RFR: 8262046: Clean up parallel class loading code and comments [v2]

[Ioi Lam]

On Wed, 31 Mar 2021 16:32:35 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> src/hotspot/share/classfile/systemDictionary.cpp line 383:
>> 
>>> 381: // the super class directly, then resume steps 4-6.
>>> 382: //
>>> 383: //
>> 
>> resolve_super_or_fail had this comment that I found difficult to understand and work through, which I did at one point, so I wanted to replace it with one that seemed less so.  It occurs to me that this is probably unhelpful also.   Maybe the whole comment from lines 351 to 381 should be replaced with a sentence like:
>> 
>> //resolve_super_or_fail adds a LOAD_SUPER placeholder to the placeholder table before calling
>> // resolve_instance_class_or_null.  ClassCircularityError is detected when a LOAD_SUPER or LOAD_INSTANCE
>> // placeholder for the same thread, class, classloader is found.
>> 
>> I'm unlikely to work through the new comment example again myself.
>
> Since nobody wants to read this example in this comment and neither do I, I'm removing it.  Debugging this using -Xlog:class+load+placeholders=debug will show the stacking of placeholders that detect CCE.

I guess it's OK to remove this comment, since most of us who have looked at this code in the past few years haven't found this comment to be helpful. Maybe you can add a comment about `-Xlog:class+load+placeholders=debug` instead?

If we do want to improve the comment, maybe we can replacing Base->Super with something like Car->Vehicle. That way it's clear which is the supertype.

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PR: https://git.openjdk.java.net/jdk/pull/3200