proposed membar simplification in c2

[Roland Westrelin]

Hi Vladimir,

Thanks for the comments.

> In general I like this idea since it is platform independent condition.
>
> There is code in macro.cpp which look for MemBarAcquire and MemBarRelease nodes
> to eliminate when eliminating locks and in memnode.cpp for scalar replaced
> object. And there is code in lcm.cpp which checks it also. I would suggest to
> add new membar nodes MemBarAcquireLock and MemBarReleaseLock instead of using
> MemBarCPUOrder.

Is this what you have in mind?
http://cr.openjdk.java.net/~roland/membar/webrev.02/

> Related note: in .ad file we have to add opposite predicate on a second version
> of membar mach node, otherwise it will be always selected by DFA regardless
> predicate value:
>
>   instruct membar_volatile() %{
>     match(MemBarVolatile);
> +  predicate(!Matcher::post_store_load_barrier(n));
>     ins_cost(4*MEMORY_REF_COST);

The costs are not the same for membar_volatile (4*MEMORY_REF_COST) and unnecessary_membar_volatile (0) so that guarantees that unnecessary_membar_volatile is tried first and that when the predicate fails membar_volatile is chosen, right?

Roland.