Move growth_left to the backing array.

PiperOrigin-RevId: 548794485
Change-Id: Ie82d5f8ad752518ef05b38144ca1e32b21c9def8

Author: ezbr

absl/container/internal/raw_hash_set.cc

@@ -19,6 +19,7 @@
 #include <cstddef>
 #include <cstring>
 
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
 #include "absl/base/dynamic_annotations.h"
 #include "absl/hash/hash.h"
@@ -27,9 +28,17 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
-// A single block of empty control bytes for tables without any slots allocated.
-// This enables removing a branch in the hot path of find().
-alignas(16) ABSL_CONST_INIT ABSL_DLL const ctrl_t kEmptyGroup[16] = {
+// We have space for `growth_left` before a single block of control bytes. A
+// single block of empty control bytes for tables without any slots allocated.
+// This enables removing a branch in the hot path of find(). In order to ensure
+// that the control bytes are aligned to 16, we have 16 bytes before the control
+// bytes even though growth_left only needs 8.
+constexpr ctrl_t ZeroCtrlT() { return static_cast<ctrl_t>(0); }
+alignas(16) ABSL_CONST_INIT ABSL_DLL const ctrl_t kEmptyGroup[32] = {
+    ZeroCtrlT(),       ZeroCtrlT(),    ZeroCtrlT(),    ZeroCtrlT(),
+    ZeroCtrlT(),       ZeroCtrlT(),    ZeroCtrlT(),    ZeroCtrlT(),
+    ZeroCtrlT(),       ZeroCtrlT(),    ZeroCtrlT(),    ZeroCtrlT(),
+    ZeroCtrlT(),       ZeroCtrlT(),    ZeroCtrlT(),    ZeroCtrlT(),
     ctrl_t::kSentinel, ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
     ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
     ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
@@ -239,12 +248,12 @@ void ClearBackingArray(CommonFields& c, const PolicyFunctions& policy,
     c.infoz().RecordStorageChanged(0, c.capacity());
   } else {
     void* set = &c;
-    (*policy.dealloc)(set, policy, c.control(), c.slots_ptr(), c.capacity());
+    (*policy.dealloc)(set, policy, c.backing_array_start(), c.slots_ptr(),
+                      c.capacity());
     c.set_control(EmptyGroup());
     c.set_generation_ptr(EmptyGeneration());
     c.set_slots(nullptr);
     c.set_capacity(0);
-    c.set_growth_left(0);
     c.infoz().RecordClearedReservation();
     assert(c.size() == 0);
     c.infoz().RecordStorageChanged(0, 0);

absl/container/internal/raw_hash_set.h

@@ -62,6 +62,8 @@
 // pseudo-struct:
 //
 //   struct BackingArray {
+//     // The number of elements we can insert before growing the capacity.
+//     size_t growth_left;
 //     // Control bytes for the "real" slots.
 //     ctrl_t ctrl[capacity];
 //     // Always `ctrl_t::kSentinel`. This is used by iterators to find when to
@@ -174,6 +176,7 @@
 
 #include <algorithm>
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
 #include <cstring>
 #include <iterator>
@@ -484,13 +487,14 @@ static_assert(ctrl_t::kDeleted == static_cast<ctrl_t>(-2),
               "ctrl_t::kDeleted must be -2 to make the implementation of "
               "ConvertSpecialToEmptyAndFullToDeleted efficient");
 
-ABSL_DLL extern const ctrl_t kEmptyGroup[16];
+// See definition comment for why this is size 32.
+ABSL_DLL extern const ctrl_t kEmptyGroup[32];
 
 // Returns a pointer to a control byte group that can be used by empty tables.
 inline ctrl_t* EmptyGroup() {
   // Const must be cast away here; no uses of this function will actually write
   // to it, because it is only used for empty tables.
-  return const_cast<ctrl_t*>(kEmptyGroup);
+  return const_cast<ctrl_t*>(kEmptyGroup + 16);
 }
 
 // Returns a pointer to a generation to use for an empty hashtable.
@@ -913,33 +917,46 @@ class CommonFields : public CommonFieldsGenerationInfo {
         // fields generates less code then calling absl::exchange per field.
         control_(that.control()),
         slots_(that.slots_ptr()),
-        size_(that.size()),
         capacity_(that.capacity()),
-        compressed_tuple_(that.growth_left(), std::move(that.infoz())) {
+        compressed_tuple_(that.size(), std::move(that.infoz())) {
     that.set_control(EmptyGroup());
     that.set_slots(nullptr);
-    that.set_size(0);
     that.set_capacity(0);
-    that.set_growth_left(0);
+    that.set_size(0);
   }
   CommonFields& operator=(CommonFields&&) = default;
 
