Reduce flat_hash_{set,map} generated code size.

This CL makes a bunch of changes (mostly to raw_hash_set which
underlies flat_hash_set and flat_hash_map). Techniques used:

* Extract code that does not depend on the specific hash table type
  into common (non-inlined) functions.
* Place ABSL_ATTRIBUTE_NOINLINE directives judiciously.
* Out-of-line some slow paths.

Reduces sizes of some large binaries by ~0.5%.

Has no significant performance impact on a few performance critical
binaries.

## Speed of fleetbench micro-benchmarks

Following is a histogram of %-age changes in
[fleetbench](https://github.com/google/fleetbench)
hot_swissmap_benchmark results. Negative numbers indicate a speedup
caused by this change. Statistically insignificant changes are mapped
to zero.

XXX Also run and merge in cold_swissmap_benchmark

Across all 351 benchmarks, the average speedup is 0.38%.
The best speedup was -25%, worst slowdown was +6.81%.

```
Count: 351  Average: -0.382764  StdDev: 3.77807
Min: -25  Median: 0.435135  Max: 6.81
---------------------------------------------
[ -25, -10)  16  4.558%   4.558% #
[  -9,  -8)   2  0.570%   5.128%
[  -8,  -7)   1  0.285%   5.413%
[  -7,  -6)   1  0.285%   5.698%
[  -6,  -5)   2  0.570%   6.268%
[  -5,  -4)   5  1.425%   7.692%
[  -4,  -3)  13  3.704%  11.396% #
[  -3,  -2)  15  4.274%  15.670% #
[  -2,  -1)  26  7.407%  23.077% ##
[  -1,   0)  14  3.989%  27.066% #
[   0,   1) 185 52.707%  79.772% ############
[   1,   2)  14  3.989%  83.761% #
[   2,   3)   8  2.279%  86.040% #
[   3,   4)   7  1.994%  88.034%
[   4,   5)  32  9.117%  97.151% ##
[   5,   6)   6  1.709%  98.860%
[   6,   7)   4  1.140% 100.000%
```

We looked at the slowdowns and they do not seem worth worrying
about. E.g., the worst one was:

```
BM_FindHit_Hot<::absl::node_hash_set,64>/set_size:4096/density:0
  2.61ns ± 1%  2.79ns ± 1%   +6.81%  (p=0.008 n=5+5)
```

## Detailed changes

* Out-of-line slow paths in hash table sampler methods.
* Explicitly unregister from sampler instead of from destructor.
* Introduced a non-templated CommonFields struct that holds some of
  the hash table fields (infoz, ctrl, slots, size, capacity). This
  struct can be passed to new non-templated helpers. The struct is
  a private base class of raw_hash_set.
* Made non-inlined InitializeSlots<> that is only templated on
  allocator and size/alignment of the slot type so that we can share
  instantiations across types that have the same size/alignment.
* Moved some infrequently called code paths into non-inlined type-erased.
  functions. Pass a suite of type-specific function pointers to these
  routines for when they need to operate on slots.
* Marked some methods as non-inlined.
* Avoid unnecessary reinitialization in destructor.
* Introduce UpdateSpine type-erased helper that is called from
  clear() and rehash().

PiperOrigin-RevId: 491413386
Change-Id: Ia5495c5a6ec73622a785a0d260e406ddb9085a7c

absl/container/internal/hashtablez_sampler.cc

@@ -14,6 +14,7 @@
 
 #include "absl/container/internal/hashtablez_sampler.h"
 
+#include <algorithm>
 #include <atomic>
 #include <cassert>
 #include <cmath>
@@ -158,6 +159,43 @@ void UnsampleSlow(HashtablezInfo* info) {
   GlobalHashtablezSampler().Unregister(info);
 }
 
+void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length) {
+#ifdef ABSL_INTERNAL_HAVE_SSE2
+  total_probe_length /= 16;
+#else
+  total_probe_length /= 8;
+#endif
+  info->total_probe_length.store(total_probe_length, std::memory_order_relaxed);
+  info->num_erases.store(0, std::memory_order_relaxed);
+  // There is only one concurrent writer, so `load` then `store` is sufficient
+  // instead of using `fetch_add`.
+  info->num_rehashes.store(
+      1 + info->num_rehashes.load(std::memory_order_relaxed),
+      std::memory_order_relaxed);
+}
+
+void RecordReservationSlow(HashtablezInfo* info, size_t target_capacity) {
+  info->max_reserve.store(
+      (std::max)(info->max_reserve.load(std::memory_order_relaxed),
+                 target_capacity),
+      std::memory_order_relaxed);
+}
+
+void RecordClearedReservationSlow(HashtablezInfo* info) {
+  info->max_reserve.store(0, std::memory_order_relaxed);
+}
+
+void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
+                              size_t capacity) {
+  info->size.store(size, std::memory_order_relaxed);
+  info->capacity.store(capacity, std::memory_order_relaxed);
+  if (size == 0) {
+    // This is a clear, reset the total/num_erases too.
+    info->total_probe_length.store(0, std::memory_order_relaxed);
+    info->num_erases.store(0, std::memory_order_relaxed);
+  }
+}
+
 void RecordInsertSlow(HashtablezInfo* info, size_t hash,
                       size_t distance_from_desired) {
   // SwissTables probe in groups of 16, so scale this to count items probes and
@@ -180,6 +218,14 @@ void RecordInsertSlow(HashtablezInfo* info, size_t hash,
   info->size.fetch_add(1, std::memory_order_relaxed);
 }
 
+void RecordEraseSlow(HashtablezInfo* info) {
+  info->size.fetch_sub(1, std::memory_order_relaxed);
+  // There is only one concurrent writer, so `load` then `store` is sufficient
+  // instead of using `fetch_add`.
+  info->num_erases.store(1 + info->num_erases.load(std::memory_order_relaxed),
+                         std::memory_order_relaxed);
+}
+
 void SetHashtablezConfigListener(HashtablezConfigListener l) {
   g_hashtablez_config_listener.store(l, std::memory_order_release);
 }

absl/container/internal/hashtablez_sampler.h

@@ -95,55 +95,19 @@ struct HashtablezInfo : public profiling_internal::Sample<HashtablezInfo> {
   size_t inline_element_size;  // How big is the slot?
 };
 
-inline void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length) {
-#ifdef ABSL_INTERNAL_HAVE_SSE2
-  total_probe_length /= 16;
-#else
-  total_probe_length /= 8;
-#endif
-  info->total_probe_length.store(total_probe_length, std::memory_order_relaxed);
-  info->num_erases.store(0, std::memory_order_relaxed);
-  // There is only one concurrent writer, so `load` then `store` is sufficient
-  // instead of using `fetch_add`.
-  info->num_rehashes.store(
-      1 + info->num_rehashes.load(std::memory_order_relaxed),
-      std::memory_order_relaxed);
-}
+void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length);
 
-inline void RecordReservationSlow(HashtablezInfo* info,
-                                  size_t target_capacity) {
-  info->max_reserve.store(
-      (std::max)(info->max_reserve.load(std::memory_order_relaxed),
-                 target_capacity),
-      std::memory_order_relaxed);
-}
+void RecordReservationSlow(HashtablezInfo* info, size_t target_capacity);
 
-inline void RecordClearedReservationSlow(HashtablezInfo* info) {
-  info->max_reserve.store(0, std::memory_order_relaxed);
-}
+void RecordClearedReservationSlow(HashtablezInfo* info);
 
-inline void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
-                                     size_t capacity) {
-  info->size.store(size, std::memory_order_relaxed);
-  info->capacity.store(capacity, std::memory_order_relaxed);
-  if (size == 0) {
-    // This is a clear, reset the total/num_erases too.
-    info->total_probe_length.store(0, std::memory_order_relaxed);
-    info->num_erases.store(0, std::memory_order_relaxed);
-  }
-}
+void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
+                              size_t capacity);
 
 void RecordInsertSlow(HashtablezInfo* info, size_t hash,
                       size_t distance_from_desired);
 
