Change the API constraints of erase(const_iterator, const_iterator) so that…

Change the API constraints of erase(const_iterator, const_iterator) so that calling erase(begin(), end()) resets reserved growth.

PiperOrigin-RevId: 551248712
Change-Id: I34755c63e3ee40da4ba7047e0d24eec567d28173

absl/container/flat_hash_map.h

@@ -235,7 +235,11 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning an
-  //   iterator pointing to `last`.
+  //   iterator pointing to `last`. The special case of calling
+  //   `erase(begin(), end())` resets the reserved growth such that if
+  //   `reserve(N)` has previously been called and there has been no intervening
+  //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
+  //   to assume that inserting N elements will not cause a rehash.
   //
   // size_type erase(const key_type& key):
   //

absl/container/flat_hash_set.h

@@ -227,7 +227,11 @@ class flat_hash_set
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning an
-  //   iterator pointing to `last`.
+  //   iterator pointing to `last`. The special case of calling
+  //   `erase(begin(), end())` resets the reserved growth such that if
+  //   `reserve(N)` has previously been called and there has been no intervening
+  //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
+  //   to assume that inserting N elements will not cause a rehash.
   //
   // size_type erase(const key_type& key):
   //

absl/container/internal/raw_hash_set.h

@@ -786,8 +786,11 @@ class CommonFieldsGenerationInfoEnabled {
  public:
   CommonFieldsGenerationInfoEnabled() = default;
   CommonFieldsGenerationInfoEnabled(CommonFieldsGenerationInfoEnabled&& that)
-      : reserved_growth_(that.reserved_growth_), generation_(that.generation_) {
+      : reserved_growth_(that.reserved_growth_),
+        reservation_size_(that.reservation_size_),
+        generation_(that.generation_) {
     that.reserved_growth_ = 0;
+    that.reservation_size_ = 0;
     that.generation_ = EmptyGeneration();
   }
   CommonFieldsGenerationInfoEnabled& operator=(
@@ -813,6 +816,8 @@ class CommonFieldsGenerationInfoEnabled {
   }
   size_t reserved_growth() const { return reserved_growth_; }
   void set_reserved_growth(size_t r) { reserved_growth_ = r; }
+  size_t reservation_size() const { return reservation_size_; }
+  void set_reservation_size(size_t r) { reservation_size_ = r; }
   GenerationType generation() const { return *generation_; }
   void set_generation(GenerationType g) { *generation_ = g; }
   GenerationType* generation_ptr() const { return generation_; }
@@ -820,10 +825,14 @@ class CommonFieldsGenerationInfoEnabled {
 
  private:
   // The number of insertions remaining that are guaranteed to not rehash due to
-  // a prior call to reserve. Note: we store reserved growth rather than
+  // a prior call to reserve. Note: we store reserved growth in addition to
   // reservation size because calls to erase() decrease size_ but don't decrease
   // reserved growth.
   size_t reserved_growth_ = 0;
+  // The maximum argument to reserve() since the container was cleared. We need
+  // to keep track of this, in addition to reserved growth, because we reset
+  // reserved growth to this when erase(begin(), end()) is called.
+  size_t reservation_size_ = 0;
   // Pointer to the generation counter, which is used to validate iterators and
   // is stored in the backing array between the control bytes and the slots.
   // Note that we can't store the generation inside the container itself and
@@ -851,6 +860,8 @@ class CommonFieldsGenerationInfoDisabled {
   void reset_reserved_growth(size_t, size_t) {}
   size_t reserved_growth() const { return 0; }
   void set_reserved_growth(size_t) {}
+  size_t reservation_size() const { return 0; }
+  void set_reservation_size(size_t) {}
   GenerationType generation() const { return 0; }
   void set_generation(GenerationType) {}
   GenerationType* generation_ptr() const { return nullptr; }
@@ -971,6 +982,12 @@ class CommonFields : public CommonFieldsGenerationInfo {
     CommonFieldsGenerationInfo::reset_reserved_growth(reservation, size());
   }
 
+  // Returns the number of control bytes set to kDeleted. For testing only.
+  size_t TombstonesCount() const {
+    return static_cast<size_t>(
+        std::count(control(), control() + capacity(), ctrl_t::kDeleted));
+  }
+
  private:
   // TODO(b/259599413): Investigate removing some of these fields:
   // - control/slots can be derived from each other
@@ -1913,6 +1930,7 @@ class raw_hash_set {
       ClearBackingArray(common(), GetPolicyFunctions(), /*reuse=*/cap < 128);
     }
     common().set_reserved_growth(0);
+    common().set_reservation_size(0);
   }
 
   inline void destroy_slots() {
@@ -2168,8 +2186,12 @@ class raw_hash_set {
     // capacity() > 0 as a precondition.
     if (empty()) return end();
     if (first == begin() && last == end()) {
+      // TODO(ezb): we access control bytes in destroy_slots so it could make
+      // sense to combine destroy_slots and ClearBackingArray to avoid cache
+      // misses when the table is large. Note that we also do this in clear().
       destroy_slots();
       ClearBackingArray(common(), GetPolicyFunctions(), /*reuse=*/true);
+      common().set_reserved_growth(common().reservation_size());
       return end();
     }
     while (first != last) {
@@ -2258,6 +2280,7 @@ class raw_hash_set {
       infoz().RecordReservation(n);
     }
     common().reset_reserved_growth(n);
+    common().set_reservation_size(n);
   }
 