   ctrl_t* control() const { return control_; }
   void set_control(ctrl_t* c) { control_ = c; }
+  void* backing_array_start() const {
+    // growth_left is stored before control bytes.
+    assert(reinterpret_cast<uintptr_t>(control()) % alignof(size_t) == 0);
+    return control() - sizeof(size_t);
+  }
+
   // Note: we can't use slots() because Qt defines "slots" as a macro.
   void* slots_ptr() const { return slots_; }
   void set_slots(void* s) { slots_ = s; }
-  size_t size() const { return size_; }
-  void set_size(size_t s) { size_ = s; }
+
+  // The number of filled slots.
+  size_t size() const { return compressed_tuple_.template get<0>(); }
+  void set_size(size_t s) { compressed_tuple_.template get<0>() = s; }
+
+  // The total number of available slots.
   size_t capacity() const { return capacity_; }
   void set_capacity(size_t c) {
     assert(c == 0 || IsValidCapacity(c));
     capacity_ = c;
   }
 
   // The number of slots we can still fill without needing to rehash.
-  size_t growth_left() const { return compressed_tuple_.template get<0>(); }
-  void set_growth_left(size_t gl) { compressed_tuple_.template get<0>() = gl; }
+  // This is stored in the heap allocation before the control bytes.
+  size_t growth_left() const {
+    return *reinterpret_cast<size_t*>(backing_array_start());
+  }
+  void set_growth_left(size_t gl) {
+    *reinterpret_cast<size_t*>(backing_array_start()) = gl;
+  }
 
   HashtablezInfoHandle& infoz() { return compressed_tuple_.template get<1>(); }
   const HashtablezInfoHandle& infoz() const {
@@ -951,32 +968,30 @@ class CommonFields : public CommonFieldsGenerationInfo {
         should_rehash_for_bug_detection_on_insert(control(), capacity());
   }
   void reset_reserved_growth(size_t reservation) {
-    CommonFieldsGenerationInfo::reset_reserved_growth(reservation, size_);
+    CommonFieldsGenerationInfo::reset_reserved_growth(reservation, size());
   }
 
  private:
   // TODO(b/259599413): Investigate removing some of these fields:
   // - control/slots can be derived from each other
-  // - growth_left can be moved into the slot array
   // - we can use 6 bits for capacity since it's always a power of two minus 1
 
-  // The control bytes (and, also, a pointer to the base of the backing array).
+  // The control bytes (and, also, a pointer near to the base of the backing
+  // array).
   //
   // This contains `capacity + 1 + NumClonedBytes()` entries, even
   // when the table is empty (hence EmptyGroup).
+  //
+  // Note that growth_left is stored immediately before this pointer.
   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;
 
-  // Bundle together growth_left and HashtablezInfoHandle to ensure EBO for
+  // Bundle together size and HashtablezInfoHandle to ensure EBO for
   // HashtablezInfoHandle when sampling is turned off.
   absl::container_internal::CompressedTuple<size_t, HashtablezInfoHandle>
       compressed_tuple_{0u, HashtablezInfoHandle{}};
@@ -1318,20 +1333,28 @@ inline void SetCtrl(const CommonFields& common, size_t i, h2_t h,
   SetCtrl(common, i, static_cast<ctrl_t>(h), slot_size);
 }
 
+// growth_left (which is a size_t) is stored with the backing array.
+constexpr size_t BackingArrayAlignment(size_t align_of_slot) {
+  return (std::max)(align_of_slot, alignof(size_t));
+}
+
+// Computes the offset from the start of the backing allocation of the control
+// bytes. growth_left is stored at the beginning of the backing array.
+inline size_t ControlOffset() { return sizeof(size_t); }
+
 // Given the capacity of a table, computes the offset (from the start of the
 // backing allocation) of the generation counter (if it exists).
 inline size_t GenerationOffset(size_t capacity) {
   assert(IsValidCapacity(capacity));
   const size_t num_control_bytes = capacity + 1 + NumClonedBytes();
-  return num_control_bytes;
+  return ControlOffset() + num_control_bytes;
 }
 
 // Given the capacity of a table, computes the offset (from the start of the
 // backing allocation) at which the slots begin.
 inline size_t SlotOffset(size_t capacity, size_t slot_align) {
   assert(IsValidCapacity(capacity));
-  const size_t num_control_bytes = capacity + 1 + NumClonedBytes();
-  return (num_control_bytes + NumGenerationBytes() + slot_align - 1) &
+  return (GenerationOffset(capacity) + NumGenerationBytes() + slot_align - 1) &
          (~slot_align + 1);
 }
 