-inline void RecordEraseSlow(HashtablezInfo* info) {
-  info->size.fetch_sub(1, std::memory_order_relaxed);
-  // There is only one concurrent writer, so `load` then `store` is sufficient
-  // instead of using `fetch_add`.
-  info->num_erases.store(
-      1 + info->num_erases.load(std::memory_order_relaxed),
-      std::memory_order_relaxed);
-}
+void RecordEraseSlow(HashtablezInfo* info);
 
 struct SamplingState {
   int64_t next_sample;
@@ -165,7 +129,10 @@ class HashtablezInfoHandle {
  public:
   explicit HashtablezInfoHandle() : info_(nullptr) {}
   explicit HashtablezInfoHandle(HashtablezInfo* info) : info_(info) {}
-  ~HashtablezInfoHandle() {
+
+  // We do not have a destructor. Caller is responsible for calling Unregister
+  // before destroying the handle.
+  void Unregister() {
     if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
     UnsampleSlow(info_);
   }
@@ -230,6 +197,7 @@ class HashtablezInfoHandle {
   explicit HashtablezInfoHandle() = default;
   explicit HashtablezInfoHandle(std::nullptr_t) {}
 
+  inline void Unregister() {}
   inline void RecordStorageChanged(size_t /*size*/, size_t /*capacity*/) {}
   inline void RecordRehash(size_t /*total_probe_length*/) {}
   inline void RecordReservation(size_t /*target_capacity*/) {}

absl/container/internal/raw_hash_set.cc

@@ -63,8 +63,156 @@ void ConvertDeletedToEmptyAndFullToDeleted(ctrl_t* ctrl, size_t capacity) {
   std::memcpy(ctrl + capacity + 1, ctrl, NumClonedBytes());
   ctrl[capacity] = ctrl_t::kSentinel;
 }
-// Extern template instantiotion for inline function.
-template FindInfo find_first_non_full(const ctrl_t*, size_t, size_t);
+// Extern template instantiation for inline function.
+template FindInfo find_first_non_full(const CommonFields&, size_t);
+
+FindInfo find_first_non_full_outofline(const CommonFields& common,
+                                       size_t hash) {
+  return find_first_non_full(common, hash);
+}
+
+// Return address of the ith slot in slots where each slot occupies slot_size.
+static inline void* SlotAddress(void* slot_array, size_t slot,
+                                size_t slot_size) {
+  return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(slot_array) +
+                                 (slot * slot_size));
+}
+
+// Return the address of the slot just after slot assuming each slot
+// has the specified size.
+static inline void* NextSlot(void* slot, size_t slot_size) {
+  return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(slot) + slot_size);
+}
+
+// Return the address of the slot just before slot assuming each slot
+// has the specified size.
+static inline void* PrevSlot(void* slot, size_t slot_size) {
+  return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(slot) - slot_size);
+}
+
+void DropDeletesWithoutResize(CommonFields& common, size_t& growth_left,
+                              const PolicyFunctions& policy, void* tmp_space) {
+  void* set = &common;
+  void* slot_array = common.slots_;
+  const size_t capacity = common.capacity_;
+  assert(IsValidCapacity(capacity));
+  assert(!is_small(capacity));
+  // Algorithm:
+  // - mark all DELETED slots as EMPTY
+  // - mark all FULL slots as DELETED
+  // - for each slot marked as DELETED
+  //     hash = Hash(element)
+  //     target = find_first_non_full(hash)
+  //     if target is in the same group
+  //       mark slot as FULL
+  //     else if target is EMPTY
+  //       transfer element to target
+  //       mark slot as EMPTY
+  //       mark target as FULL
+  //     else if target is DELETED
+  //       swap current element with target element
+  //       mark target as FULL
+  //       repeat procedure for current slot with moved from element (target)
+  ctrl_t* ctrl = common.control_;
+  ConvertDeletedToEmptyAndFullToDeleted(ctrl, capacity);
+  auto hasher = policy.hash_slot;
+  auto transfer = policy.transfer;
+  const size_t slot_size = policy.slot_size;
+
+  size_t total_probe_length = 0;
+  void* slot_ptr = SlotAddress(slot_array, 0, slot_size);
+  for (size_t i = 0; i != capacity;
+       ++i, slot_ptr = NextSlot(slot_ptr, slot_size)) {
+    assert(slot_ptr == SlotAddress(slot_array, i, slot_size));
+    if (!IsDeleted(ctrl[i])) continue;
+    const size_t hash = (*hasher)(set, slot_ptr);
+    const FindInfo target = find_first_non_full(common, hash);
+    const size_t new_i = target.offset;
+    total_probe_length += target.probe_length;
+
+    // Verify if the old and new i fall within the same group wrt the hash.
+    // If they do, we don't need to move the object as it falls already in the
+    // best probe we can.
+    const size_t probe_offset = probe(common, hash).offset();
+    const auto probe_index = [probe_offset, capacity](size_t pos) {
+      return ((pos - probe_offset) & capacity) / Group::kWidth;
+    };
+
+    // Element doesn't move.
+    if (ABSL_PREDICT_TRUE(probe_index(new_i) == probe_index(i))) {
+      SetCtrl(common, i, H2(hash), slot_size);
+      continue;
+    }
+
+    void* new_slot_ptr = SlotAddress(slot_array, new_i, slot_size);
+    if (IsEmpty(ctrl[new_i])) {
+      // Transfer element to the empty spot.
+      // SetCtrl poisons/unpoisons the slots so we have to call it at the
+      // right time.
+      SetCtrl(common, new_i, H2(hash), slot_size);
+      (*transfer)(set, new_slot_ptr, slot_ptr);
+      SetCtrl(common, i, ctrl_t::kEmpty, slot_size);
+    } else {
+      assert(IsDeleted(ctrl[new_i]));
+      SetCtrl(common, new_i, H2(hash), slot_size);
+      // Until we are done rehashing, DELETED marks previously FULL slots.
+
+      // Swap i and new_i elements.
+      (*transfer)(set, tmp_space, new_slot_ptr);
+      (*transfer)(set, new_slot_ptr, slot_ptr);
+      (*transfer)(set, slot_ptr, tmp_space);
+
+      // repeat the processing of the ith slot
+      --i;
+      slot_ptr = PrevSlot(slot_ptr, slot_size);
+    }
+  }
+  ResetGrowthLeft(common, growth_left);
+  common.infoz().RecordRehash(total_probe_length);
+}
+
+void EraseMetaOnly(CommonFields& c, size_t& growth_left, ctrl_t* it,
+                   size_t slot_size) {
+  assert(IsFull(*it) && "erasing a dangling iterator");
+  --c.size_;
+  const auto index = static_cast<size_t>(it - c.control_);
+  const size_t index_before = (index - Group::kWidth) & c.capacity_;
+  const auto empty_after = Group(it).MaskEmpty();
+  const auto empty_before = Group(c.control_ + index_before).MaskEmpty();
+
+  // We count how many consecutive non empties we have to the right and to the
+  // left of `it`. If the sum is >= kWidth then there is at least one probe
+  // window that might have seen a full group.
+  bool was_never_full =
+      empty_before && empty_after &&
+      static_cast<size_t>(empty_after.TrailingZeros() +
+                          empty_before.LeadingZeros()) < Group::kWidth;
+
+  SetCtrl(c, index, was_never_full ? ctrl_t::kEmpty : ctrl_t::kDeleted,
+          slot_size);
+  growth_left += (was_never_full ? 1 : 0);
+  c.infoz().RecordErase();
+}
+
+void ClearBackingArray(CommonFields& c, size_t& growth_left,
+                       const PolicyFunctions& policy, bool reuse) {
+  if (reuse) {
+    c.size_ = 0;
+    ResetCtrl(c, growth_left, policy.slot_size);
+    c.infoz().RecordStorageChanged(0, c.capacity_);
+  } else {
+    void* set = &c;
+    (*policy.dealloc)(set, policy, c.control_, c.slots_, c.capacity_);
+    c.control_ = EmptyGroup();
+    c.slots_ = nullptr;
+    c.size_ = 0;
+    c.capacity_ = 0;
+    growth_left = 0;
+    c.infoz().RecordClearedReservation();
+    assert(c.size_ == 0);
+    c.infoz().RecordStorageChanged(0, 0);
+  }
+}
 
