[9] RFR (XS): 8155635: C2: assert(flat != TypePtr::BOTTOM) failed: cannot alias-analyze an untyped ptr

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8155635/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8155635

SplitIf transformation can produce untyped pointers when slitting AddP 
nodes for unsafe accesses through a Phi which merges non-null & null values:
     AddP ... (Phi (ConP #NULL) (CheckCastPP Oop:...:NotNull))

Proposed fix is to enable oop pointer to raw pointer conversion for 
absolute addresses.

I also experimented with blocking SplitIf transformation is such cases, 
but the transformation seems viable and considerably simplifies the 
graph: X-shaped control flow is untangled by eliminating redundant and 
the transformation sharpens types on both branches.

I checked specifically how Phi merges raw & oop pointers after the split 
and it works fine.

Testing: failing test, JPRT, RBT (hs-tier0-comp.js).

Thanks!

Best regards,
Vladimir Ivanov

PS: though AddP (Phi #NULL #NotNull) shape is common, I wasn't able to 
write a simplified test case which triggers SplitIf transformation.