WIP: contrib: universal atomic operations

src/contrib/atomic.h

+/*  Copyright (C) 2019 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+
+    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
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/*!
+ * \brief Multiplatform atomic operations.
+ */
+
+#pragma once
+
+#ifdef HAVE_ATOMIC                   /* C11 */
+ #define ATOMIC_SET(dst, val) __atomic_store_n(&(dst), (val), __ATOMIC_RELAXED)
+ #define ATOMIC_GET(src)      __atomic_load_n(&(src), __ATOMIC_RELAXED)
+ #define ATOMIC_ADD(dst, val) __atomic_add_fetch(&(dst), (val), __ATOMIC_RELAXED)
+ #define ATOMIC_SUB(dst, val) __atomic_sub_fetch(&(dst), (val), __ATOMIC_RELAXED)
+#elif HAVE_SYNC_ATOMIC               /* GCC, partial support only. */
+ #define ATOMIC_SET(dst, val) ((dst) = (val))
+ #define ATOMIC_GET(src)      __sync_fetch_and_or(&(src), 0)
+ #define ATOMIC_ADD(dst, val) __sync_add_and_fetch(&(dst), (val))
+ #define ATOMIC_SUB(dst, val) __sync_sub_and_fetch(&(dst), (val))
+#else                                /* Fallback, non-atomic. */
+ #define ATOMIC_SET(dst, val) ((dst) = (val))
+ #define ATOMIC_GET(src)      (src)
+ #define ATOMIC_ADD(dst, val) ((dst) += (val))
+ #define ATOMIC_SUB(dst, val) ((dst) -= (val))
+#endif