Replace memory barriers to better reflect Java needs.

Replaces barriers that enforce ordering of one access type
(e.g. Load) with respect to another (e.g. store) with more general
ones that better reflect both Java requirements and actual hardware
barrier/fence instructions.  The old code was inconsistent and
unclear about which barriers implied which others.  Sometimes
multiple barriers were generated and then eliminated;
sometimes it was assumed that certain barriers implied others.
The new barriers closely parallel those in C++11, though, for now,
we use something closer to the old naming.

Bug: 14685856

Change-Id: Ie1c80afe3470057fc6f2b693a9831dfe83add831

1 files changed, 6 insertions, 8 deletions

diff --git a/compiler/dex/quick/arm/utility_arm.cc b/compiler/dex/quick/arm/utility_arm.cc
index 2d5e291442..9cbf7b89f2 100644
--- a/compiler/dex/quick/arm/utility_arm.cc
+++ b/compiler/dex/quick/arm/utility_arm.cc
@@ -986,10 +986,7 @@ LIR* ArmMir2Lir::LoadBaseDisp(RegStorage r_base, int displacement, RegStorage r_
   }
 
   if (UNLIKELY(is_volatile == kVolatile)) {
-    // Without context sensitive analysis, we must issue the most conservative barriers.
-    // In this case, either a load or store may follow so we issue both barriers.
-    GenMemBarrier(kLoadLoad);
-    GenMemBarrier(kLoadStore);
+    GenMemBarrier(kLoadAny);
   }
 
   return load;
@@ -1091,8 +1088,8 @@ LIR* ArmMir2Lir::StoreBaseDispBody(RegStorage r_base, int displacement, RegStora
 LIR* ArmMir2Lir::StoreBaseDisp(RegStorage r_base, int displacement, RegStorage r_src,
                                OpSize size, VolatileKind is_volatile) {
   if (UNLIKELY(is_volatile == kVolatile)) {
-    // There might have been a store before this volatile one so insert StoreStore barrier.
-    GenMemBarrier(kStoreStore);
+    // Ensure that prior accesses become visible to other threads first.
+    GenMemBarrier(kAnyStore);
   }
 
   LIR* store;
@@ -1135,8 +1132,9 @@ LIR* ArmMir2Lir::StoreBaseDisp(RegStorage r_base, int displacement, RegStorage r
   }
 
   if (UNLIKELY(is_volatile == kVolatile)) {
-    // A load might follow the volatile store so insert a StoreLoad barrier.
-    GenMemBarrier(kStoreLoad);
+    // Preserve order with respect to any subsequent volatile loads.
+    // We need StoreLoad, but that generally requires the most expensive barrier.
+    GenMemBarrier(kAnyAny);
   }
 
   return store;