>
+    void Set(U&& aValue)
+    {
+      mMatch = true;
+      Data() = std::forward<U>(aValue);
+    }
+
+    void Remove()
+    {
+      if (mMatch) {
+        Data() = ValueType{};
+        mMatch = false;
+      }
+    }
+
+  private:
+    ValueType* mEntry; // Location of the entry in the cache.
+    bool mMatch;       // Whether the value matched.
+  };
+
+  // Retrieves an entry from the cache. Can be used to test if an entry is
+  // present, update the entry to a new value, or remove the entry if one was
+  // matched.
+  Entry Lookup(const KeyType& aKey)
+  {
+    using EmptyChecker = detail::EmptyChecker<ValueType>;
+
+    auto entry = RawEntry(aKey);
+    bool match = EmptyChecker::IsNotEmpty(*entry) && Cache::Match(aKey, *entry);
+    return Entry(entry, match);
+  }
+
+private:
+    MOZ_ALWAYS_INLINE ValueType* RawEntry(const KeyType& aKey)
+    {
+      return &mCache[Cache::Hash(aKey) % Size];
+    }
+
+    ValueType mCache[Size] = {};
+};
+
+} // namespace mozilla
+
+#endif // mozilla_mrucache_h

xpcom/ds/moz.build

@@ -87,6 +87,7 @@ EXPORTS.mozilla += [
     'AtomArray.h',
     'Dafsa.h',
     'IncrementalTokenizer.h',
+    'MruCache.h',
     'Observer.h',
     'SimpleEnumerator.h',
     'StickyTimeDuration.h',