@@ -1356,13 +1379,15 @@ ABSL_ATTRIBUTE_NOINLINE void InitializeSlots(CommonFields& c, Alloc alloc) {
           : 0;
 
   const size_t cap = c.capacity();
+  const size_t alloc_size = AllocSize(cap, SizeOfSlot, AlignOfSlot);
+  // growth_left (which is a size_t) is stored with the backing array.
   char* mem = static_cast<char*>(
-      Allocate<AlignOfSlot>(&alloc, AllocSize(cap, SizeOfSlot, AlignOfSlot)));
+      Allocate<BackingArrayAlignment(AlignOfSlot)>(&alloc, alloc_size));
   const GenerationType old_generation = c.generation();
   c.set_generation_ptr(
       reinterpret_cast<GenerationType*>(mem + GenerationOffset(cap)));
   c.set_generation(NextGeneration(old_generation));
-  c.set_control(reinterpret_cast<ctrl_t*>(mem));
+  c.set_control(reinterpret_cast<ctrl_t*>(mem + ControlOffset()));
   c.set_slots(mem + SlotOffset(cap, AlignOfSlot));
   ResetCtrl(c, SizeOfSlot);
   if (sample_size) {
@@ -1385,8 +1410,8 @@ struct PolicyFunctions {
   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);
+  void (*dealloc)(void* set, const PolicyFunctions& policy,
+                  void* backing_array_start, void* slot_array, size_t n);
 };
 
 // ClearBackingArray clears the backing array, either modifying it in place,
@@ -1405,14 +1430,14 @@ void EraseMetaOnly(CommonFields& c, ctrl_t* it, size_t slot_size);
 template <size_t AlignOfSlot>
 ABSL_ATTRIBUTE_NOINLINE void DeallocateStandard(void*,
                                                 const PolicyFunctions& policy,
-                                                ctrl_t* ctrl, void* slot_array,
-                                                size_t n) {
+                                                void* backing_array_start,
+                                                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));
+  Deallocate<BackingArrayAlignment(AlignOfSlot)>(
+      &alloc, backing_array_start, AllocSize(n, policy.slot_size, AlignOfSlot));
 }
 
 // For trivially relocatable types we use memcpy directly. This allows us to
@@ -1773,7 +1798,9 @@ class raw_hash_set {
 
   raw_hash_set(const raw_hash_set& that, const allocator_type& a)
       : raw_hash_set(0, that.hash_ref(), that.eq_ref(), a) {
-    reserve(that.size());
+    const size_t size = that.size();
+    if (size == 0) return;
+    reserve(size);
     // Because the table is guaranteed to be empty, we can do something faster
     // than a full `insert`.
     for (const auto& v : that) {
@@ -1784,8 +1811,8 @@ class raw_hash_set {
       common().maybe_increment_generation_on_insert();
       infoz().RecordInsert(hash, target.probe_length);
     }
-    common().set_size(that.size());
-    set_growth_left(growth_left() - that.size());
+    common().set_size(size);
+    set_growth_left(growth_left() - size);
   }
 
   ABSL_ATTRIBUTE_NOINLINE raw_hash_set(raw_hash_set&& that) noexcept(
@@ -1838,8 +1865,8 @@ class raw_hash_set {
 
     // Unpoison before returning the memory to the allocator.
     SanitizerUnpoisonMemoryRegion(slot_array(), sizeof(slot_type) * cap);
-    Deallocate<alignof(slot_type)>(
-        &alloc_ref(), control(),
+    Deallocate<BackingArrayAlignment(alignof(slot_type))>(
+        &alloc_ref(), common().backing_array_start(),
         AllocSize(cap, sizeof(slot_type), alignof(slot_type)));
 
     infoz().Unregister();
@@ -2470,8 +2497,8 @@ class raw_hash_set {
     if (old_capacity) {
       SanitizerUnpoisonMemoryRegion(old_slots,
                                     sizeof(slot_type) * old_capacity);
-      Deallocate<alignof(slot_type)>(
-          &alloc_ref(), old_ctrl,
+      Deallocate<BackingArrayAlignment(alignof(slot_type))>(
+          &alloc_ref(), old_ctrl - ControlOffset(),
           AllocSize(old_capacity, sizeof(slot_type), alignof(slot_type)));
     }
     infoz().RecordRehash(total_probe_length);
@@ -2707,15 +2734,15 @@ class raw_hash_set {
                            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) {
+  static void dealloc_fn(void* set, const PolicyFunctions&,
+                         void* backing_array_start, void* slot_mem, size_t n) {
     auto* h = static_cast<raw_hash_set*>(set);
 
     // Unpoison before returning the memory to the allocator.
     SanitizerUnpoisonMemoryRegion(slot_mem, sizeof(slot_type) * n);
 
-    Deallocate<alignof(slot_type)>(
-        &h->alloc_ref(), ctrl,
+    Deallocate<BackingArrayAlignment(alignof(slot_type))>(
+        &h->alloc_ref(), backing_array_start,
         AllocSize(n, sizeof(slot_type), alignof(slot_type)));
   }