Optimize RepeatedPtrField constructors.

Writing zeros is faster as they are often available in the cache as well as it can be done with less instructions on common platforms.

```
BM_RepeatedPtrField_Ctor                125ns ± 3%   118ns ± 3%  -5.34%  (p=0.000 n=217+219)
```

PiperOrigin-RevId: 576126835

src/google/protobuf/repeated_ptr_field.cc

@@ -20,6 +20,7 @@
 #include "absl/log/absl_check.h"
 #include "google/protobuf/arena.h"
 #include "google/protobuf/implicit_weak_message.h"
+#include "google/protobuf/message_lite.h"
 #include "google/protobuf/port.h"
 #include "google/protobuf/repeated_field.h"
 
@@ -34,10 +35,11 @@ namespace internal {
 void** RepeatedPtrFieldBase::InternalExtend(int extend_amount) {
   ABSL_DCHECK(extend_amount > 0);
   constexpr size_t ptr_size = sizeof(rep()->elements[0]);
-  int new_capacity = total_size_ + extend_amount;
+  int capacity = Capacity();
+  int new_capacity = capacity + extend_amount;
   Arena* arena = GetArena();
   new_capacity = internal::CalculateReserveSize<void*, kRepHeaderSize>(
-      total_size_, new_capacity);
+      capacity, new_capacity);
   ABSL_CHECK_LE(
       static_cast<int64_t>(new_capacity),
       static_cast<int64_t>(
@@ -45,7 +47,6 @@ void** RepeatedPtrFieldBase::InternalExtend(int extend_amount) {
       << "Requested size is too large to fit into size_t.";
   size_t bytes = kRepHeaderSize + ptr_size * new_capacity;
   Rep* new_rep;
-  void* old_tagged_ptr = tagged_rep_or_elem_;
   if (arena == nullptr) {
     internal::SizedPtr res = internal::AllocateAtLeast(bytes);
     new_capacity = static_cast<int>((res.n - kRepHeaderSize) / ptr_size);
@@ -55,18 +56,17 @@ void** RepeatedPtrFieldBase::InternalExtend(int extend_amount) {
   }
 
   if (using_sso()) {
-    new_rep->allocated_size = old_tagged_ptr != nullptr ? 1 : 0;
-    new_rep->elements[0] = old_tagged_ptr;
+    new_rep->allocated_size = tagged_rep_or_elem_ != nullptr ? 1 : 0;
+    new_rep->elements[0] = tagged_rep_or_elem_;
   } else {
-    Rep* old_rep =
-        reinterpret_cast<Rep*>(reinterpret_cast<uintptr_t>(old_tagged_ptr) - 1);
+    Rep* old_rep = rep();
     if (old_rep->allocated_size > 0) {
       memcpy(new_rep->elements, old_rep->elements,
              old_rep->allocated_size * ptr_size);
     }
     new_rep->allocated_size = old_rep->allocated_size;
 
-    size_t old_size = total_size_ * ptr_size + kRepHeaderSize;
+    size_t old_size = capacity * ptr_size + kRepHeaderSize;
     if (arena == nullptr) {
       internal::SizedDelete(old_rep, old_size);
     } else {
@@ -76,13 +76,14 @@ void** RepeatedPtrFieldBase::InternalExtend(int extend_amount) {
 
   tagged_rep_or_elem_ =
       reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(new_rep) + 1);
-  total_size_ = new_capacity;
+  capacity_proxy_ = new_capacity - kSSOCapacity;
   return &new_rep->elements[current_size_];
 }
 
 void RepeatedPtrFieldBase::Reserve(int capacity) {
-  if (capacity > total_size_) {
-    InternalExtend(capacity - total_size_);
+  int delta = capacity - Capacity();
+  if (delta > 0) {
+    InternalExtend(delta);
   }
 }
 
@@ -98,7 +99,7 @@ void RepeatedPtrFieldBase::DestroyProtos() {
     for (int i = 0; i < n; i++) {
       delete static_cast<MessageLite*>(elements[i]);
     }
-    const size_t size = total_size_ * sizeof(elements[0]) + kRepHeaderSize;
+    const size_t size = Capacity() * sizeof(elements[0]) + kRepHeaderSize;
     internal::SizedDelete(r, size);
   }
 
