Perf: SATB and WB coalescing

[Aleksey Shipilev]

On 01/10/2018 12:35 PM, Roman Kennke wrote:
> Ah!
> I made something like this a while ago and it hasn't gone in back then:
> http://cr.openjdk.java.net/~rkennke/gc-phase-flag/webrev.01/

Okay! That looks like a good start. Now we "only" need to cover all other phases, and fix up the
codegen to make use of "test 0xOFF(TLS), mask". :)

I still think the phases themselves are inconvenient to encode, because they don't say everything
about the heap. For example, you would want to disambiguate the idle phase that has forwarded
objects waiting for CM-with-UR to fix stuff up, and idle phase where everything is fixed up. Maybe
just introducing separate "idle" and "idle-need-fixup" phases would be enough?

Then we can approach compiler checking for "idle" state, and optimize the happy path accordingly.

Thanks,
-Aleksey