Allow tables to be in read-only sections.

PiperOrigin-RevId: 841862343

Author: ckennelly

src/google/protobuf/generated_message_tctable_decl.h

@@ -334,7 +334,7 @@ struct alignas(uint64_t) TcParseTableBase {
   // Table entry for fast-path tailcall dispatch handling.
   struct FastFieldEntry {
     // Target function for dispatch:
-    mutable std::atomic<TailCallParseFunc> target_atomic;
+    TailCallParseFunc target_function;
 
     // Field data used during parse:
     TcFieldData bits;
@@ -344,25 +344,14 @@ struct alignas(uint64_t) TcParseTableBase {
 
     // Constant initializes this instance
     constexpr FastFieldEntry(TailCallParseFunc func, TcFieldData bits)
-        : target_atomic(func), bits(bits) {}
+        : target_function(func), bits(bits) {}
 
     // FastFieldEntry is copy-able and assignable, which is intended
     // mainly for testing and debugging purposes.
-    FastFieldEntry(const FastFieldEntry& rhs) noexcept
-        : FastFieldEntry(rhs.target(), rhs.bits) {}
-    FastFieldEntry& operator=(const FastFieldEntry& rhs) noexcept {
-      SetTarget(rhs.target());
-      bits = rhs.bits;
-      return *this;
-    }
+    FastFieldEntry(const FastFieldEntry& rhs) noexcept = default;
+    FastFieldEntry& operator=(const FastFieldEntry& rhs) noexcept = default;
 
-    // Protocol buffer code should use these relaxed accessors.
-    TailCallParseFunc target() const {
-      return target_atomic.load(std::memory_order_relaxed);
-    }
-    void SetTarget(TailCallParseFunc func) const {
-      return target_atomic.store(func, std::memory_order_relaxed);
-    }
+    TailCallParseFunc target() const { return target_function; }
   };
   // There is always at least one table entry.
   const FastFieldEntry* fast_entry(size_t idx) const {