RFR(S): 8028471: PPC64 (part 215): opto: Extend ImplicitNullCheck optimization.

[Lindenmaier, Goetz]

Hi Vladimir, 

thanks for the quick review!

I updated the webrev: 
http://cr.openjdk.java.net/~goetz/webrevs/8028471-0-zpage/

I made an additional method with the name you proposed.
I prefer two methods, as one tests whether the operation
traps, the other checks how the narrow oop was decoded --
two different things.  Should be optimized by the C-compiler
anyways. 

No ShouldNotReachHere for AIX.  I tried to improve the comment
instead.

Sorry for the wrong indents.

Best regards,
  Goetz.

-----Original Message-----
From: Vladimir Kozlov [mailto:vladimir.kozlov at oracle.com] 
Sent: Wednesday, November 20, 2013 9:04 PM
To: Lindenmaier, Goetz; 'hotspot-dev at openjdk.java.net'; 'ppc-aix-port-dev at openjdk.java.net'
Subject: Re: RFR(S): 8028471: PPC64 (part 215): opto: Extend ImplicitNullCheck optimization.

Hi, Goetz

Can you move all checks in lcm.cpp into new method and rename it to 
needs_explicit_null_check_for_read()? So the condition will be simple:

if (!was_store && needs_explicit_null_check_for_read(val)) {
   continue;
}

You should also add ShouldNotReachHere() for AIX near Unimplemented() 
for non AIX.

And, please, fix indents in new method.

Thanks,
Vladimir

On 11/20/13 2:11 AM, Lindenmaier, Goetz wrote:
> Hi,
>
> ImplicitNullChecks did not work on platforms where the zero
> page is only write protected.
>
> This change adds os property 'zero_page_read_protected' and extends
> the ImplicitNullCheck optimization to consider only stores if
> this property is not true. If a decoded compressed oop will access the
> guard page before the heap, Loads work again.
> This is needed on AIX, where the page at address '0' is only write-protected.
>
> Please review and test this change:
> http://cr.openjdk.java.net/~goetz/webrevs/8028471-0-zpage/
>
> I tagged this S, as a majority of the patches apply to the ppc platform files.
>
> Best regards,
>    Goetz.
>