proposed membar simplification in c2

[Tom Rodriguez]

On Jul 22, 2011, at 3:10 AM, Roland Westrelin wrote:

> 
> 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/

That looks good.  I think keeping the release/acquire distinction was a good idea for later optimization.

tom

> 
>> 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.