 }  // namespace container_internal
 ABSL_NAMESPACE_END

absl/container/internal/raw_hash_set.h

@@ -717,6 +717,61 @@ using Group = GroupAArch64Impl;
 using Group = GroupPortableImpl;
 #endif
 
+// CommonFields hold the fields in raw_hash_set that do not depend
+// on template parameters. This allows us to conveniently pass all
+// of this state to helper functions as a single argument.
+//
+// We make HashtablezInfoHandle a base class to take advantage of
+// the empty base-class optimization when sampling is turned off.
+class CommonFields : public HashtablezInfoHandle {
+ public:
+  CommonFields() = default;
+
+  // Not copyable
+  CommonFields(const CommonFields&) = delete;
+  CommonFields& operator=(const CommonFields&) = delete;
+
+  // Movable
+  CommonFields(CommonFields&& that)
+      : HashtablezInfoHandle(
+            std::move(static_cast<HashtablezInfoHandle&&>(that))),
+        // Explicitly copying fields into "this" and then resetting "that"
+        // fields generates less code then calling absl::exchange per field.
+        control_(that.control_),
+        slots_(that.slots_),
+        size_(that.size_),
+        capacity_(that.capacity_) {
+    that.control_ = EmptyGroup();
+    that.slots_ = nullptr;
+    that.size_ = 0;
+    that.capacity_ = 0;
+  }
+  CommonFields& operator=(CommonFields&&) = default;
+
+  // TODO(b/259599413): Investigate removing some of these fields:
+  // - control/slots can be derived from each other
+  // - size can be moved into the slot array
+
+  // The control bytes (and, also, a pointer to the base of the backing array).
+  //
+  // This contains `capacity + 1 + NumClonedBytes()` entries, even
+  // when the table is empty (hence EmptyGroup).
+  ctrl_t* control_ = EmptyGroup();
+
+  // The beginning of the slots, located at `SlotOffset()` bytes after
+  // `control`. May be null for empty tables.
+  void* slots_ = nullptr;
+
+  // The number of filled slots.
+  size_t size_ = 0;
+
+  // The total number of available slots.
+  size_t capacity_ = 0;
+
+  HashtablezInfoHandle& infoz() { return *this; }
+  const HashtablezInfoHandle& infoz() const { return *this; }
+};
+
 // Returns he number of "cloned control bytes".
 //
 // This is the number of control bytes that are present both at the beginning
@@ -866,9 +921,10 @@ struct FindInfo {
 // `ShouldInsertBackwards()` for small tables.
 inline bool is_small(size_t capacity) { return capacity < Group::kWidth - 1; }
 
-// Begins a probing operation on `ctrl`, using `hash`.
-inline probe_seq<Group::kWidth> probe(const ctrl_t* ctrl, size_t hash,
-                                      size_t capacity) {
+// Begins a probing operation on `common.control`, using `hash`.
+inline probe_seq<Group::kWidth> probe(const CommonFields& common, size_t hash) {
+  const ctrl_t* ctrl = common.control_;
+  const size_t capacity = common.capacity_;
   return probe_seq<Group::kWidth>(H1(hash, ctrl), capacity);
 }
 
@@ -880,9 +936,9 @@ inline probe_seq<Group::kWidth> probe(const ctrl_t* ctrl, size_t hash,
 // NOTE: this function must work with tables having both empty and deleted
 // slots in the same group. Such tables appear during `erase()`.
 template <typename = void>
-inline FindInfo find_first_non_full(const ctrl_t* ctrl, size_t hash,
-                                    size_t capacity) {
-  auto seq = probe(ctrl, hash, capacity);
+inline FindInfo find_first_non_full(const CommonFields& common, size_t hash) {
+  auto seq = probe(common, hash);
+  const ctrl_t* ctrl = common.control_;
   while (true) {
     Group g{ctrl + seq.offset()};
     auto mask = g.MaskEmptyOrDeleted();
@@ -892,55 +948,68 @@ inline FindInfo find_first_non_full(const ctrl_t* ctrl, size_t hash,
       // In debug build we will randomly insert in either the front or back of
       // the group.
       // TODO(kfm,sbenza): revisit after we do unconditional mixing
-      if (!is_small(capacity) && ShouldInsertBackwards(hash, ctrl)) {
+      if (!is_small(common.capacity_) && ShouldInsertBackwards(hash, ctrl)) {
         return {seq.offset(mask.HighestBitSet()), seq.index()};
       }
 #endif
       return {seq.offset(mask.LowestBitSet()), seq.index()};
     }
     seq.next();
-    assert(seq.index() <= capacity && "full table!");
+    assert(seq.index() <= common.capacity_ && "full table!");
   }
 }
 
 // Extern template for inline function keep possibility of inlining.
 // When compiler decided to not inline, no symbols will be added to the
 // corresponding translation unit.
-extern template FindInfo find_first_non_full(const ctrl_t*, size_t, size_t);
+extern template FindInfo find_first_non_full(const CommonFields&, size_t);
+
+// Non-inlined version of find_first_non_full for use in less
+// performance critical routines.
+FindInfo find_first_non_full_outofline(const CommonFields&, size_t);
+
+inline void ResetGrowthLeft(CommonFields& common, size_t& growth_left) {
+  growth_left = CapacityToGrowth(common.capacity_) - common.size_;
+}
 
 // Sets `ctrl` to `{kEmpty, kSentinel, ..., kEmpty}`, marking the entire
 // array as marked as empty.
-inline void ResetCtrl(size_t capacity, ctrl_t* ctrl, const void* slot,
+inline void ResetCtrl(CommonFields& common, size_t& growth_left,
                       size_t slot_size) {
+  const size_t capacity = common.capacity_;
+  ctrl_t* ctrl = common.control_;
   std::memset(ctrl, static_cast<int8_t>(ctrl_t::kEmpty),
               capacity + 1 + NumClonedBytes());
   ctrl[capacity] = ctrl_t::kSentinel;
-  SanitizerPoisonMemoryRegion(slot, slot_size * capacity);
+  SanitizerPoisonMemoryRegion(common.slots_, slot_size * capacity);
+  ResetGrowthLeft(common, growth_left);
 }
 
 // Sets `ctrl[i]` to `h`.
 //
 // Unlike setting it directly, this function will perform bounds checks and
 // mirror the value to the cloned tail if necessary.
-inline void SetCtrl(size_t i, ctrl_t h, size_t capacity, ctrl_t* ctrl,
-                    const void* slot, size_t slot_size) {
+inline void SetCtrl(const CommonFields& common, size_t i, ctrl_t h,
+                    size_t slot_size) {
+  const size_t capacity = common.capacity_;
   assert(i < capacity);
 
-  auto* slot_i = static_cast<const char*>(slot) + i * slot_size;
+  auto* slot_i = static_cast<const char*>(common.slots_) + i * slot_size;
   if (IsFull(h)) {
     SanitizerUnpoisonMemoryRegion(slot_i, slot_size);
   } else {
     SanitizerPoisonMemoryRegion(slot_i, slot_size);
   }
 
+  ctrl_t* ctrl = common.control_;
   ctrl[i] = h;
   ctrl[((i - NumClonedBytes()) & capacity) + (NumClonedBytes() & capacity)] = h;
 }
 
 // Overload for setting to an occupied `h2_t` rather than a special `ctrl_t`.
-inline void SetCtrl(size_t i, h2_t h, size_t capacity, ctrl_t* ctrl,
-                    const void* slot, size_t slot_size) {
-  SetCtrl(i, static_cast<ctrl_t>(h), capacity, ctrl, slot, slot_size);
+inline void SetCtrl(const CommonFields& common, size_t i, h2_t h,
+                    size_t slot_size) {
+  SetCtrl(common, i, static_cast<ctrl_t>(h), slot_size);
 }
 
 // Given the capacity of a table, computes the offset (from the start of the
@@ -957,6 +1026,82 @@ inline size_t AllocSize(size_t capacity, size_t slot_size, size_t slot_align) {
   return SlotOffset(capacity, slot_align) + capacity * slot_size;
 }
 
