[aarch64-port-dev ] RFR: 8080293: AARCH64: Remove unnecessary dmbs from generated CAS code

[Andrew Dinn]

On 25/08/15 18:14, Vladimir Kozlov wrote:
> Okay, I agree to have only one predicate. So I am fine with version A).

Thanks, Vladimir. So, that is now as provided in the latest posted webrev:

  http://cr.openjdk.java.net/~adinn/8080293/webrev.01/

> PS: "first rule will have a lower" - should compareAndSwapI be first then?

Sorry, I think the problem here is that I explained the status of the
original patch in a rather confusing way. I am not sure it matters all
that much which rule appears first. Or do you really want the lower cost
rule to appear before the higher cost one? . . .

>> However, looking again at the code I believe I have the costs (and hence
>> the predicates) attached to the wrong rules in each pair. For example,
>> currently the rules include the following details
>>
>>    compareAndSwapIAcq -- does not emit dmb instructions
>>      no predicate
>>      cost  (2 * VOLATILE_REF_COST )
>>
>>    compareAndSwapI -- emits dmb instructions
>>      predicate(!needs_acquiring_load_exclusive(n))
>>      cost VOLATILE_REF_COST


. . . what I meant by that comment was that this:

  - The optimization implemented in this patch is based on an assumption
that a generation strategy using dmb -- i.e. the one encoded by
compareAndSwapI -- will execute more slowly, or at least no faster, than
a generation strategy using stlr -- i.e. the one encoded by
compareAndSwapIAcq.

  - The text above displays the original costs and predicates used to
enforce the required rule selection.

  - In that version the /costs/ are the wrong way round with respect to
the /motivating assumption/ i.e. compareAndSwapI has a lower cost than
compareAndSwapIAcq.

In version A the costs reflect the motivating assumption  i.e. for each
X in {I, L, P, N} rule compareAndSwapXAcq has a lower cost than
compareAndSwapX.

However, it is also true that for each X in {I, L, P, N} rule
compareAndSwapX appears earlier than compareAndSwapXAcq.

regards,


Andrew Dinn
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