[PATCH 2/3][ARM][PR target/65697] Strengthen barriers for compare-and-swap builtin.

[Matthew Wahab <matthew dot wahab at arm dot com>]

This is the ARM version of the patches to strengthen memory barriers for the
__sync builtins on ARMv8 targets
(https://gcc.gnu.org/ml/gcc-patches/2015-05/msg01989.html).

This patch changes the code generated for __sync_type_compare_and_swap to remove
the acquire-barrier from the load and end the operation with a fence. This also
strengthens the acquire barrier generated for __sync_lock_test_and_set which,
like compare-and-swap, is implemented as a form of atomic exchange.

Tested as part of a series for arm-none-linux-gnueabihf with check-gcc.

Ok for trunk?
Matthew

gcc/
2015-06-22  Matthew Wahab  <matthew.wahab@arm.com>

	PR Target/65697
	* config/armc/arm.c (arm_split_compare_and_swap): For ARMv8, replace an
	initial acquire barrier with a final full barrier.

From ddb9a45acda7bb64d91c446bc40afe4b78fcc1e1 Mon Sep 17 00:00:00 2001
From: Matthew Wahab <matthew.wahab@arm.com>
Date: Fri, 22 May 2015 13:36:39 +0100
Subject: [PATCH 2/3] [ARM] Strengthen barriers for compare-and-swap builtin.

Change-Id: I43381b2ea88492f807d85a73d233369334c99881
---
 gcc/config/arm/arm.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 94118f4..4610ff6 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -27603,6 +27603,8 @@ arm_split_compare_and_swap (rtx operands[])
   scratch = operands[7];
   mode = GET_MODE (mem);
 
+  bool is_armv8_sync = arm_arch8 && is_mm_sync (mod_s);
+
   bool use_acquire = TARGET_HAVE_LDACQ
                      && !(is_mm_relaxed (mod_s) || is_mm_consume (mod_s)
 			  || is_mm_release (mod_s));
@@ -27611,6 +27613,11 @@ arm_split_compare_and_swap (rtx operands[])
                      && !(is_mm_relaxed (mod_s) || is_mm_consume (mod_s)
 			  || is_mm_acquire (mod_s));
 
+  /* For ARMv8, the load-acquire is too weak for __sync memory orders.  Instead,
+     a full barrier is emitted after the store-release.  */
+  if (is_armv8_sync)
+    use_acquire = false;
+
   /* Checks whether a barrier is needed and emits one accordingly.  */
   if (!(use_acquire || use_release))
     arm_pre_atomic_barrier (mod_s);
@@ -27651,7 +27658,8 @@ arm_split_compare_and_swap (rtx operands[])
     emit_label (label2);
 
   /* Checks whether a barrier is needed and emits one accordingly.  */
-  if (!(use_acquire || use_release))
+  if (is_armv8_sync
+      || !(use_acquire || use_release))
     arm_post_atomic_barrier (mod_s);
 
   if (is_mm_relaxed (mod_f))
-- 
1.9.1