Skip write of `_cached_size_` when the value to write is `0` and the value was

already `0`.
This prevents writes to empty messages, and it is required for moving default
instances to `rodata`.

PiperOrigin-RevId: 655593321

src/google/protobuf/message_lite.h

@@ -88,6 +88,7 @@ class PROTOBUF_EXPORT CachedSize {
   constexpr CachedSize() noexcept : atom_(Scalar{}) {}
   // NOLINTNEXTLINE(google-explicit-constructor)
   constexpr CachedSize(Scalar desired) noexcept : atom_(desired) {}
+
 #if PROTOBUF_BUILTIN_ATOMIC
   constexpr CachedSize(const CachedSize& other) = default;
 
@@ -96,6 +97,15 @@ class PROTOBUF_EXPORT CachedSize {
   }
 
   void Set(Scalar desired) const noexcept {
+    // Avoid writing the value when it is zero. This prevents writing to gloabl
+    // default instances, which might be in readonly memory.
+    if (ABSL_PREDICT_FALSE(desired == 0)) {
+      if (Get() == 0) return;
+    }
+    __atomic_store_n(&atom_, desired, __ATOMIC_RELAXED);
+  }
+
+  void SetNonZero(Scalar desired) const noexcept {
     __atomic_store_n(&atom_, desired, __ATOMIC_RELAXED);
   }
 #else
@@ -110,6 +120,15 @@ class PROTOBUF_EXPORT CachedSize {
   }
 
   void Set(Scalar desired) const noexcept {
+    // Avoid writing the value when it is zero. This prevents writing to gloabl
+    // default instances, which might be in readonly memory.
+    if (ABSL_PREDICT_FALSE(desired == 0)) {
+      if (Get() == 0) return;
+    }
+    atom_.store(desired, std::memory_order_relaxed);
+  }
+
+  void SetNonZero(Scalar desired) const noexcept {
     atom_.store(desired, std::memory_order_relaxed);
   }
 #endif

src/google/protobuf/wire_format_lite.cc

@@ -840,7 +840,7 @@ size_t WireFormatLite::Int32SizeWithPackedTagSize(
   }
   size_t res;
   PROTOBUF_ALWAYS_INLINE_CALL res = Int32Size(value);
-  cached_size.Set(ToCachedSize(res));
+  cached_size.SetNonZero(ToCachedSize(res));
   return tag_size + res + Int32Size(static_cast<int32_t>(res));
 }
 size_t WireFormatLite::Int64SizeWithPackedTagSize(
@@ -852,7 +852,7 @@ size_t WireFormatLite::Int64SizeWithPackedTagSize(
   }
   size_t res;
   PROTOBUF_ALWAYS_INLINE_CALL res = Int64Size(value);
-  cached_size.Set(ToCachedSize(res));
+  cached_size.SetNonZero(ToCachedSize(res));
   return tag_size + res + Int32Size(static_cast<int32_t>(res));
 }
 size_t WireFormatLite::UInt32SizeWithPackedTagSize(
@@ -864,7 +864,7 @@ size_t WireFormatLite::UInt32SizeWithPackedTagSize(
   }
   size_t res;
   PROTOBUF_ALWAYS_INLINE_CALL res = UInt32Size(value);
-  cached_size.Set(ToCachedSize(res));
+  cached_size.SetNonZero(ToCachedSize(res));
   return tag_size + res + Int32Size(static_cast<int32_t>(res));
 }
 size_t WireFormatLite::UInt64SizeWithPackedTagSize(
@@ -876,7 +876,7 @@ size_t WireFormatLite::UInt64SizeWithPackedTagSize(
   }
   size_t res;
   PROTOBUF_ALWAYS_INLINE_CALL res = UInt64Size(value);
-  cached_size.Set(ToCachedSize(res));
+  cached_size.SetNonZero(ToCachedSize(res));
   return tag_size + res + Int32Size(static_cast<int32_t>(res));
 }
 size_t WireFormatLite::SInt32SizeWithPackedTagSize(
@@ -888,7 +888,7 @@ size_t WireFormatLite::SInt32SizeWithPackedTagSize(
   }
   size_t res;
   PROTOBUF_ALWAYS_INLINE_CALL res = SInt32Size(value);
-  cached_size.Set(ToCachedSize(res));
+  cached_size.SetNonZero(ToCachedSize(res));
   return tag_size + res + Int32Size(static_cast<int32_t>(res));
 }
 size_t WireFormatLite::SInt64SizeWithPackedTagSize(
@@ -900,7 +900,7 @@ size_t WireFormatLite::SInt64SizeWithPackedTagSize(
   }
   size_t res;
   PROTOBUF_ALWAYS_INLINE_CALL res = SInt64Size(value);
-  cached_size.Set(ToCachedSize(res));
+  cached_size.SetNonZero(ToCachedSize(res));
   return tag_size + res + Int32Size(static_cast<int32_t>(res));
 }
 size_t WireFormatLite::EnumSizeWithPackedTagSize(
@@ -912,7 +912,7 @@ size_t WireFormatLite::EnumSizeWithPackedTagSize(
   }
   size_t res;
   PROTOBUF_ALWAYS_INLINE_CALL res = EnumSize(value);
-  cached_size.Set(ToCachedSize(res));
+  cached_size.SetNonZero(ToCachedSize(res));
   return tag_size + res + Int32Size(static_cast<int32_t>(res));
 }