RFR: 8233506: ZGC: the load for Reference.get() can be converted to a load for strong refs

[Tobias Hartmann]

Hi Erik,

On 08.11.19 16:15, erik.osterlund at oracle.com wrote:
> On 11/8/19 9:38 AM, Tobias Hartmann wrote:
>> Do we still need that logic with your change?
> 
> Nope! :) Because the access nodes have their barrier data populated before transformation,
> describing the semantics of the produced access, we don't care what GVN gives us after
> transformation. Whatever it gives us has the correct semantics for the corresponding access that
> produced it.

Can we remove that code then? I mean the one you referred to with "We already have code that tries
to determine if the load we got out from the GVN transformation looks like a load that was created
in the BarrierSetC2 factory function".

> Sure, will file another RFE for that.
> Regarding the assert, it's not obvious what it would look like, since the assert in the
> transformation code has to know exactly what nodes are expected to have GC data. For example, a
> LoadP might be used to read any pointer, including but not limited to oops. And some oops don't need
> barriers like the threadOop due to being processed in safepoints. So given a LoadP node for example,
> I don't know if we can determine whether it should or should not have GC data.

Right, that doesn't seem feasible.

Ship it!

Best regards,
Tobias