MDEV-22456 after-merge fix: introduce Atomic_relaxed

In the merge 9e6e43551fc61bc34152f8d60f5d72f0d3814787
we made Atomic_counter a more generic wrapper of std::atomic
so that dict_index_t would support the implicit assignment operator.

It is better to revert the changes to Atomic_counter and
instead introduce Atomic_relaxed as a generic wrapper to std::atomic.

Unlike Atomic_counter, we will not define operator++, operator+=
or similar, because we want to make the operations more explicit
in the users of Atomic_wrapper, because unlike loads and stores,
atomic read-modify-write operations always incur some overhead.

1 files changed, 45 insertions, 0 deletions

diff --git a/include/my_atomic.h b/include/my_atomic.h
index 1c54c24d455..88f6746ba3d 100644
--- a/include/my_atomic.h
+++ b/include/my_atomic.h
@@ -2,6 +2,7 @@
 #define MY_ATOMIC_INCLUDED
 
 /* Copyright (c) 2006, 2010, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2018, 2020, MariaDB
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -169,4 +170,48 @@
   my_atomic_casptr((P), (E), (D))
 #endif
 
+#ifdef __cplusplus
+#include <atomic>
+/**
+  A wrapper for std::atomic, defaulting to std::memory_order_relaxed.
+
+  When it comes to atomic loads or stores at std::memory_order_relaxed
+  on IA-32 or AMD64, this wrapper is only introducing some constraints
+  to the C++ compiler, to prevent some optimizations of loads or
+  stores.
+
+  On POWER and ARM, atomic loads and stores involve different instructions
+  from normal loads and stores and will thus incur some overhead.
+
+  Because atomic read-modify-write operations will always incur
+  overhead, we intentionally do not define
+  operator++(), operator--(), operator+=(), operator-=(), or similar,
+  to make the overhead stand out in the users of this code.
+*/
+template <typename Type> class Atomic_relaxed
+{
+  std::atomic<Type> m;
+public:
+  Atomic_relaxed(const Atomic_relaxed<Type> &rhs)
+  { m.store(rhs, std::memory_order_relaxed); }
+  Atomic_relaxed(Type val) : m(val) {}
+  Atomic_relaxed() {}
+
+  operator Type() const { return m.load(std::memory_order_relaxed); }
+  Type operator=(const Type val)
+  { m.store(val, std::memory_order_relaxed); return val; }
+  Type operator=(const Atomic_relaxed<Type> &rhs) { return *this= Type{rhs}; }
+  Type fetch_add(const Type i, std::memory_order o= std::memory_order_relaxed)
+  { return m.fetch_add(i, o); }
+  Type fetch_sub(const Type i, std::memory_order o= std::memory_order_relaxed)
+  { return m.fetch_sub(i, o); }
+  bool compare_exchange_strong(Type& i1, const Type i2,
+                               std::memory_order o1= std::memory_order_relaxed,
+                               std::memory_order o2= std::memory_order_relaxed)
+  { return m.compare_exchange_strong(i1, i2, o1, o2); }
+  Type exchange(const Type i, std::memory_order o= std::memory_order_relaxed)
+  { return m.exchange(i, o); }
+};
+#endif /* __cplusplus */
+
 #endif /* MY_ATOMIC_INCLUDED */