+template <typename Alloc, size_t SizeOfSlot, size_t AlignOfSlot>
+ABSL_ATTRIBUTE_NOINLINE void InitializeSlots(CommonFields& c,
+                                             size_t& growth_left, Alloc alloc) {
+  assert(c.capacity_);
+  // Folks with custom allocators often make unwarranted assumptions about the
+  // behavior of their classes vis-a-vis trivial destructability and what
+  // calls they will or won't make.  Avoid sampling for people with custom
+  // allocators to get us out of this mess.  This is not a hard guarantee but
+  // a workaround while we plan the exact guarantee we want to provide.
+  const size_t sample_size =
+      (std::is_same<Alloc, std::allocator<char>>::value && c.slots_ == nullptr)
+          ? SizeOfSlot
+          : 0;
+
+  const size_t cap = c.capacity_;
+  char* mem = static_cast<char*>(
+      Allocate<AlignOfSlot>(&alloc, AllocSize(cap, SizeOfSlot, AlignOfSlot)));
+  c.control_ = reinterpret_cast<ctrl_t*>(mem);
+  c.slots_ = mem + SlotOffset(cap, AlignOfSlot);
+  ResetCtrl(c, growth_left, SizeOfSlot);
+  if (sample_size) {
+    c.infoz() = Sample(sample_size);
+  }
+  c.infoz().RecordStorageChanged(c.size_, cap);
+}
+
+// PolicyFunctions bundles together some information for a particular
+// raw_hash_set<T, ...> instantiation. This information is passed to
+// type-erased functions that want to do small amounts of type-specific
+// work.
+struct PolicyFunctions {
+  size_t slot_size;
+  size_t slot_align;
+
+  // Return the hash of the pointed-to slot.
+  size_t (*hash_slot)(void* set, void* slot);
+
+  // Transfer the contents of src_slot to dst_slot.
+  void (*transfer)(void* set, void* dst_slot, void* src_slot);
+
+  // Deallocate the specified backing store which is sized for n slots.
+  void (*dealloc)(void* set, const PolicyFunctions& policy, ctrl_t* ctrl,
+                  void* slot_array, size_t n);
+};
+
+// ClearBackingArray clears the backing array, either modifying it in place,
+// or creating a new one based on the value of "reuse".
+// REQUIRES: c.capacity > 0
+void ClearBackingArray(CommonFields& c, size_t& growth_left,
+                       const PolicyFunctions& policy, bool reuse);
+
+// Type-erased version of raw_hash_set::erase_meta_only.
+void EraseMetaOnly(CommonFields& c, size_t& growth_left, ctrl_t* it,
+                   size_t slot_size);
+
+// Function to place in PolicyFunctions::dealloc for raw_hash_sets
+// that are using std::allocator. This allows us to share the same
+// function body for raw_hash_set instantiations that have the
+// same slot alignment.
+template <size_t AlignOfSlot>
+ABSL_ATTRIBUTE_NOINLINE void DeallocateStandard(void*,
+                                                const PolicyFunctions& policy,
+                                                ctrl_t* ctrl, void* slot_array,
+                                                size_t n) {
+  // Unpoison before returning the memory to the allocator.
+  SanitizerUnpoisonMemoryRegion(slot_array, policy.slot_size * n);
+
+  std::allocator<char> alloc;
+  Deallocate<AlignOfSlot>(&alloc, ctrl,
+                          AllocSize(n, policy.slot_size, AlignOfSlot));
+}
+
+// Type-erased version of raw_hash_set::drop_deletes_without_resize.
+void DropDeletesWithoutResize(CommonFields& common, size_t& growth_left,
+                              const PolicyFunctions& policy, void* tmp_space);
+
 // A SwissTable.
 //
 // Policy: a policy defines how to perform different operations on
@@ -978,7 +1123,7 @@ inline size_t AllocSize(size_t capacity, size_t slot_size, size_t slot_align) {
 // the storage of the hashtable will be allocated and the elements will be
 // constructed and destroyed.
 template <class Policy, class Hash, class Eq, class Alloc>
-class raw_hash_set {
+class raw_hash_set : private CommonFields {
   using PolicyTraits = hash_policy_traits<Policy>;
   using KeyArgImpl =
       KeyArg<IsTransparent<Eq>::value && IsTransparent<Hash>::value>;
@@ -1181,14 +1326,13 @@ class raw_hash_set {
           std::is_nothrow_default_constructible<key_equal>::value&&
               std::is_nothrow_default_constructible<allocator_type>::value) {}
 
-  explicit raw_hash_set(size_t bucket_count,
-                        const hasher& hash = hasher(),
-                        const key_equal& eq = key_equal(),
-                        const allocator_type& alloc = allocator_type())
-      : ctrl_(EmptyGroup()),
-        settings_(0u, HashtablezInfoHandle(), hash, eq, alloc) {
+  ABSL_ATTRIBUTE_NOINLINE explicit raw_hash_set(
+      size_t bucket_count, const hasher& hash = hasher(),
+      const key_equal& eq = key_equal(),
+      const allocator_type& alloc = allocator_type())
+      : settings_(0u, hash, eq, alloc) {
     if (bucket_count) {
-      capacity_ = NormalizeCapacity(bucket_count);
+      common().capacity_ = NormalizeCapacity(bucket_count);
       initialize_slots();
     }
   }
@@ -1295,47 +1439,31 @@ class raw_hash_set {
     // than a full `insert`.
     for (const auto& v : that) {
       const size_t hash = PolicyTraits::apply(HashElement{hash_ref()}, v);
-      auto target = find_first_non_full(ctrl_, hash, capacity_);
-      SetCtrl(target.offset, H2(hash), capacity_, ctrl_, slots_,
-              sizeof(slot_type));
+      auto target = find_first_non_full_outofline(common(), hash);
+      SetCtrl(common(), target.offset, H2(hash), sizeof(slot_type));
       emplace_at(target.offset, v);
       infoz().RecordInsert(hash, target.probe_length);
     }
-    size_ = that.size();
+    common().size_ = that.size();
     growth_left() -= that.size();
   }
 
-  raw_hash_set(raw_hash_set&& that) noexcept(
+  ABSL_ATTRIBUTE_NOINLINE raw_hash_set(raw_hash_set&& that) noexcept(
       std::is_nothrow_copy_constructible<hasher>::value&&
           std::is_nothrow_copy_constructible<key_equal>::value&&
               std::is_nothrow_copy_constructible<allocator_type>::value)
-      : ctrl_(absl::exchange(that.ctrl_, EmptyGroup())),
-        slots_(absl::exchange(that.slots_, nullptr)),
-        size_(absl::exchange(that.size_, size_t{0})),
-        capacity_(absl::exchange(that.capacity_, size_t{0})),
-        // Hash, equality and allocator are copied instead of moved because
-        // `that` must be left valid. If Hash is std::function<Key>, moving it
-        // would create a nullptr functor that cannot be called.
+      :  // Hash, equality and allocator are copied instead of moved because
+         // `that` must be left valid. If Hash is std::function<Key>, moving it
+         // would create a nullptr functor that cannot be called.
+        CommonFields(std::move(that)),
         settings_(absl::exchange(that.growth_left(), size_t{0}),
-                  absl::exchange(that.infoz(), HashtablezInfoHandle()),
-                  that.hash_ref(),
-                  that.eq_ref(),
-                  that.alloc_ref()) {}
+                  that.hash_ref(), that.eq_ref(), that.alloc_ref()) {}
 
   raw_hash_set(raw_hash_set&& that, const allocator_type& a)
-      : ctrl_(EmptyGroup()),
-        slots_(nullptr),
-        size_(0),
-        capacity_(0),
-        settings_(0, HashtablezInfoHandle(), that.hash_ref(), that.eq_ref(),
-                  a) {
+      : settings_(0, that.hash_ref(), that.eq_ref(), a) {
     if (a == that.alloc_ref()) {
-      std::swap(ctrl_, that.ctrl_);
-      std::swap(slots_, that.slots_);
-      std::swap(size_, that.size_);
-      std::swap(capacity_, that.capacity_);
+      std::swap(common(), that.common());
       std::swap(growth_left(), that.growth_left());
-      std::swap(infoz(), that.infoz());
     } else {
       reserve(that.size());
       // Note: this will copy elements of dense_set and unordered_set instead of
@@ -1364,7 +1492,19 @@ class raw_hash_set {
         typename AllocTraits::propagate_on_container_move_assignment());
   }
 
