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

[Aleksey Shipilev]

On 12/3/20 12:50 AM, Vladimir Ivanov wrote:
>> We need to draw the line in the sand somewhere. I can try to do what
>> MemBarCPUOrder does, but honestly it feels like a conceptual step back.
> 
> That's the point we disagree on from the very beginning.

Here is where I was confused. The discussion never mentioned the word "disagree" up until now.

You asked for clarifications why call-based approach was used, thanked me for explanations, and then 
went on suggesting other directions for experimentation. It did not read as "I disagree with current 
approach, here is the approach I would like to see tried", but rather as "Looks okay, and here is 
something we could also try". There is a world of difference between these two statements.

> I think we have much better understanding now what is essential for
> Blackhole and it can be implemented with something simple yet sufficient
> to do the job.

Right! New iteration hopefully does what you suggest. And it is indeed much simpler than call-based 
approach, thank you.

> The nodes ignored during matching aren't removed, but stays intact in
> the graph surrounded by newly created mach nodes. (Otherwise,
> MemBarCPUOrder wouldn't have been able to succeed at its job of ordering
> the memory graph.)

OK, that is good to know. I thought only Mach nodes are left after the matching. New commits have 
the Blackhole formatter.

-- 
Thanks,
-Aleksey