@@ -115,7 +116,9 @@ void* RepeatedPtrFieldBase::AddOutOfLineHelper(void* obj) {
     ExchangeCurrentSize(1);
     return tagged_rep_or_elem_ = obj;
   }
-  if (using_sso() || rep()->allocated_size == total_size_) {
+  // Not using `AllocatedSizeAtCapacity` because it's already known that
+  // `tagged_rep_or_elem_ != nullptr`.
+  if (using_sso() || rep()->allocated_size >= Capacity()) {
     InternalExtend(1);  // Equivalent to "Reserve(total_size_ + 1)"
   }
   Rep* r = rep();
@@ -134,7 +137,7 @@ void* RepeatedPtrFieldBase::AddOutOfLineHelper(ElementFactory factory) {
   } else {
     absl::PrefetchToLocalCache(rep());
   }
-  if (PROTOBUF_PREDICT_FALSE(current_size_ == total_size_)) {
+  if (PROTOBUF_PREDICT_FALSE(SizeAtCapacity())) {
     InternalExtend(1);
   } else {
     Rep* r = rep();

src/google/protobuf/repeated_ptr_field.h

@@ -177,12 +177,12 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
   constexpr RepeatedPtrFieldBase()
       : tagged_rep_or_elem_(nullptr),
         current_size_(0),
-        total_size_(kSSOCapacity),
+        capacity_proxy_(0),
         arena_(nullptr) {}
   explicit RepeatedPtrFieldBase(Arena* arena)
       : tagged_rep_or_elem_(nullptr),
         current_size_(0),
-        total_size_(kSSOCapacity),
+        capacity_proxy_(0),
         arena_(arena) {}
 
   RepeatedPtrFieldBase(const RepeatedPtrFieldBase&) = delete;
@@ -198,7 +198,13 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
 
   bool empty() const { return current_size_ == 0; }
   int size() const { return current_size_; }
-  int Capacity() const { return total_size_; }
+  // Returns the size of the buffer with pointers to elements.
+  //
+  // Note:
+  //
+  //   * prefer `SizeAtCapacity()` to `size() == Capacity()`;
+  //   * prefer `AllocatedSizeAtCapacity()` to `allocated_size() == Capacity()`.
+  int Capacity() const { return capacity_proxy_ + kSSOCapacity; }
 
   template <typename TypeHandler>
   const Value<TypeHandler>& at(int index) const {
@@ -271,7 +277,7 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
     }
     if (!using_sso()) {
       internal::SizedDelete(rep(),
-                            total_size_ * sizeof(elems[0]) + kRepHeaderSize);
+                            Capacity() * sizeof(elems[0]) + kRepHeaderSize);
     }
   }
 
@@ -417,7 +423,7 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
     size_t allocated_bytes =
         using_sso()
             ? 0
-            : static_cast<size_t>(total_size_) * sizeof(void*) + kRepHeaderSize;
+            : static_cast<size_t>(Capacity()) * sizeof(void*) + kRepHeaderSize;
     const int n = allocated_size();
     void* const* elems = elements();
     for (int i = 0; i < n; ++i) {
@@ -451,11 +457,11 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
   void UnsafeArenaAddAllocated(Value<TypeHandler>* value) {
     ABSL_DCHECK_NE(value, nullptr);
     // Make room for the new pointer.
-    if (current_size_ == total_size_) {
+    if (SizeAtCapacity()) {
       // The array is completely full with no cleared objects, so grow it.
-      Reserve(total_size_ + 1);
+      InternalExtend(1);
       ++rep()->allocated_size;
-    } else if (allocated_size() == total_size_) {
+    } else if (AllocatedSizeAtCapacity()) {
       // There is no more space in the pointer array because it contains some
       // cleared objects awaiting reuse.  We don't want to grow the array in
       // this case because otherwise a loop calling AddAllocated() followed by
@@ -539,41 +545,41 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
   void AddAllocatedInternal(Value<TypeHandler>* value, std::true_type) {
     Arena* element_arena = TypeHandler::GetArena(value);
     Arena* arena = GetArena();
-    if (arena == element_arena && allocated_size() < total_size_) {
-      // Fast path: underlying arena representation (tagged pointer) is equal to
-      // our arena pointer, and we can add to array without resizing it (at
-      // least one slot that is not allocated).
-      void** elems = elements();
-      if (current_size_ < allocated_size()) {
-        // Make space at [current] by moving first allocated element to end of
-        // allocated list.
-        elems[allocated_size()] = elems[current_size_];
-      }
-      elems[ExchangeCurrentSize(current_size_ + 1)] = value;
-      if (!using_sso()) ++rep()->allocated_size;
-    } else {
+    if (arena != element_arena || AllocatedSizeAtCapacity()) {
       AddAllocatedSlowWithCopy<TypeHandler>(value, element_arena, arena);
+      return;
+    }
+    // Fast path: underlying arena representation (tagged pointer) is equal to
+    // our arena pointer, and we can add to array without resizing it (at
+    // least one slot that is not allocated).
+    void** elems = elements();
+    if (current_size_ < allocated_size()) {
+      // Make space at [current] by moving first allocated element to end of
+      // allocated list.
+      elems[allocated_size()] = elems[current_size_];
     }
+    elems[ExchangeCurrentSize(current_size_ + 1)] = value;
+    if (!using_sso()) ++rep()->allocated_size;
   }
 
