RFR: 8331194: NPE in ArrayCreationTree.java with -XX:-UseCompressedOops [v2]

[Cesar Soares Lucas]

> Please, review this PR to fix issue in debug information serialization encountered when RAM reduces a Phi for which one of the inputs is an object already scalar replaced.
> 
> **Details:**
> 
> Consider class `Picture` that has two reference fields, `first` and `second` of type `Point`. In a random method in the application an object `obj` of this class is created, and the fields of this object are initialized such that `first` is assigned a new object whereas `second` receives the output of a `Phi` node merging the object assigned to `first` and some other allocation. Also, assumes `obj` is used as debug information in an `uncommon_trap` and none of these objects escapes. I.e., we have a scenario like this:
> 
> 
> Picture obj       = new Picture(); // allocation A
>         obj.first = new Point(); // allocation B
> 
> Point    p2    = obj.first;
> if (<cond>) p2 = new Point(); // allocation C
> 
> obj.second = p2;
> 
> <trap>
> 
> 
> After one iteration of EA+SR, Allocation `A` will be scalar replaced and debug information in `<Trap>` adjusted accordingly. The description of field `second` in the debug information on `<Trap>` will, however, still involve a `Phi` node between allocation `B` and `C`. In the next iteration of EA+SR the `Phi` node for field `second` will be reduced by RAM and debug information will be adjusted accordingly. So far nothing is wrong.
> 
> The issue happens because the existing code in `Process_OopMap_Node` to serialize debug information was missing a check. Simply, the check for setting `is_root` of an `ObjectValue` wasn't checking that the `ObjectValue` might be a description of a field of a scalar replaced object. It was only checking whether `ObjectValue` was a local, stack or monitor. Consequently, the allocation assigned to `obj.first` (yes, `first`) was **not** being marked as _root_.
> 
> But the issue only manifested if the `<trap>` was exercised **AND** the result of `<cond>` was _true_. If the result of `<cond>` was `false` when the trap was exercised, then no problem would happen. The reason is, when `<cond>` is _true_ the `select` method in [ObjectMergeValue](https://github.com/openjdk/jdk/blob/1472124489c841642996ae984e21c533ffec8091/src/hotspot/share/code/debugInfo.cpp#L237) would flag, correctly, that the allocation inside the `if` needs to be rematerialized and the other input of the ObjectMergeValue shouldn't be rematerialized because `_is_root == false`, meaning it's just a candidate for rematerialization.
> 
> Fixing the check for setting `_is_root` solv...

Cesar Soares Lucas has updated the pull request incrementally with two additional commits since the last revision:

 - Update src/hotspot/share/opto/output.cpp
   
   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>
 - Update src/hotspot/share/opto/machnode.hpp
   
   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20087/files
  - new: https://git.openjdk.org/jdk/pull/20087/files/38f047a6..5e441707

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20087&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20087&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20087.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20087/head:pull/20087

PR: https://git.openjdk.org/jdk/pull/20087