RFR: 8265322: C2: Simplify control inputs for BarrierSetC2::obj_allocate

[Vladimir Kozlov]

On Fri, 16 Apr 2021 03:06:19 GMT, Yi Yang <yyang at openjdk.org> wrote:

> While just using BarrierSetC2::obj_allocate in https://github.com/openjdk/valhalla/pull/385, I noticed the control input `ctrl` for obj_allocate is not so much necessary. This PR simplifies control inputs for BarrierSetC2::obj_allocate. In most cases, it doesn't change anything since `toobig_false` is equivalent to `ctrl`. In other cases, `toobig_false` is created for Unsafe.allocateInstance while instance size is not statically known, `ctrl` would become control input of IfNode whose projects are `toobig_false` and `toobig_true`, old eden_end and old_eden_top can simply accept `toobig_false` as their control input rather than `ctrl`.

> > From compiler code POV the fix is reasonable and correct.
> Do you think it's reasonable to move PhaseMacroExpand::set_eden_pointers to BarrierSetC2? It seems that that's GC knowledge area about how to set eden_top/eden_end w or w/o turning UseTLAB.

It is up to GC group but it is common code for all GCs. As I understand BarrierSetC2 is used for GC variations code.

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PR: https://git.openjdk.java.net/jdk/pull/3529