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meson: Define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 on GNU/Linux if needed

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armv5 Linux systems implement __sync_bool_compare_and_swap() and
friends by calling a function provided by the kernel. This is not
technically an atomic intrinsic, so gcc doesn't define
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 in this case, but it's good
enough for us. Extend the current Android special case to cover
GNU/Linux too.

The possibilities are:

* __sync_foo detected and __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 predefined:
  calls to __atomic_foo or __sync_foo primitives are inlined into user
  code by gatomic.h

* __sync_foo detected but __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 not
  predefined: user code has an extern reference to g_atomic_foo(),
  which calls __atomic_foo or __sync_foo because we defined
  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 when compiling GLib itself

* Windows: user code has an extern reference to g_atomic_foo(),
  which calls InterlockedFoo()

* !defined(G_ATOMIC_LOCK_FREE): user code has an extern reference to
  g_atomic_foo(), which emulates atomic operations with a mutex

Signed-off-by: Simon McVittie <smcv@collabora.com>
Closes: #1576
This commit is contained in:
Simon McVittie 2018-10-30 17:20:34 +00:00 committed by Philip Withnall
parent 7aa52a7d53
commit 7d72a6763c
1 changed files with 2 additions and 2 deletions
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@ -1571,8 +1571,8 @@ atomicdefine = '''
# We know that we can always use real ("lock free") atomic operations with MSVC
if cc.get_id() == 'msvc' or cc.links(atomictest, name : 'atomic ops')
have_atomic_lock_free = true
if host_system == 'android' and not cc.compiles(atomicdefine, name : 'atomic ops define')
# When building for armv5 on Android, gcc 4.9 provides
if (host_system == 'android' or host_system == 'linux') and not cc.compiles(atomicdefine, name : 'atomic ops define')
# When building for armv5 on Linux, gcc provides
# __sync_bool_compare_and_swap but doesn't define
# __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4
glib_conf.set('__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4', true)