RFR(M): 8212243: More gc interface tweaks for arraycopy and some other smaller changes in preparation for Shenandoah

[Roman Kennke]

>> So basically, you could make a new C2PostParseTimeAccess (name?!) that 
>> you wrap this information in, and send it through the 
>> BarrierSetC2::load_at backend.
>> Similar how there is a GraphKit::access_load_at helper for baking the 
>> context object, you can have a helper (somewhere at your convenience) 
>> for baking the C2PostParseTimeAccess.
> 
> So you're suggesting an implementation of load and store that would work
> after parsing. We don't know how to do that in general. Arraycopy
> happens to be a special case where can emit the right barriers and
> that's why it has a special entry point.
> 
> An object field load for us is:
> 
> a' = rb(a);
> v = a'.f
> 
> an object field store is:
> 
> b' = wb(b);
> pre_barrier();
> b'.f = wb(v);
> 
> We have no way today of emitting the pre_barrier after parsing. In the
> case of this patch, we don't need the pre_barrier because the copy is to
> a newly allocated object.
> 
> The load_store barrier for arraycopy is wb(v) only because the rb(a) and
> wb(b) are emitted at parse time right before the arraycopy.
> 
> So what you're suggesting would be to move that stuff under a generic
> api which would be quite a bit more work and would actually not be
> generic because we have no way of implementing the generic api today.

Yeah, this is a very good point. arraycopy replacement by load/store
series is not the same as emitting 'standalone' loads or stores: we know
we don't need RB and WB on the source and target arrays (we already have
that), and we know we don't need certain other things (e.g. SATB
pre-barrier) because of dst being new (in case of clone). Seems
tricky/impossible to do this generically. I'd rather have a special API
for this particular arraycopy optimization.

Roman

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