RFR: 8252505: C1/C2 compiler support for blackholes [v11]

[Aleksey Shipilev]

On Tue, 1 Dec 2020 12:23:44 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Cleanup BlackholeCallGenerator
>
> src/hotspot/share/c1/c1_GraphBuilder.cpp line 3469:
> 
>> 3467:   Values* args = state()->pop_arguments(callee->arg_size());
>> 3468: 
>> 3469:   // Blackhole everything except the receiver itself
> 
> Does it make sense to limit blackhole candidates to static methods only? 
> Otherwise, you end up with a null check in the generated code.

Unfortunately, not. In JMH, `Blackhole` methods are instance methods (they have state), and that plays nicely with compatibility: when JIT fails to provide blackholing, it would naturally fall back to old behavior. I think null check for receiver is the fair behavior here, as subsequent optimization may expect receiver null-checked after the call. I have not seen null-checks in my `perfasm` runs with C1, though.

-------------

PR: https://git.openjdk.java.net/jdk/pull/1203