Please review fix for 6716785: implicit null checks not triggering with CompressedOops

[Coleen Phillimore - Sun Microsystems]

Vladimir Kozlov wrote:
> Coleen,
>
> Why you added os::is_implicit_NPE() method?
>
> All MacroAssembler::needs_explicit_null_check() methods
> have  the code for compressed oops implicit NPE.
Oh, now I see that.  You must have added it.  It's not in sparc versions 
though (and I can't figure out why we have so many variants of the same 
function!).
>
> Also you removed _heap_base setting in universe.cpp.
> Where it is set now?
It's set in protect_noaccess_prefix in virtualspace.cpp

Coleen
>
> Thanks,
> Vladimir
>
> Coleen Phillimore - Sun Microsystems wrote:
>>
>> 6716785: implicit null checks not triggering with CompressedOops
>> Summary: allocate alignment-sized page(s) below java heap so that 
>> memory accesses at heap_base+1page give signal and cause an implicit 
>> null check
>> Reviewed-by:
>>
>> Webrev:
>> http://webrev.invokedynamic.info/coleenp/6716785/
>>
>> Note in this webrev, the change in arguments.cpp to enable 
>> UseCompressedOops by default will be reverted for checking into HS13 
>> (it's still off by default) for 6u6p.  Should I enable it in our HS14 
>> source base?
>>
>> Thanks!
>> Coleen