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

[Christian Thalinger]

On Wed, 2009-10-28 at 16:13 -0700, Vladimir Kozlov 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.

Using size_t for base (which is an address) feels a little strange but
otherwise looks good.

-- Christian