MDEV-26467 fixup: Prefer fetch_add() to fetch_or() on IA-32 and AMD64

storage/innobase/include/srw_lock.h

@@ -226,8 +226,18 @@ class ssux_lock_impl final
   void wr_lock()
   {
     writer.wr_lock();
+#if defined __i386__||defined __x86_64__||defined _M_IX86||defined _M_IX64
+    /* On IA-32 and AMD64, this type of fetch_or() can only be implemented
+    as a loop around LOCK CMPXCHG. In this particular case, setting the
+    most significant bit using fetch_add() is equivalent, and is
+    translated into a simple LOCK XADD. */
+    static_assert(WRITER == 1U << 31, "compatibility");
+    if (uint32_t lk= readers.fetch_add(WRITER, std::memory_order_acquire))
+      wr_wait(lk);
+#else
     if (uint32_t lk= readers.fetch_or(WRITER, std::memory_order_acquire))
       wr_wait(lk);
+#endif
   }
 
   void u_wr_upgrade()