Using LoadAcquire & StoreRelease instructions

[Andrew Haley]

Just to recap: when we have LoadAcquire & StoreRelease instructions we
don't also need fences.  We respect MemNode::unordered in the C2
aarch64.ad and generate LoadAcquire & StoreRelease.  I have changed
HotSpot in a few places so that we can disable the separate fences.

However, there are two places in the HotSpot code base where I've had
to conditionalize on AArch64 because a store is marked as a release
where we don't need it to be.

The first is a store to a non-volatile OOP field, which I think you
said was for IA-64, because IA-64 does not have a store fence at the
end of object initialization.  I understand that argument and it makes
sense, but can we make this IA64_ONLY, or is it wanted for other
architectures as well?

--- old/src/share/vm/opto/memnode.hpp	2014-11-21 12:09:22.766963837 -0500
+++ new/src/share/vm/opto/memnode.hpp	2014-11-21 12:09:22.546983320 -0500
@@ -503,6 +503,10 @@
   // Conservatively release stores of object references in order to
   // ensure visibility of object initialization.
   static inline MemOrd release_if_reference(const BasicType t) {
+    // AArch64 doesn't need a release store here because object
+    // initialization contains the necessary barriers.
+    AARCH64_ONLY(return unordered);
+
     const MemOrd mo = (t == T_ARRAY ||
                        t == T_ADDRESS || // Might be the address of an object reference (`boxing').
                        t == T_OBJECT) ? release : unordered;


The second is for release stores into the card table, which I believe
are not needed on any architecture.  (G1 is irrelevant here: it has
its own card table code, with the fences it needs.)  If updating the
card table really does need to be a release store then we must insert
a fence here for every architecture.  However, I don't think we do,
and the release here has a significant performance impact.

--- old/src/share/vm/opto/graphKit.cpp	2014-11-21 12:09:20.017207376 -0500
+++ new/src/share/vm/opto/graphKit.cpp	2014-11-21 12:09:19.787227745 -0500
@@ -3813,7 +3813,8 @@

   // Smash zero into card
   if( !UseConcMarkSweepGC ) {
-    __ store(__ ctrl(), card_adr, zero, bt, adr_type, MemNode::release);
+    __ store(__ ctrl(), card_adr, zero, bt, adr_type,
+             NOT_AARCH64(MemNode::release) AARCH64_ONLY(MemNode::unordered));
   } else {
     // Specialized path for CM store barrier
     __ storeCM(__ ctrl(), card_adr, zero, oop_store, adr_idx, bt, adr_type);


So, while I am perfectly happy to just disable these for AArch64, it
would be better for all concerned to have a resolution of this.

Thanks,
Andrew.