-  ~raw_hash_set() { destroy_slots(/*reset=*/false); }
+  ~raw_hash_set() {
+    const size_t cap = capacity();
+    if (!cap) return;
+    destroy_slots();
+
+    // Unpoison before returning the memory to the allocator.
+    SanitizerUnpoisonMemoryRegion(slot_array(), sizeof(slot_type) * cap);
+    Deallocate<alignof(slot_type)>(
+        &alloc_ref(), control(),
+        AllocSize(cap, sizeof(slot_type), alignof(slot_type)));
+
+    infoz().Unregister();
+  }
 
   iterator begin() {
     auto it = iterator_at(0);
@@ -1381,8 +1521,8 @@ class raw_hash_set {
   const_iterator cend() const { return end(); }
 
   bool empty() const { return !size(); }
-  size_t size() const { return size_; }
-  size_t capacity() const { return capacity_; }
+  size_t size() const { return common().size_; }
+  size_t capacity() const { return common().capacity_; }
   size_t max_size() const { return (std::numeric_limits<size_t>::max)(); }
 
   ABSL_ATTRIBUTE_REINITIALIZES void clear() {
@@ -1393,22 +1533,25 @@ class raw_hash_set {
     // compared to destruction of the elements of the container. So we pick the
     // largest bucket_count() threshold for which iteration is still fast and
     // past that we simply deallocate the array.
-    if (capacity_ > 127) {
-      destroy_slots(/*reset=*/true);
-
-      infoz().RecordClearedReservation();
-    } else if (capacity_) {
-      for (size_t i = 0; i != capacity_; ++i) {
-        if (IsFull(ctrl_[i])) {
-          PolicyTraits::destroy(&alloc_ref(), slots_ + i);
-        }
+    const size_t cap = capacity();
+    if (cap == 0) {
+      // Already guaranteed to be empty; so nothing to do.
+    } else {
+      destroy_slots();
+      ClearBackingArray(common(), growth_left(), kPolicyFunctions,
+                        /*reuse=*/cap < 128);
+    }
+  }
+
+  inline void destroy_slots() {
+    const size_t cap = capacity();
+    const ctrl_t* ctrl = control();
+    slot_type* slot = slot_array();
+    for (size_t i = 0; i != cap; ++i) {
+      if (IsFull(ctrl[i])) {
+        PolicyTraits::destroy(&alloc_ref(), slot + i);
       }
-      size_ = 0;
-      ResetCtrl(capacity_, ctrl_, slots_, sizeof(slot_type));
-      reset_growth_left();
     }
-    assert(empty());
-    infoz().RecordStorageChanged(0, capacity_);
   }
 
   // This overload kicks in when the argument is an rvalue of insertable and
@@ -1596,7 +1739,7 @@ class raw_hash_set {
   iterator lazy_emplace(const key_arg<K>& key, F&& f) {
     auto res = find_or_prepare_insert(key);
     if (res.second) {
-      slot_type* slot = slots_ + res.first;
+      slot_type* slot = slot_array() + res.first;
       std::forward<F>(f)(constructor(&alloc_ref(), &slot));
       assert(!slot);
     }
@@ -1692,24 +1835,19 @@ class raw_hash_set {
       IsNoThrowSwappable<allocator_type>(
           typename AllocTraits::propagate_on_container_swap{})) {
     using std::swap;
-    swap(ctrl_, that.ctrl_);
-    swap(slots_, that.slots_);
-    swap(size_, that.size_);
-    swap(capacity_, that.capacity_);
+    swap(common(), that.common());
     swap(growth_left(), that.growth_left());
     swap(hash_ref(), that.hash_ref());
     swap(eq_ref(), that.eq_ref());
-    swap(infoz(), that.infoz());
     SwapAlloc(alloc_ref(), that.alloc_ref(),
               typename AllocTraits::propagate_on_container_swap{});
   }
 
   void rehash(size_t n) {
-    if (n == 0 && capacity_ == 0) return;
-    if (n == 0 && size_ == 0) {
-      destroy_slots(/*reset=*/true);
-      infoz().RecordStorageChanged(0, 0);
-      infoz().RecordClearedReservation();
+    if (n == 0 && capacity() == 0) return;
+    if (n == 0 && size() == 0) {
+      ClearBackingArray(common(), growth_left(), kPolicyFunctions,
+                        /*reuse=*/false);
       return;
     }
 
@@ -1717,7 +1855,7 @@ class raw_hash_set {
     // power-of-2-minus-1, so bitor is good enough.
     auto m = NormalizeCapacity(n | GrowthToLowerboundCapacity(size()));
     // n == 0 unconditionally rehashes as per the standard.
-    if (n == 0 || m > capacity_) {
+    if (n == 0 || m > capacity()) {
       resize(m);
 
       // This is after resize, to ensure that we have completed the allocation
@@ -1762,9 +1900,9 @@ class raw_hash_set {
     // Avoid probing if we won't be able to prefetch the addresses received.
 #ifdef ABSL_INTERNAL_HAVE_PREFETCH
     prefetch_heap_block();
-    auto seq = probe(ctrl_, hash_ref()(key), capacity_);
-    base_internal::PrefetchT0(ctrl_ + seq.offset());
-    base_internal::PrefetchT0(slots_ + seq.offset());
+    auto seq = probe(common(), hash_ref()(key));
+    base_internal::PrefetchT0(control() + seq.offset());
+    base_internal::PrefetchT0(slot_array() + seq.offset());
 #endif  // ABSL_INTERNAL_HAVE_PREFETCH
   }
 
@@ -1777,18 +1915,20 @@ class raw_hash_set {
   // called heterogeneous key support.
   template <class K = key_type>
   iterator find(const key_arg<K>& key, size_t hash) {
-    auto seq = probe(ctrl_, hash, capacity_);
+    auto seq = probe(common(), hash);
+    slot_type* slot_ptr = slot_array();
+    const ctrl_t* ctrl = control();
     while (true) {
-      Group g{ctrl_ + seq.offset()};
+      Group g{ctrl + seq.offset()};
       for (uint32_t i : g.Match(H2(hash))) {
         if (ABSL_PREDICT_TRUE(PolicyTraits::apply(
                 EqualElement<K>{key, eq_ref()},
-                PolicyTraits::element(slots_ + seq.offset(i)))))
+                PolicyTraits::element(slot_ptr + seq.offset(i)))))
           return iterator_at(seq.offset(i));
       }
       if (ABSL_PREDICT_TRUE(g.MaskEmpty())) return end();
       seq.next();
-      assert(seq.index() <= capacity_ && "full table!");
+      assert(seq.index() <= capacity() && "full table!");
     }
   }
   template <class K = key_type>
@@ -1826,9 +1966,9 @@ class raw_hash_set {
     return {it, it};
   }
 
-  size_t bucket_count() const { return capacity_; }
+  size_t bucket_count() const { return capacity(); }
   float load_factor() const {
-    return capacity_ ? static_cast<double>(size()) / capacity_ : 0.0;
+    return capacity() ? static_cast<double>(size()) / capacity() : 0.0;
   }
   float max_load_factor() const { return 1.0f; }
   void max_load_factor(float) {
@@ -1915,7 +2055,8 @@ class raw_hash_set {
     std::pair<iterator, bool> operator()(const K& key, Args&&...) && {
       auto res = s.find_or_prepare_insert(key);
       if (res.second) {
-        PolicyTraits::transfer(&s.alloc_ref(), s.slots_ + res.first, &slot);
+        PolicyTraits::transfer(&s.alloc_ref(), s.slot_array() + res.first,
+                               &slot);
       } else if (do_destroy) {
         PolicyTraits::destroy(&s.alloc_ref(), &slot);
       }
@@ -1931,104 +2072,43 @@ class raw_hash_set {
   // This merely updates the pertinent control byte. This can be used in
   // conjunction with Policy::transfer to move the object to another place.
   void erase_meta_only(const_iterator it) {
-    assert(IsFull(*it.inner_.ctrl_) && "erasing a dangling iterator");
-    --size_;
-    const size_t index = static_cast<size_t>(it.inner_.ctrl_ - ctrl_);
-    const size_t index_before = (index - Group::kWidth) & capacity_;
-    const auto empty_after = Group(it.inner_.ctrl_).MaskEmpty();
-    const auto empty_before = Group(ctrl_ + index_before).MaskEmpty();
-
-    // We count how many consecutive non empties we have to the right and to the
-    // left of `it`. If the sum is >= kWidth then there is at least one probe
-    // window that might have seen a full group.
-    bool was_never_full =
-        empty_before && empty_after &&
-        static_cast<size_t>(empty_after.TrailingZeros() +
-                            empty_before.LeadingZeros()) < Group::kWidth;
-
-    SetCtrl(index, was_never_full ? ctrl_t::kEmpty : ctrl_t::kDeleted,
-            capacity_, ctrl_, slots_, sizeof(slot_type));
-    growth_left() += was_never_full;
-    infoz().RecordErase();
+    EraseMetaOnly(common(), growth_left(), it.inner_.ctrl_, sizeof(slot_type));
   }
 