   // AddAllocated version that does not implement arena-safe copying behavior.
   template <typename TypeHandler>
   void AddAllocatedInternal(Value<TypeHandler>* value, std::false_type) {
-    if (allocated_size() < total_size_) {
-      // Fast path: underlying arena representation (tagged pointer) is equal to
-      // our arena pointer, and we can add to array without resizing it (at
-      // least one slot that is not allocated).
-      void** elems = elements();
-      if (current_size_ < allocated_size()) {
-        // Make space at [current] by moving first allocated element to end of
-        // allocated list.
-        elems[allocated_size()] = elems[current_size_];
-      }
-      elems[ExchangeCurrentSize(current_size_ + 1)] = value;
-      if (!using_sso()) ++rep()->allocated_size;
-    } else {
+    if (AllocatedSizeAtCapacity()) {
       UnsafeArenaAddAllocated<TypeHandler>(value);
+      return;
     }
+    // Fast path: underlying arena representation (tagged pointer) is equal to
+    // our arena pointer, and we can add to array without resizing it (at
+    // least one slot that is not allocated).
+    void** elems = elements();
+    if (current_size_ < allocated_size()) {
+      // Make space at [current] by moving first allocated element to end of
+      // allocated list.
+      elems[allocated_size()] = elems[current_size_];
+    }
+    elems[ExchangeCurrentSize(current_size_ + 1)] = value;
+    if (!using_sso()) ++rep()->allocated_size;
   }
 
   // Slowpath handles all cases, copying if necessary.
@@ -715,6 +721,25 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
   inline int ExchangeCurrentSize(int new_size) {
     return std::exchange(current_size_, new_size);
   }
+  inline bool SizeAtCapacity() const {
+    // Harden invariant size() <= allocated_size() <= Capacity().
+    ABSL_DCHECK_LE(size(), allocated_size());
+    ABSL_DCHECK_LE(allocated_size(), Capacity());
+    // This is equivalent to `current_size_ == Capacity()`.
+    // Assuming `Capacity()` function is inlined, compiler is likely to optimize
+    // away "+ kSSOCapacity" and reduce it to "current_size_ > capacity_proxy_"
+    // which is an instruction less than "current_size_ == capacity_proxy_ + 1".
+    return current_size_ >= Capacity();
+  }
+  inline bool AllocatedSizeAtCapacity() const {
+    // Harden invariant size() <= allocated_size() <= Capacity().
+    ABSL_DCHECK_LE(size(), allocated_size());
+    ABSL_DCHECK_LE(allocated_size(), Capacity());
+    // This combines optimization mentioned in `SizeAtCapacity()` and simplifies
+    // `allocated_size()` in sso case.
+    return using_sso() ? (tagged_rep_or_elem_ != nullptr)
+                       : rep()->allocated_size >= Capacity();
+  }
 
   void* const* elements() const {
     return using_sso() ? &tagged_rep_or_elem_ : +rep()->elements;
@@ -800,8 +825,7 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
 
   // Ensures that capacity is big enough to store one more allocated element.
   inline void MaybeExtend() {
-    if (using_sso() ? (tagged_rep_or_elem_ != nullptr)
-                    : (rep()->allocated_size == total_size_)) {
+    if (AllocatedSizeAtCapacity()) {
       ABSL_DCHECK_EQ(allocated_size(), Capacity());
       InternalExtend(1);
     } else {
@@ -812,11 +836,11 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
   // Ensures that capacity is at least `n` elements.
   // Returns a pointer to the element directly beyond the last element.
   inline void** InternalReserve(int n) {
-    if (n <= total_size_) {
+    if (n <= Capacity()) {
       void** elements = using_sso() ? &tagged_rep_or_elem_ : rep()->elements;
       return elements + current_size_;
     }
-    return InternalExtend(n - total_size_);
+    return InternalExtend(n - Capacity());
   }
 
   // Internal helper for Add: adds "obj" as the next element in the
@@ -838,7 +862,7 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
   // significant performance for memory-sensitive workloads.
   void* tagged_rep_or_elem_;
   int current_size_;
-  int total_size_;
+  int capacity_proxy_;  // we store `capacity - kSSOCapacity` as an optimization
   Arena* arena_;
 };