RFR: 8219101: Stop using ICStubs for CompiledIC transitions to clean

[Erik Österlund]

Hi,

When transitioning CompiledICs to clean, we sometimes need to use 
transitional states (using JIT-compiled ICStubs).
The trick is that today when we clean ICs, we atomically and logically 
write NULL to the data section, and set the destination to some resolve 
stub. In reality, the NULL gets translated to Universe::non_oop_word. 
But the point is that in order to set both data and destination 
atomically, we use an ICStub.

...but we don't need to set them both.

When the inline cache is cleaned, the data part is no longer relevant, 
and could really be set to any nonsense value, as long as subsequent 
transitions away from that cleaned inline cache 
(monomorphic/interpreted) uses a transitional state to do so.
I suspect that at some point in time, when inline caches pointed at oops 
in perm gen or something, it was crucial that the data part of the 
inline cache made sense to the GC. This is not the case today, and we 
can just ignore the value pretending it doesn't exist instead.

The benefit with deferring the use of transitional state to when someone 
performs a subsequent transition away from clean (set_to_monomorphic), 
is that all code paths where cleaning inline caches fail can be removed. 
This in particular simplifies the sweeper and ZGC nmethod unloading 
code, that needs to handle failures in inline cache cleaning due to 
running out of ICStubs forcing us to trigger safepoints to refill IC 
stubs. It should also reduce the overall number of safepoints due to 
running out of ICStubs.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8219101

Webrev:
http://cr.openjdk.java.net/~eosterlund/8219101/webrev.00/

Testing: mach5 hs-tier1-5.

Thanks,
/Erik