   // Extension API: support for heterogeneous keys.

absl/container/internal/raw_hash_set_test.cc

@@ -60,6 +60,10 @@ struct RawHashSetTestOnlyAccess {
   static auto GetSlots(const C& c) -> decltype(c.slot_array()) {
     return c.slot_array();
   }
+  template <typename C>
+  static size_t CountTombstones(const C& c) {
+    return c.common().TombstonesCount();
+  }
 };
 
 namespace {
@@ -472,6 +476,28 @@ struct MinimumAlignmentUint8Table
   using Base::Base;
 };
 
+// Allows for freezing the allocator to expect no further allocations.
+template <typename T>
+struct FreezableAlloc : std::allocator<T> {
+  explicit FreezableAlloc(bool* f) : frozen(f) {}
+
+  template <typename U>
+  explicit FreezableAlloc(const FreezableAlloc<U>& other)
+      : frozen(other.frozen) {}
+
+  template <class U>
+  struct rebind {
+    using other = FreezableAlloc<U>;
+  };
+
+  T* allocate(size_t n) {
+    EXPECT_FALSE(*frozen);
+    return std::allocator<T>::allocate(n);
+  }
+
+  bool* frozen;
+};
+
 struct BadFastHash {
   template <class T>
   size_t operator()(const T&) const {
@@ -479,6 +505,13 @@ struct BadFastHash {
   }
 };
 
+struct BadHashFreezableIntTable
+    : raw_hash_set<IntPolicy, BadFastHash, std::equal_to<int64_t>,
+                   FreezableAlloc<int64_t>> {
+  using Base = typename BadHashFreezableIntTable::raw_hash_set;
+  using Base::Base;
+};
+
 struct BadTable : raw_hash_set<IntPolicy, BadFastHash, std::equal_to<int>,
                                std::allocator<int>> {
   using Base = typename BadTable::raw_hash_set;
@@ -512,6 +545,7 @@ TEST(Table, EmptyFunctorOptimization) {
 
   struct GenerationData {
     size_t reserved_growth;
+    size_t reservation_size;
     GenerationType* generation;
   };
 
@@ -2387,6 +2421,30 @@ TEST(Table, ReservedGrowthUpdatesWhenTableDoesntGrow) {
   EXPECT_EQ(*it, 0);
 }
 
+TEST(Table, EraseBeginEndResetsReservedGrowth) {
+  bool frozen = false;
+  BadHashFreezableIntTable t{FreezableAlloc<int64_t>(&frozen)};
+  t.reserve(100);
+  const size_t cap = t.capacity();
+  frozen = true;  // no further allocs allowed
+
+  for (int i = 0; i < 10; ++i) {
+    // Create a long run (hash function returns constant).
+    for (int j = 0; j < 100; ++j) t.insert(j);
+    // Erase elements from the middle of the long run, which creates tombstones.
+    for (int j = 30; j < 60; ++j) t.erase(j);
+    EXPECT_EQ(t.size(), 70);
+    EXPECT_EQ(t.capacity(), cap);
+    ASSERT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 30);
+
+    t.erase(t.begin(), t.end());
+
+    EXPECT_EQ(t.size(), 0);
+    EXPECT_EQ(t.capacity(), cap);
+    ASSERT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 0);
+  }
+}
+
 TEST(Table, GenerationInfoResetsOnClear) {
   if (!SwisstableGenerationsEnabled()) GTEST_SKIP() << "Generations disabled.";
   if (kMsvc) GTEST_SKIP() << "MSVC doesn't support | in regexp.";

absl/container/node_hash_map.h

@@ -226,7 +226,11 @@ class node_hash_map
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning an
-  //   iterator pointing to `last`.
+  //   iterator pointing to `last`. The special case of calling
+  //   `erase(begin(), end())` resets the reserved growth such that if
+  //   `reserve(N)` has previously been called and there has been no intervening
+  //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
+  //   to assume that inserting N elements will not cause a rehash.
   //
   // size_type erase(const key_type& key):
   //

absl/container/node_hash_set.h

@@ -218,7 +218,11 @@ class node_hash_set
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning an
-  //   iterator pointing to `last`.
+  //   iterator pointing to `last`. The special case of calling
+  //   `erase(begin(), end())` resets the reserved growth such that if
+  //   `reserve(N)` has previously been called and there has been no intervening
+  //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
+  //   to assume that inserting N elements will not cause a rehash.
   //
   // size_type erase(const key_type& key):
   //