RFR: 8259713: Fix comments about ResetNoHandleMark in deoptimization

[Coleen Phillimore]

On Thu, 14 Jan 2021 07:09:58 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Unfortunately this is not entirely what I want to say.
>> I want to say that you *must* have a HandleMark if you have ResetNoHandleMark in a JRT_LEAF.
>> You need a ResetNoHandleMark to create Handles but there are no asserts to make sure you have a HandleMark.
>> ie:
>> 
>>      ResetNoHandleMark rnhm;
>>      Handle h(THREAD, obj);  // OK no problem
>> 
>> But the Handle will leak once this function returns (until the next JRT_ENTRY anyway).
>> 
>>     // This code wants to allocate handles which is okay in a leaf method,
>>     // but JRT_LEAF contains a NoHandleMark so we have to work around
>>     // that with ResetNoHandleMark in order to create a Handles here.
>>     // There must also be a HandleMark here to clean up any handles created in this scope.
>> 
>> How about that?
>
> My suggestion:
> // JRT_LEAF methods don't normally allocate handles and there is a
> // NoHandleMark to enforce that. It is actually safe to use Handles
> // in a JRT_LEAF method, and sometimes desirable, but to do so we
> // must use ResetNoHandleMark to bypass the NoHandleMark, and
> // then use a HandleMark to ensure any Handles we do create are
> // cleaned up in this scope.
> I still question why such a function declares itself a LEAF instead of just being a JRT_ENTRY but that is a different argument. :)
> Cheers,
> David

Ok, your comment says the same thing as mine and includes what I want to say, so I'll use your comment.
I'm pretty sure this function wants a NoSafepointVerifier but some of the calls it uses might need handles, because we were pretty aggressive about making arguments into handles.

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PR: https://git.openjdk.java.net/jdk/pull/2069