[14] RFR (S): 8226411: C2: Avoid memory barriers around off-heap unsafe accesses

[Roman Kennke]

Hi Vladimir,

This is cool.

Does it affect this:
https://bugs.openjdk.java.net/browse/JDK-8220714

Thanks,
Roman

> http://cr.openjdk.java.net/~vlivanov/8226411/webrev.00/
> https://bugs.openjdk.java.net/browse/JDK-8226411
> 
> There were a number of fixes in C2 support for unsafe accesses recently
> which led to additional memory barriers around them. It improved
> stability, but in some cases it was redundant. One of important use
> cases which regressed is off-heap accesses [1]. The barriers around them
> are redundant because they are serialized on raw memory and don't
> intersect with any on-heap accesses.
> 
> Proposed fix skips memory barriers around unsafe accesses which are
> provably off-heap (base == NULL).
> 
> It (almost completely) recovers performance on the microbenchmark
> provided in JDK-8224182 [1].
> 
> Testing: tier1-6.
> 
> Best regards,
> Vladimir Ivanov
> 
> [1] https://bugs.openjdk.java.net/browse/JDK-8224182
>