Add stdatomic fallback for GCC 4.7/4.8

This compiler version is still shipped with the latest version of
some distros, like RHEL/CentOS, sadly.

This implementation requires on the __atomic functions.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

include/compat/gcc/stdatomic.h

+/*
+* Copyright © 2018, VideoLAN and dav1d authors
+* All rights reserved.
+*
+* Redistribution and use in source and binary forms, with or without
+* modification, are permitted provided that the following conditions are met:
+*
+* 1. Redistributions of source code must retain the above copyright notice, this
+*    list of conditions and the following disclaimer.
+*
+* 2. Redistributions in binary form must reproduce the above copyright notice,
+*    this list of conditions and the following disclaimer in the documentation
+*    and/or other materials provided with the distribution.
+*
+* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+* ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+* ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+#ifndef GCCVER_STDATOMIC_H_
+#define GCCVER_STDATOMIC_H_
+
+#if !defined(__cplusplus)
+
+typedef int atomic_int;
+typedef unsigned int atomic_uint;
+
+#define memory_order_relaxed __ATOMIC_RELAXED
+#define memory_order_acquire __ATOMIC_ACQUIRE
+
+#define atomic_init(p_a, v)           do { *(p_a) = (v); } while(0)
+#define atomic_store(p_a, v)          __atomic_store_n(p_a, v, __ATOMIC_SEQ_CST)
+#define atomic_load(p_a)              __atomic_load_n(p_a, __ATOMIC_SEQ_CST)
+#define atomic_load_explicit(p_a, mo) __atomic_load_n(p_a, mo)
+#define atomic_fetch_add(p_a, inc)    __atomic_fetch_add(p_a, inc, __ATOMIC_SEQ_CST)
+#define atomic_fetch_sub(p_a, dec)    __atomic_fetch_sub(p_a, dec, __ATOMIC_SEQ_CST)
+
+#endif /* !defined(__cplusplus) */
+
+#endif /* GCCVER_STDATOMIC_H_ */

meson.build

@@ -108,6 +108,11 @@ if not cc.check_header('stdatomic.h')
         stdatomic_dependency = declare_dependency(
             include_directories : include_directories('include/compat'),
         )
+    elif cc.compiles('''int main() { int v = 0; return __atomic_fetch_add(&v, 1, __ATOMIC_SEQ_CST); }''',
+                     name : 'GCC-style atomics', args : test_args)
+        stdatomic_dependency = declare_dependency(
+            include_directories : include_directories('include/compat/gcc'),
+        )
     else
         error('Atomics not supported')
     endif