[Added PR number and updated patches]

On Aarch64, the __sync builtins are implemented using the __atomic operations
and barriers. This makes the the __sync builtins inconsistent with their
documentation which requires stronger barriers than those for the __atomic
builtins.

The difference between __sync and __atomic builtins is that the restrictions
imposed by a __sync operation's barrier apply to all memory references while the
restrictions of an __atomic operation's barrier only need to apply to a
subset. This affects Aarch64 in particular because, although its implementation
of __atomic builtins is correct, the barriers generated are too weak for the
__sync builtins.

The affected __sync builtins are the __sync_fetch_and_op (and
__sync_op_and_fetch) functions, __sync_compare_and_swap and
__sync_lock_test_and_set. This and a following patch modifies the code generated
for these functions to weaken initial load-acquires to a simple load and to add
a final fence to prevent code-hoisting. The last patch will add tests for the
code generated by the Aarch64 backend for the __sync builtins.

- Full barriers:  __sync_fetch_and_op, __sync_op_and_fetch
   __sync_*_compare_and_swap

   [load-acquire; code; store-release]
   becomes
   [load; code ; store-release; fence].

- Acquire barriers:  __sync_lock_test_and_set

   [load-acquire; code; store]
   becomes
   [load; code; store; fence]

The code generated for release barriers and for the __atomic builtins is
unchanged.

This patch changes the code generated for __sync_fetch_and_<op> and
__sync_<op>_and_fetch builtins.

Tested with check-gcc for aarch64-none-linux-gnu.

Ok for trunk?
Matthew

gcc/
2015-05-22  Matthew Wahab  <matthew.wahab@arm.com>

         PR target/65697
	* config/aarch64/aarch64.c (aarch64_emit_post_barrier): New.
	(aarch64_split_atomic_op): Check for __sync memory models, emit
	appropriate initial and final barriers.