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

[Erik Österlund]

Hi Per,

Thanks for the partial review!

/Erik

On 2019-02-18 15:40, Per Liden wrote:
> On 02/18/2019 01:01 PM, Erik Österlund wrote:
>> 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/
>
> Not a complete review, but the ZGC change looks awesome ;)
>
> /Per
>
>>
>> Testing: mach5 hs-tier1-5.
>>
>> Thanks,
>> /Erik