xpcom/tests/gtest/TestMruCache.cpp

@@ -0,0 +1,393 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+
+#include "gtest/gtest.h"
+
+#include "mozilla/MruCache.h"
+#include "nsString.h"
+
+using namespace mozilla;
+
+// A few MruCache implementations to use during testing.
+struct IntMap : public MruCache<int, int, IntMap>
+{
+  static HashNumber Hash(const KeyType& aKey) { return aKey - 1; }
+  static bool Match(const KeyType& aKey, const ValueType& aVal) { return aKey == aVal; }
+};
+
+struct UintPtrMap : public MruCache<uintptr_t, int*, UintPtrMap>
+{
+  static HashNumber Hash(const KeyType& aKey) { return aKey - 1; }
+  static bool Match(const KeyType& aKey, const ValueType& aVal) { return aKey == (KeyType)aVal; }
+};
+
+struct StringStruct
+{
+  nsCString mKey;
+  nsCString mOther;
+};
+
+struct StringStructMap : public MruCache<nsCString, StringStruct, StringStructMap>
+{
+  static HashNumber Hash(const KeyType& aKey) { return *aKey.BeginReading() - 1; }
+  static bool Match(const KeyType& aKey, const ValueType& aVal) { return aKey == aVal.mKey; }
+};
+
+// Helper for emulating convertable holders such as RefPtr.
+template <typename T>
+struct Convertable
+{
+  T mItem;
+  operator T() const { return mItem; }
+};
+
+// Helper to create a StringStructMap key.
+nsCString MakeStringKey(char aKey)
+{
+  nsCString key;
+  key.Append(aKey);
+  return key;
+}
+
+TEST(MruCache, TestNullChecker)
+{
+  using mozilla::detail::EmptyChecker;
+
+  {
+    int test = 0;
+    EXPECT_TRUE(EmptyChecker<decltype(test)>::IsNotEmpty(test));
+
+    test = 42;
+    EXPECT_TRUE(EmptyChecker<decltype(test)>::IsNotEmpty(test));
+  }
+
+  {
+    const char* test = "abc";
+    EXPECT_TRUE(EmptyChecker<decltype(test)>::IsNotEmpty(test));
+
+    test = nullptr;
+    EXPECT_FALSE(EmptyChecker<decltype(test)>::IsNotEmpty(test));
+  }
+
+  {
+    int foo = 42;
+    int* test = &foo;
+    EXPECT_TRUE(EmptyChecker<decltype(test)>::IsNotEmpty(test));
+
+    test = nullptr;
+    EXPECT_FALSE(EmptyChecker<decltype(test)>::IsNotEmpty(test));
+  }
+}
+
+TEST(MruCache, TestEmptyCache)
+{
+  {
+    // Test a basic empty cache.
+    IntMap mru;
+
+    // Make sure the default values are set.
+    for (int i = 1; i < 32; i++) {
+      auto p = mru.Lookup(i);
+
+      // Shouldn't be found.
+      EXPECT_FALSE(p);
+    }
+  }
+
+  {
+    // Test an empty cache with pointer values.
+    UintPtrMap mru;
+
+    // Make sure the default values are set.
+    for (uintptr_t i = 1; i < 32; i++) {
+      auto p = mru.Lookup(i);
+
+      // Shouldn't be found.
+      EXPECT_FALSE(p);
+    }
+  }
+
+  {
+    // Test an empty cache with more complex structure.
+    StringStructMap mru;
+
+    // Make sure the default values are set.
+    for (char i = 1; i < 32; i++) {
+      const nsCString key = MakeStringKey(i);
+      auto p = mru.Lookup(key);
+
+      // Shouldn't be found.
+      EXPECT_FALSE(p);
+    }
+  }
+}
+
+TEST(MruCache, TestPut)
+{
+  IntMap mru;
+
+  // Fill it up.
+  for (int i = 1; i < 32; i++) {
+    mru.Put(i, i);
+  }
+
+  // Now check each value.
+  for (int i = 1; i < 32; i++) {
+    auto p = mru.Lookup(i);
+
+    // Should be found.
+    EXPECT_TRUE(p);
+    EXPECT_EQ(p.Data(), i);
+  }
+}
+
+TEST(MruCache, TestPutConvertable)
+{
+  UintPtrMap mru;
+
+  // Fill it up.
+  for (uintptr_t i = 1; i < 32; i++) {
+    Convertable<int*> val{(int*)i};
+    mru.Put(i, val);
+  }
+
+  // Now check each value.
+  for (uintptr_t i = 1; i < 32; i++) {
+    auto p = mru.Lookup(i);
+
+    // Should be found.
+    EXPECT_TRUE(p);
+    EXPECT_EQ(p.Data(), (int*)i);
+  }
+}
+
+TEST(MruCache, TestOverwriting)
+{
+  // Test overwrting
+  IntMap mru;
+
+  // 1-31 should be overwritten by 32-63
+  for (int i = 1; i < 63; i++) {
+    mru.Put(i, i);
+  }
+
+  // Look them up.
+  for (int i = 32; i < 63; i++) {
+    auto p = mru.Lookup(i);
+
+    // Should be found.
+    EXPECT_TRUE(p);
+    EXPECT_EQ(p.Data(), i);
+  }
+}
+
+TEST(MruCache, TestRemove)
+{
+  {
+    IntMap mru;
+
+    // Fill it up.
+    for (int i = 1; i < 32; i++) {
+      mru.Put(i, i);
+    }
+
+    // Now remove each value.
+    for (int i = 1; i < 32; i++) {
