RFR(S): 8224580: Matcher can cause oop field/array element to be reloaded

[Roman Kennke]

Hi Roland,

The change looks reasonable to me.

I have run tests with the patch and can confirm that the original bug
went away. I've also run a bunch of other tests and workloads and looks
good too.

Roman


> http://cr.openjdk.java.net/~roland/8224580/webrev.00/
> 
> With Shenandoah, we have a prototype where, rather than using an extra
> header field for the forwarding pointer, we use the mark word. When the
> shenandoah barrier is applied to an oop, the oop is first checked to be
> in cset which is done by converting the oop to an integer and extracting
> some of the bits of the address and then, if that test fails, the
> forwarding pointer is loaded by masking off some bits in the mark
> word. We're seeing a case where, in compiled code, the in cset test and
> the mark word load for the same oop use 2 different loads, i.e. the load
> of the field/array element is duplicated, the oop is reloaded between
> the in cset test and loading of the forwarding pointer. This only with
> compressed oops.
> 
> I traced it down to the matcher, where the LoadN node for the
> field/array element is not marked as shared and so is emitted
> twice. This happens because the LoadN result is only used through a
> decodeN which has multiple uses. I see the matcher has code to deal with
> that situation:
> 
> if( mop == Op_AddP && m->in(AddPNode::Base)->is_DecodeNarrowPtr()) {
>   // Bases used in addresses must be shared but since
>   // they are shared through a DecodeN they may appear
>   // to have a single use so force sharing here.
>   set_shared(m->in(AddPNode::Base)->in(1));
> }
> 
> But that logic doesn't trigger in our case because the mark word load is
> at offset 0 so there's not AddP. The fix I propose is to always mark the
> input of a DecodeN as shared if the DecodeN is shared.
> 
> Roland.
> 

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