Request for reviews (S): 6892186: 6896084: VM does not reserve protected page below heap for compressed oops implicit null checks

[John Coomes]

Vladimir Kozlov (Vladimir.Kozlov at Sun.COM) wrote:
> ...
> http://cr.openjdk.java.net/~kvn/6896084/webrev.00
> 
> Fixed 6896084: VM does not reserve protected page below heap for compressed oops implicit null checks
> 
> Problem:
> VM should reserve protected page below heap for compressed oops
> implicit null checks in compiled code (see 6716785).
> After zero based compressed oops changes (6791178) the page is
> not reserved because undefined narrow_oop_base (NULL by default)
> is used in ReservedHeapSpace() to determine if the page is needed.
> 
> Solution:
> Set narrow_oop_base and narrow_oop_use_implicit_null_checks
> values according to compressed oops encoding mode in
> Universe::preferred_heap_base() which is called before
> ReservedHeapSpace() constructors.

Your changes look good to me.  Some comment typos/nits:

788       // Set not NULL value to indicate the need of narrow_oop_base.

The comment didn't help me understand why it had to be non-null.
Maybe "Set to a non-NULL value so the ReservedSpace ctor computes the
correct no-access prefix."

Not your change, but the conditional expression in ReservedHeapSpace
ctor that uses this should be made into a static function, e.g.,
no_access_prefix_size(...).  I'll add it to my cleanups list.

794         // addressing mode, when large pages are specified on windows.

FWIW, the comma isn't necessary.

-John