re PR target/65697 (__atomic memory barriers not strong enough for __sync builtins)

2015-06-29  Matthew Wahab  <matthew.wahab@arm.com>

	PR target/65697
	* config/armc/arm.c (arm_split_atomic_op): For ARMv8, replace an
	initial acquire barrier with final barrier.

From-SVN: r225132

gcc/ChangeLog

+2015-06-29  Matthew Wahab  <matthew.wahab@arm.com>
+
+	PR target/65697
+	* config/armc/arm.c (arm_split_atomic_op): For ARMv8, replace an
+	initial acquire barrier with final barrier.
+
 2015-06-29  Richard Henderson  <rth@redhat.com>
 
 	* config/i386/constraints.md (Bf): New constraint.

gcc/config/arm/arm.c

@@ -27679,6 +27679,8 @@ arm_split_atomic_op (enum rtx_code code, rtx old_out, rtx new_out, rtx mem,
   rtx_code_label *label;
   rtx x;
 
+  bool is_armv8_sync = arm_arch8 && is_mm_sync (model);
+
   bool use_acquire = TARGET_HAVE_LDACQ
                      && !(is_mm_relaxed (model) || is_mm_consume (model)
 			  || is_mm_release (model));
@@ -27687,6 +27689,11 @@ arm_split_atomic_op (enum rtx_code code, rtx old_out, rtx new_out, rtx mem,
                      && !(is_mm_relaxed (model) || is_mm_consume (model)
 			  || is_mm_acquire (model));
 
+  /* For ARMv8, a 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 (model);
@@ -27757,7 +27764,8 @@ arm_split_atomic_op (enum rtx_code code, rtx old_out, rtx new_out, rtx mem,
   emit_unlikely_jump (gen_cbranchsi4 (x, cond, const0_rtx, label));
 
   /* 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 (model);
 }