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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>

diff: cannot open b/include/compat/gcc//null: file does not exist: 'b/include/compat/gcc//null'

--- /dev/null

+++ b/include/compat/gcc/stdatomic.h

@@ -1,0 +1,47 @@

+/*

+* 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_ */

--- a/meson.build

+++ b/meson.build

@@ -108,6 +108,11 @@

         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