RFR: 8209189: Make CompiledMethod::do_unloading more concurrent

[Erik Österlund]

Hi,

Today the nmethods are unloaded either serially or in parallel due to GC 
(and e.g. class unloading). With ZGC concurrent class unloading, a 
concurrent fashion is required. Rather than inventing yet a third way of 
unloading nmethods due to class unloading, it would be ideal if there 
could be one unified way of doing it that makes everyone happy.

To solve this problem in a more general way, a new member function on 
CompiledMethod is added: is_unloading(). It returns whether a 
CompiledMethod has broken oops. In order to not have to iterate over all 
the oops every time this question is asked, it caches the result, so it 
needs to be computed only once per "epoch". Every time a CodeCache 
unloading is triggered by the GC, a new epoch is started, which forces 
re-computation of whether the CompiledMethod is_unloading() the first 
time it is called.

So by having is_unloading() be lazily evaluated, it is now possible to 
build a do_unloading() method on nmethod that simply makes the nmethod 
unloaded if it is_unloading(), and otherwise cleans the various caches. 
Cleaning e.g. the inline caches of said nmethod, uses the same 
is_unloading() method on its targets to figure out if the IC cache 
should be cleaned or not. Again, the epoched caching mechanism makes 
sure we don't recompute this value.

The new do_unloading() method may be called in both serial, parallel and 
concurrent contexts. So I removed the parallel variation of this that we 
had before, that unnecessarily postponed the unloading due to not having 
computed whether the nmethod should be unloaded or not. Since that is 
now computed on-demand lazily, there is no longer a need for postponing 
this, nor to have two phases for parallel nmethod unloading.

While there is more work involved in making concurrent nmethod unloading 
work, this is a good cleanup towards that goal.

Bug ID:
https://bugs.openjdk.java.net/browse/JDK-8209189

Webrev:
cr.openjdk.java.net/~eosterlund/8209189/webrev.00/

Thanks,
/Erik