+      // Should be present.
+      auto p = mru.Lookup(i);
+      EXPECT_TRUE(p);
+
+      mru.Remove(i);
+
+      // Should no longer match.
+      p = mru.Lookup(i);
+      EXPECT_FALSE(p);
+    }
+  }
+
+  {
+    UintPtrMap mru;
+
+    // Fill it up.
+    for (uintptr_t i = 1; i < 32; i++) {
+      mru.Put(i, (int*)i);
+    }
+
+    // Now remove each value.
+    for (uintptr_t i = 1; i < 32; i++) {
+      // Should be present.
+      auto p = mru.Lookup(i);
+      EXPECT_TRUE(p);
+
+      mru.Remove(i);
+
+      // Should no longer match.
+      p = mru.Lookup(i);
+      EXPECT_FALSE(p);
+    }
+  }
+
+  {
+    StringStructMap mru;
+
+    // Fill it up.
+    for (char i = 1; i < 32; i++) {
+      const nsCString key = MakeStringKey(i);
+      mru.Put(key, StringStruct{key, NS_LITERAL_CSTRING("foo")});
+    }
+
+    // Now remove each value.
+    for (char i = 1; i < 32; i++) {
+      const nsCString key = MakeStringKey(i);
+
+      // Should be present.
+      auto p = mru.Lookup(key);
+      EXPECT_TRUE(p);
+
+      mru.Remove(key);
+
+      // Should no longer match.
+      p = mru.Lookup(key);
+      EXPECT_FALSE(p);
+    }
+  }
+}
+
+TEST(MruCache, TestClear)
+{
+  IntMap mru;
+
+  // Fill it up.
+  for (int i = 1; i < 32; i++) {
+    mru.Put(i, i);
+  }
+
+  // Empty it.
+  mru.Clear();
+
+  // Now check each value.
+  for (int i = 1; i < 32; i++) {
+    auto p = mru.Lookup(i);
+
+    // Should not be found.
+    EXPECT_FALSE(p);
+  }
+}
+
+TEST(MruCache, TestLookupMissingAndSet)
+{
+  IntMap mru;
+
+  // Value not found.
+  auto p = mru.Lookup(1);
+  EXPECT_FALSE(p);
+
+  // Set it.
+  p.Set(1);
+  EXPECT_TRUE(p);
+  EXPECT_EQ(p.Data(), 1);
+
+  // Look it up again.
+  p = mru.Lookup(1);
+  EXPECT_TRUE(p);
+  EXPECT_EQ(p.Data(), 1);
+
+  // Test w/ a convertable value.
+  p = mru.Lookup(2);
+  EXPECT_FALSE(p);
+
+  // Set it.
+  Convertable<int> val{2};
+  p.Set(val);
+  EXPECT_TRUE(p);
+  EXPECT_EQ(p.Data(), 2);
+
+  // Look it up again.
+  p = mru.Lookup(2);
+  EXPECT_TRUE(p);
+  EXPECT_EQ(p.Data(), 2);
+}
+
+TEST(MruCache, TestLookupAndOverwrite)
+{
+  IntMap mru;
+
+  // Set 1.
+  mru.Put(1, 1);
+
+  // Lookup a key that maps the 1's entry.
+  auto p = mru.Lookup(32);
+  EXPECT_FALSE(p); // not a match
+
+  // Now overwrite the entry.
+  p.Set(32);
+  EXPECT_TRUE(p);
+  EXPECT_EQ(p.Data(), 32);
+
+  // 1 should be gone now.
+  p = mru.Lookup(1);
+  EXPECT_FALSE(p);
+
+  // 32 should be found.
+  p = mru.Lookup(32);
+  EXPECT_TRUE(p);
+  EXPECT_EQ(p.Data(), 32);
+}
+
+TEST(MruCache, TestLookupAndRemove)
+{
+  IntMap mru;
+
+  // Set 1.
+  mru.Put(1, 1);
+
+  auto p = mru.Lookup(1);
+  EXPECT_TRUE(p);
+  EXPECT_EQ(p.Data(), 1);
+
+  // Now remove it.
+  p.Remove();
+  EXPECT_FALSE(p);
+
+  p = mru.Lookup(1);
+  EXPECT_FALSE(p);
+}
+
+TEST(MruCache, TestLookupNotMatchedAndRemove)
+{
+  IntMap mru;
+
+  // Set 1.
+  mru.Put(1, 1);
+
+  // Lookup a key that matches 1's entry.
+  auto p = mru.Lookup(32);
+  EXPECT_FALSE(p);
+
+  // Now attempt to remove it.
+  p.Remove();
+
+  // Make sure 1 is still there.
+  p = mru.Lookup(1);
+  EXPECT_TRUE(p);
+  EXPECT_EQ(p.Data(), 1);
+}
+
+TEST(MruCache, TestLookupAndSetWithMove)
+{
+  StringStructMap mru;
+
+  const nsCString key = MakeStringKey((char)1);
+  StringStruct val{key, NS_LITERAL_CSTRING("foo")};
+
+  auto p = mru.Lookup(key);
+  EXPECT_FALSE(p);
+  p.Set(std::move(val));
+
+  EXPECT_TRUE(p.Data().mKey == key);
+  EXPECT_TRUE(p.Data().mOther == NS_LITERAL_CSTRING("foo"));
+}

xpcom/tests/gtest/TestObserverArray.cpp

@@ -6,6 +6,7 @@
 
 #include "nsTObserverArray.h"
 #include "gtest/gtest.h"
+#include "mozilla/ArrayUtils.h"
 
 using namespace mozilla;
 

xpcom/tests/gtest/TestTArray.cpp

@@ -6,6 +6,7 @@
 
 #include "nsTArray.h"
 #include "gtest/gtest.h"
+#include "mozilla/ArrayUtils.h"
 
 using namespace mozilla;
 

xpcom/tests/gtest/moz.build

@@ -29,6 +29,7 @@ UNIFIED_SOURCES += [
     'TestLogCommandLineHandler.cpp',
     'TestMoveString.cpp',
     'TestMozPromise.cpp',
+    'TestMruCache.cpp',
     'TestMultiplexInputStream.cpp',
     'TestNonBlockingAsyncInputStream.cpp',
     'TestNsDeque.cpp',