RFR(XS): ZGC: C2: Missing null check in LoadBarrier::bottom_type

[Nils Eliasson]

Thank you, Erik

// Nils


On 2018-05-29 17:20, Erik Österlund wrote:
> Looks good.
>
> /Erik
>
> On 2018-05-29 16:30, Nils Eliasson wrote:
>> Hi,
>>
>> Please review,
>>
>> Stefan reported a crash when using -XX:+TraceIterativeGVN. When 
>> printing the types of the nodes, a loadbarrier is encountered that no 
>> longer has a preceding load (in(oop) is null), and we crash when we 
>> try to call bottom_type on that.
>>
>> We don't see this in normal runs because all useless barriers are 
>> eliminated before checking types. That is not the case when debug 
>> printing.
>>
>> Fix returns top (no possible value) if in_oop is null.
>>
>> http://cr.openjdk.java.net/~neliasso/zgc/fix_lb_bottom_type/webrev/
>>
>> Regards,
>>
>> Nils
>>
>