   // Allocates a backing array for `self` and initializes its control bytes.
-  // This reads `capacity_` and updates all other fields based on the result of
+  // This reads `capacity` and updates all other fields based on the result of
   // the allocation.
   //
-  // This does not free the currently held array; `capacity_` must be nonzero.
-  void initialize_slots() {
-    assert(capacity_);
-    // Folks with custom allocators often make unwarranted assumptions about the
-    // behavior of their classes vis-a-vis trivial destructability and what
-    // calls they will or wont make.  Avoid sampling for people with custom
-    // allocators to get us out of this mess.  This is not a hard guarantee but
-    // a workaround while we plan the exact guarantee we want to provide.
-    //
+  // This does not free the currently held array; `capacity` must be nonzero.
+  inline void initialize_slots() {
     // People are often sloppy with the exact type of their allocator (sometimes
     // it has an extra const or is missing the pair, but rebinds made it work
-    // anyway).  To avoid the ambiguity, we work off SlotAlloc which we have
-    // bound more carefully.
-    if (std::is_same<SlotAlloc, std::allocator<slot_type>>::value &&
-        slots_ == nullptr) {
-      infoz() = Sample(sizeof(slot_type));
-    }
-
-    char* mem = static_cast<char*>(Allocate<alignof(slot_type)>(
-        &alloc_ref(),
-        AllocSize(capacity_, sizeof(slot_type), alignof(slot_type))));
-    ctrl_ = reinterpret_cast<ctrl_t*>(mem);
-    slots_ = reinterpret_cast<slot_type*>(
-        mem + SlotOffset(capacity_, alignof(slot_type)));
-    ResetCtrl(capacity_, ctrl_, slots_, sizeof(slot_type));
-    reset_growth_left();
-    infoz().RecordStorageChanged(size_, capacity_);
-  }
-
-  // Destroys all slots in the backing array, frees the backing array,
-  // If reset is true, also clears all top-level book-keeping data.
-  //
-  // This essentially implements `map = raw_hash_set();`.
-  void destroy_slots(bool reset) {
-    if (!capacity_) return;
-    for (size_t i = 0; i != capacity_; ++i) {
-      if (IsFull(ctrl_[i])) {
-        PolicyTraits::destroy(&alloc_ref(), slots_ + i);
-      }
-    }
-
-    // Unpoison before returning the memory to the allocator.
-    SanitizerUnpoisonMemoryRegion(slots_, sizeof(slot_type) * capacity_);
-    Deallocate<alignof(slot_type)>(
-        &alloc_ref(), ctrl_,
-        AllocSize(capacity_, sizeof(slot_type), alignof(slot_type)));
-    if (reset) {
-      ctrl_ = EmptyGroup();
-      slots_ = nullptr;
-      size_ = 0;
-      capacity_ = 0;
-      growth_left() = 0;
-    }
+    // anyway).
+    using CharAlloc =
+        typename absl::allocator_traits<Alloc>::template rebind_alloc<char>;
+    InitializeSlots<CharAlloc, sizeof(slot_type), alignof(slot_type)>(
+        common(), growth_left(), CharAlloc(alloc_ref()));
   }
 
-  void resize(size_t new_capacity) {
+  ABSL_ATTRIBUTE_NOINLINE void resize(size_t new_capacity) {
     assert(IsValidCapacity(new_capacity));
-    auto* old_ctrl = ctrl_;
-    auto* old_slots = slots_;
-    const size_t old_capacity = capacity_;
-    capacity_ = new_capacity;
+    auto* old_ctrl = control();
+    auto* old_slots = slot_array();
+    const size_t old_capacity = common().capacity_;
+    common().capacity_ = new_capacity;
     initialize_slots();
 
+    auto* new_slots = slot_array();
     size_t total_probe_length = 0;
     for (size_t i = 0; i != old_capacity; ++i) {
       if (IsFull(old_ctrl[i])) {
         size_t hash = PolicyTraits::apply(HashElement{hash_ref()},
                                           PolicyTraits::element(old_slots + i));
-        auto target = find_first_non_full(ctrl_, hash, capacity_);
+        auto target = find_first_non_full(common(), hash);
         size_t new_i = target.offset;
         total_probe_length += target.probe_length;
-        SetCtrl(new_i, H2(hash), capacity_, ctrl_, slots_, sizeof(slot_type));
-        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, old_slots + i);
+        SetCtrl(common(), new_i, H2(hash), sizeof(slot_type));
+        PolicyTraits::transfer(&alloc_ref(), new_slots + new_i, old_slots + i);
       }
     }
     if (old_capacity) {
@@ -2044,70 +2124,10 @@ class raw_hash_set {
   // Prunes control bytes to remove as many tombstones as possible.
   //
   // See the comment on `rehash_and_grow_if_necessary()`.
-  void drop_deletes_without_resize() ABSL_ATTRIBUTE_NOINLINE {
-    assert(IsValidCapacity(capacity_));
-    assert(!is_small(capacity_));
-    // Algorithm:
-    // - mark all DELETED slots as EMPTY
-    // - mark all FULL slots as DELETED
-    // - for each slot marked as DELETED
-    //     hash = Hash(element)
-    //     target = find_first_non_full(hash)
-    //     if target is in the same group
-    //       mark slot as FULL
-    //     else if target is EMPTY
-    //       transfer element to target
-    //       mark slot as EMPTY
-    //       mark target as FULL
-    //     else if target is DELETED
-    //       swap current element with target element
-    //       mark target as FULL
-    //       repeat procedure for current slot with moved from element (target)
-    ConvertDeletedToEmptyAndFullToDeleted(ctrl_, capacity_);
-    alignas(slot_type) unsigned char raw[sizeof(slot_type)];
-    size_t total_probe_length = 0;
-    slot_type* slot = reinterpret_cast<slot_type*>(&raw);
-    for (size_t i = 0; i != capacity_; ++i) {
-      if (!IsDeleted(ctrl_[i])) continue;
-      const size_t hash = PolicyTraits::apply(
-          HashElement{hash_ref()}, PolicyTraits::element(slots_ + i));
-      const FindInfo target = find_first_non_full(ctrl_, hash, capacity_);
-      const size_t new_i = target.offset;
-      total_probe_length += target.probe_length;
-
-      // Verify if the old and new i fall within the same group wrt the hash.
-      // If they do, we don't need to move the object as it falls already in the
-      // best probe we can.
-      const size_t probe_offset = probe(ctrl_, hash, capacity_).offset();
-      const auto probe_index = [probe_offset, this](size_t pos) {
-        return ((pos - probe_offset) & capacity_) / Group::kWidth;
-      };
-
-      // Element doesn't move.
-      if (ABSL_PREDICT_TRUE(probe_index(new_i) == probe_index(i))) {
-        SetCtrl(i, H2(hash), capacity_, ctrl_, slots_, sizeof(slot_type));
-        continue;
-      }
-      if (IsEmpty(ctrl_[new_i])) {
-        // Transfer element to the empty spot.
-        // SetCtrl poisons/unpoisons the slots so we have to call it at the
-        // right time.
-        SetCtrl(new_i, H2(hash), capacity_, ctrl_, slots_, sizeof(slot_type));
-        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, slots_ + i);
-        SetCtrl(i, ctrl_t::kEmpty, capacity_, ctrl_, slots_, sizeof(slot_type));
-      } else {
-        assert(IsDeleted(ctrl_[new_i]));
-        SetCtrl(new_i, H2(hash), capacity_, ctrl_, slots_, sizeof(slot_type));
-        // Until we are done rehashing, DELETED marks previously FULL slots.
-        // Swap i and new_i elements.
-        PolicyTraits::transfer(&alloc_ref(), slot, slots_ + i);
-        PolicyTraits::transfer(&alloc_ref(), slots_ + i, slots_ + new_i);
-        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, slot);
-        --i;  // repeat
-      }
-    }
-    reset_growth_left();
-    infoz().RecordRehash(total_probe_length);
+  inline void drop_deletes_without_resize() {
+    // Stack-allocate space for swapping elements.
+    alignas(slot_type) unsigned char tmp[sizeof(slot_type)];
+    DropDeletesWithoutResize(common(), growth_left(), kPolicyFunctions, tmp);
   }
 
