[aarch64-port-dev ] volatiles and membars in jdk7 aarch64.ad

[Andrew Dinn]

When I first backported the aarch64.ad rules to JDK7 I deleted all the
special case volatile load/store rules, removed the 'treat_as_volatile'
predicates from the normal case load/store rules and relied on the
membar rules to ensure synchronization occurs where needed. That was
because the non-volatile case was isolated through the use of predicates
which called treat_as_volatile() and the latter relies on being able to
call memnode->is_volatile(). JDK7 has no such method.

While modifying JDK7 to add the pipeline model I noticed that the latest
jdk8 aarch64.ad relies on predicates which call  memnode->is_unordered()
to isolate the non-volatile cases. Now, JDK7 /does/ include this method.
It might, therefore, be appropriate to rejig the jdk7 rule set to
re-include the volatile rules (i.e. ruels which use load acquire and
store release) and reinstate a predicate for the normal case load/store
rules based on is_unordered(). However, I am not sure about what this
implies for the membar rules or, perhaps more germane, for the shared
code which generates the required membar operations.

What has been done to jdk8 to avoid generating memory barriers for
volatile operations. Are there any shared code changes I would need to
backport in order to ensure that we don't end up executing unnecessary
memory barriers for cases where we would also end up using a load
acquire or store release?

Is it a good idea to try this?

regards,


Andrew Dinn
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