Call RepeatedField::Reserve for small packed enums.

It's a pretty good guess that if all enum values are in [0, 128), that all the
varint values will be one byte long.  So we can call Reserve() on the
RepeatedField, saving some malloc's while parsing.

PiperOrigin-RevId: 533489111

src/google/protobuf/generated_message_tctable_lite.cc

@@ -1389,13 +1389,23 @@ const char* TcParser::PackedEnumSmallRange(PROTOBUF_TC_PARAM_DECL) {
   auto* field = &RefAt<RepeatedField<int32_t>>(msg, data.offset());
   const uint8_t max = data.aux_idx();
 
-  return ctx->ReadPackedVarint(ptr, [=](int32_t v) {
-    if (PROTOBUF_PREDICT_FALSE(min > v || v > max)) {
-      AddUnknownEnum(msg, table, FastDecodeTag(saved_tag), v);
-    } else {
-      field->Add(v);
-    }
-  });
+  return ctx->ReadPackedVarint(
+      ptr,
+      [=](int32_t v) {
+        if (PROTOBUF_PREDICT_FALSE(min > v || v > max)) {
+          AddUnknownEnum(msg, table, FastDecodeTag(saved_tag), v);
+        } else {
+          // The size_callback below ensures that we have enough capacity.
+          field->AddAlreadyReserved(v);
+        }
+      },
+      /*size_callback=*/
+      [=](int32_t size_bytes) {
+        // For enums that fit in one varint byte, optimistically assume that all
+        // the values are one byte long (i.e. no large unknown values).  If so,
+        // we know exactly how many values we're going to get.
+        field->Reserve(field->size() + size_bytes);
+      });
 }
 
 PROTOBUF_NOINLINE const char* TcParser::FastEr0P1(PROTOBUF_TC_PARAM_DECL) {

src/google/protobuf/parse_context.h

@@ -245,7 +245,12 @@ class PROTOBUF_EXPORT EpsCopyInputStream {
   PROTOBUF_NODISCARD const char* ReadPackedFixed(const char* ptr, int size,
                                                  RepeatedField<T>* out);
   template <typename Add>
-  PROTOBUF_NODISCARD const char* ReadPackedVarint(const char* ptr, Add add);
+  PROTOBUF_NODISCARD const char* ReadPackedVarint(const char* ptr, Add add) {
+    return ReadPackedVarint(ptr, add, [](int) {});
+  }
+  template <typename Add, typename SizeCb>
+  PROTOBUF_NODISCARD const char* ReadPackedVarint(const char* ptr, Add add,
+                                                  SizeCb size_callback);
 
   uint32_t LastTag() const { return last_tag_minus_1_ + 1; }
   bool ConsumeEndGroup(uint32_t start_tag) {
@@ -1235,9 +1240,12 @@ const char* ReadPackedVarintArray(const char* ptr, const char* end, Add add) {
   return ptr;
 }
 
-template <typename Add>
-const char* EpsCopyInputStream::ReadPackedVarint(const char* ptr, Add add) {
+template <typename Add, typename SizeCb>
+const char* EpsCopyInputStream::ReadPackedVarint(const char* ptr, Add add,
+                                                 SizeCb size_callback) {
   int size = ReadSize(&ptr);
+  size_callback(size);
+
   GOOGLE_PROTOBUF_PARSER_ASSERT(ptr);
   int chunk_size = static_cast<int>(buffer_end_ - ptr);
   while (size > chunk_size) {