   // Called whenever the table *might* need to conditionally grow.
@@ -2116,14 +2136,13 @@ class raw_hash_set {
   // growth is unnecessary, because vacating tombstones is beneficial for
   // performance in the long-run.
   void rehash_and_grow_if_necessary() {
-    if (capacity_ == 0) {
-      resize(1);
-    } else if (capacity_ > Group::kWidth &&
-               // Do these calcuations in 64-bit to avoid overflow.
-               size() * uint64_t{32} <= capacity_ * uint64_t{25}) {
+    const size_t cap = capacity();
+    if (cap > Group::kWidth &&
+        // Do these calcuations in 64-bit to avoid overflow.
+        size() * uint64_t{32} <= cap* uint64_t{25}) {
       // Squash DELETED without growing if there is enough capacity.
       //
-      // Rehash in place if the current size is <= 25/32 of capacity_.
+      // Rehash in place if the current size is <= 25/32 of capacity.
       // Rationale for such a high factor: 1) drop_deletes_without_resize() is
       // faster than resize, and 2) it takes quite a bit of work to add
       // tombstones.  In the worst case, seems to take approximately 4
@@ -2141,8 +2160,8 @@ class raw_hash_set {
       //
       // Here is output of an experiment using the BM_CacheInSteadyState
       // benchmark running the old case (where we rehash-in-place only if we can
-      // reclaim at least 7/16*capacity_) vs. this code (which rehashes in place
-      // if we can recover 3/32*capacity_).
+      // reclaim at least 7/16*capacity) vs. this code (which rehashes in place
+      // if we can recover 3/32*capacity).
       //
       // Note that although in the worst-case number of rehashes jumped up from
       // 15 to 190, but the number of operations per second is almost the same.
@@ -2165,23 +2184,24 @@ class raw_hash_set {
       drop_deletes_without_resize();
     } else {
       // Otherwise grow the container.
-      resize(capacity_ * 2 + 1);
+      resize(cap * 2 + 1);
     }
   }
 
   bool has_element(const value_type& elem) const {
     size_t hash = PolicyTraits::apply(HashElement{hash_ref()}, elem);
-    auto seq = probe(ctrl_, hash, capacity_);
+    auto seq = probe(common(), hash);
+    const ctrl_t* ctrl = control();
     while (true) {
-      Group g{ctrl_ + seq.offset()};
+      Group g{ctrl + seq.offset()};
       for (uint32_t i : g.Match(H2(hash))) {
-        if (ABSL_PREDICT_TRUE(PolicyTraits::element(slots_ + seq.offset(i)) ==
-                              elem))
+        if (ABSL_PREDICT_TRUE(
+                PolicyTraits::element(slot_array() + seq.offset(i)) == elem))
           return true;
       }
       if (ABSL_PREDICT_TRUE(g.MaskEmpty())) return false;
       seq.next();
-      assert(seq.index() <= capacity_ && "full table!");
+      assert(seq.index() <= capacity() && "full table!");
     }
     return false;
   }
@@ -2206,18 +2226,19 @@ class raw_hash_set {
   std::pair<size_t, bool> find_or_prepare_insert(const K& key) {
     prefetch_heap_block();
     auto hash = hash_ref()(key);
-    auto seq = probe(ctrl_, hash, capacity_);
+    auto seq = probe(common(), hash);
+    const ctrl_t* ctrl = control();
     while (true) {
-      Group g{ctrl_ + seq.offset()};
+      Group g{ctrl + seq.offset()};
       for (uint32_t i : g.Match(H2(hash))) {
         if (ABSL_PREDICT_TRUE(PolicyTraits::apply(
                 EqualElement<K>{key, eq_ref()},
-                PolicyTraits::element(slots_ + seq.offset(i)))))
+                PolicyTraits::element(slot_array() + seq.offset(i)))))
           return {seq.offset(i), false};
       }
       if (ABSL_PREDICT_TRUE(g.MaskEmpty())) break;
       seq.next();
-      assert(seq.index() <= capacity_ && "full table!");
+      assert(seq.index() <= capacity() && "full table!");
     }
     return {prepare_insert(hash), true};
   }
@@ -2227,16 +2248,15 @@ class raw_hash_set {
   //
   // REQUIRES: At least one non-full slot available.
   size_t prepare_insert(size_t hash) ABSL_ATTRIBUTE_NOINLINE {
-    auto target = find_first_non_full(ctrl_, hash, capacity_);
+    auto target = find_first_non_full(common(), hash);
     if (ABSL_PREDICT_FALSE(growth_left() == 0 &&
-                           !IsDeleted(ctrl_[target.offset]))) {
+                           !IsDeleted(control()[target.offset]))) {
       rehash_and_grow_if_necessary();
-      target = find_first_non_full(ctrl_, hash, capacity_);
+      target = find_first_non_full(common(), hash);
     }
-    ++size_;
-    growth_left() -= IsEmpty(ctrl_[target.offset]);
-    SetCtrl(target.offset, H2(hash), capacity_, ctrl_, slots_,
-            sizeof(slot_type));
+    ++common().size_;
+    growth_left() -= IsEmpty(control()[target.offset]);
+    SetCtrl(common(), target.offset, H2(hash), sizeof(slot_type));
     infoz().RecordInsert(hash, target.probe_length);
     return target.offset;
   }
@@ -2251,7 +2271,7 @@ class raw_hash_set {
   // POSTCONDITION: *m.iterator_at(i) == value_type(forward<Args>(args)...).
   template <class... Args>
   void emplace_at(size_t i, Args&&... args) {
-    PolicyTraits::construct(&alloc_ref(), slots_ + i,
+    PolicyTraits::construct(&alloc_ref(), slot_array() + i,
                             std::forward<Args>(args)...);
 
     assert(PolicyTraits::apply(FindElement{*this}, *iterator_at(i)) ==
@@ -2259,16 +2279,14 @@ class raw_hash_set {
            "constructed value does not match the lookup key");
   }
 
-  iterator iterator_at(size_t i) { return {ctrl_ + i, slots_ + i}; }
-  const_iterator iterator_at(size_t i) const { return {ctrl_ + i, slots_ + i}; }
+  iterator iterator_at(size_t i) { return {control() + i, slot_array() + i}; }
+  const_iterator iterator_at(size_t i) const {
+    return {control() + i, slot_array() + i};
+  }
 
  private:
   friend struct RawHashSetTestOnlyAccess;
 
-  void reset_growth_left() {
-    growth_left() = CapacityToGrowth(capacity()) - size_;
-  }
-
   // The number of slots we can still fill without needing to rehash.
   //
   // This is stored separately due to tombstones: we do not include tombstones
@@ -2284,46 +2302,76 @@ class raw_hash_set {
   // Prefetch the heap-allocated memory region to resolve potential TLB misses.
   // This is intended to overlap with execution of calculating the hash for a
   // key.
-  void prefetch_heap_block() const {
-    base_internal::PrefetchT2(ctrl_);
-  }
+  void prefetch_heap_block() const { base_internal::PrefetchT2(control()); }
+
+  CommonFields& common() { return *this; }
+  const CommonFields& common() const { return *this; }
 
