RFE: 8023597: Optimize G1 barriers code for unsafe load_store

[Doerr, Martin]

Hi,

I did some more tests with our proposed webrev
http://cr.openjdk.java.net/~goetz/webrevs/opto_g1_barr/
and it happened that the assertion
"assert(pre_val->bottom_type()->basic_type() == T_OBJECT, "or we shouldn't be here");"

was firing when running jvm2008 derby on linuxx86_64.



What happened is that the C2 was inserting a barrier of kind=LibraryCallKit::LS_cmpxchg,

but oldval was a ConP node with bottom_type()->basic_type() = T_ADDRESS.



I believe there's nothing logically wrong with the change.

Should the assertion be modified or should we filter out cases in which oldval

does not contain a valid Oop?



I guess some advice from C2 and G1 experts is needed.



Best regards,

Martin


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