RFR: 8261268: LOAD_INSTANCE placeholders unneeded for parallelCapable class loaders

[Coleen Phillimore]

On Tue, 9 Feb 2021 02:33:19 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> See CR for more details.  This optimizes some code for class loading.
>> Tested with tier1-6.
>
> src/hotspot/share/classfile/placeholders.cpp line 107:
> 
>> 105: // Doubly-linked list of Threads per action for class/classloader pair
>> 106: // Class circularity support: links in thread before loading superclass
>> 107: // bootstrap loader support:  links in a thread before load_instance_class
> 
> I don't think the loader is what the comment is referring to. The reference to bootstrapsearchpath relates to this comment in SD::resolve_instance_class_or_null:
>     // add placeholder entry to record loading instance class
>     // Five cases:
>     // All cases need to prevent modifying bootclasssearchpath
>     // in parallel with a classload of same classname

I don't see any code that does this though.  The code used to compute_loader_lock_object() and return NULL in JVMTI where it was modifying the bootclass search path.  I wonder if that code should have a wait on the placeholder.  I'll investigate.

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PR: https://git.openjdk.java.net/jdk/pull/2469