-  HashtablezInfoHandle& infoz() { return settings_.template get<1>(); }
+  ctrl_t* control() const { return common().control_; }
+  slot_type* slot_array() const {
+    return static_cast<slot_type*>(common().slots_);
+  }
+  HashtablezInfoHandle& infoz() { return common().infoz(); }
 
-  hasher& hash_ref() { return settings_.template get<2>(); }
-  const hasher& hash_ref() const { return settings_.template get<2>(); }
-  key_equal& eq_ref() { return settings_.template get<3>(); }
-  const key_equal& eq_ref() const { return settings_.template get<3>(); }
-  allocator_type& alloc_ref() { return settings_.template get<4>(); }
+  hasher& hash_ref() { return settings_.template get<1>(); }
+  const hasher& hash_ref() const { return settings_.template get<1>(); }
+  key_equal& eq_ref() { return settings_.template get<2>(); }
+  const key_equal& eq_ref() const { return settings_.template get<2>(); }
+  allocator_type& alloc_ref() { return settings_.template get<3>(); }
   const allocator_type& alloc_ref() const {
-    return settings_.template get<4>();
+    return settings_.template get<3>();
   }
 
-  // TODO(alkis): Investigate removing some of these fields:
-  // - ctrl/slots can be derived from each other
-  // - size can be moved into the slot array
+  // Make type-specific functions for this type's PolicyFunctions struct.
+  static size_t hash_slot_fn(void* set, void* slot) {
+    auto* h = static_cast<raw_hash_set*>(set);
+    return PolicyTraits::apply(
+        HashElement{h->hash_ref()},
+        PolicyTraits::element(static_cast<slot_type*>(slot)));
+  }
+  static void transfer_slot_fn(void* set, void* dst, void* src) {
+    auto* h = static_cast<raw_hash_set*>(set);
+    PolicyTraits::transfer(&h->alloc_ref(), static_cast<slot_type*>(dst),
+                           static_cast<slot_type*>(src));
+  }
+  // Note: dealloc_fn will only be used if we have a non-standard allocator.
+  static void dealloc_fn(void* set, const PolicyFunctions&, ctrl_t* ctrl,
+                         void* slot_mem, size_t n) {
+    auto* h = static_cast<raw_hash_set*>(set);
 
-  // The control bytes (and, also, a pointer to the base of the backing array).
-  //
-  // This contains `capacity_ + 1 + NumClonedBytes()` entries, even
-  // when the table is empty (hence EmptyGroup).
-  ctrl_t* ctrl_ = EmptyGroup();
-  // The beginning of the slots, located at `SlotOffset()` bytes after
-  // `ctrl_`. May be null for empty tables.
-  slot_type* slots_ = nullptr;
+    // Unpoison before returning the memory to the allocator.
+    SanitizerUnpoisonMemoryRegion(slot_mem, sizeof(slot_type) * n);
 
-  // The number of filled slots.
-  size_t size_ = 0;
+    Deallocate<alignof(slot_type)>(
+        &h->alloc_ref(), ctrl,
+        AllocSize(n, sizeof(slot_type), alignof(slot_type)));
+  }
+
+  static constexpr PolicyFunctions kPolicyFunctions = {
+      sizeof(slot_type),
+      alignof(slot_type),
+      &raw_hash_set::hash_slot_fn,
+      &raw_hash_set::transfer_slot_fn,
+      (std::is_same<SlotAlloc, std::allocator<slot_type>>::value
+           ? &DeallocateStandard<alignof(slot_type)>
+           : &raw_hash_set::dealloc_fn),
+  };
 
-  // The total number of available slots.
-  size_t capacity_ = 0;
-  absl::container_internal::CompressedTuple<size_t /* growth_left */,
-                                            HashtablezInfoHandle, hasher,
+  // Bundle together growth_left (number of slots that can be filled without
+  // rehashing) plus other objects which might be empty. CompressedTuple will
+  // ensure that sizeof is not affected by any of the empty fields that occur
+  // after growth_left.
+  absl::container_internal::CompressedTuple<size_t /* growth_left */, hasher,
                                             key_equal, allocator_type>
-      settings_{0u, HashtablezInfoHandle{}, hasher{}, key_equal{},
-                allocator_type{}};
+      settings_{0u, hasher{}, key_equal{}, allocator_type{}};
 };
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+template <class Policy, class Hash, class Eq, class Alloc>
+constexpr PolicyFunctions
+    raw_hash_set<Policy, Hash, Eq, Alloc>::kPolicyFunctions;
+#endif
+
 // Erases all elements that satisfy the predicate `pred` from the container `c`.
 template <typename P, typename H, typename E, typename A, typename Predicate>
 typename raw_hash_set<P, H, E, A>::size_type EraseIf(
@@ -2349,14 +2397,15 @@ struct HashtableDebugAccess<Set, absl::void_t<typename Set::raw_hash_set>> {
                              const typename Set::key_type& key) {
     size_t num_probes = 0;
     size_t hash = set.hash_ref()(key);
-    auto seq = probe(set.ctrl_, hash, set.capacity_);
+    auto seq = probe(set.common(), hash);
+    const ctrl_t* ctrl = set.control();
     while (true) {
-      container_internal::Group g{set.ctrl_ + seq.offset()};
+      container_internal::Group g{ctrl + seq.offset()};
       for (uint32_t i : g.Match(container_internal::H2(hash))) {
         if (Traits::apply(
                 typename Set::template EqualElement<typename Set::key_type>{
                     key, set.eq_ref()},
-                Traits::element(set.slots_ + seq.offset(i))))
+                Traits::element(set.slot_array() + seq.offset(i))))
           return num_probes;
         ++num_probes;
       }
@@ -2367,7 +2416,7 @@ struct HashtableDebugAccess<Set, absl::void_t<typename Set::raw_hash_set>> {
   }
 
   static size_t AllocatedByteSize(const Set& c) {
-    size_t capacity = c.capacity_;
+    size_t capacity = c.capacity();
     if (capacity == 0) return 0;
     size_t m = AllocSize(capacity, sizeof(Slot), alignof(Slot));
 
@@ -2375,9 +2424,10 @@ struct HashtableDebugAccess<Set, absl::void_t<typename Set::raw_hash_set>> {
     if (per_slot != ~size_t{}) {
       m += per_slot * c.size();
     } else {
+      const ctrl_t* ctrl = c.control();
       for (size_t i = 0; i != capacity; ++i) {
-        if (container_internal::IsFull(c.ctrl_[i])) {
-          m += Traits::space_used(c.slots_ + i);
+        if (container_internal::IsFull(ctrl[i])) {
+          m += Traits::space_used(c.slot_array() + i);
         }
       }
     }

absl/container/internal/raw_hash_set_benchmark.cc

@@ -477,6 +477,24 @@ void BM_DropDeletes(benchmark::State& state) {
 }
 BENCHMARK(BM_DropDeletes);
 
+void BM_Resize(benchmark::State& state) {
+  // For now just measure a small cheap hash table since we
+  // are mostly interested in the overhead of type-erasure
+  // in resize().
+  constexpr int kElements = 64;
+  const int kCapacity = kElements * 2;
+
+  IntTable table;
+  for (int i = 0; i < kElements; i++) {
+    table.insert(i);
+  }
+  for (auto unused : state) {
+    table.rehash(0);
+    table.rehash(kCapacity);
+  }
+}
+BENCHMARK(BM_Resize);
+
 }  // namespace
 }  // namespace container_internal
 ABSL_NAMESPACE_END

absl/container/internal/raw_hash_set_test.cc

@@ -56,8 +56,8 @@ namespace container_internal {
 
 struct RawHashSetTestOnlyAccess {
   template <typename C>
-  static auto GetSlots(const C& c) -> decltype(c.slots_) {
-    return c.slots_;
+  static auto GetSlots(const C& c) -> decltype(c.slot_array()) {
+    return c.slot_array();
   }
 };
 
@@ -455,12 +455,12 @@ TEST(Table, EmptyFunctorOptimization) {
   static_assert(std::is_empty<std::allocator<int>>::value, "");
 
   struct MockTable {
+    void* infoz;
     void* ctrl;
     void* slots;
     size_t size;
     size_t capacity;
     size_t growth_left;
-    void* infoz;
   };
   struct MockTableInfozDisabled {
     void* ctrl;