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

[Emanuel Peter]

On Tue, 9 Jul 2024 00:06:33 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

> 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...

test/hotspot/jtreg/compiler/c2/TestReduceAllocationAndNestedScalarized.java line 41:

> 39:  *                   -Xcomp
> 40:  *                   -server
> 41:  *                   compiler.c2.TestReduceAllocationAndNestedScalarized

Drive by comment: are all these `requires` absolutely necessary? Could there be a run without them? Is the test expected to fail with graal or C1? Or any external flags?

Could we have a run without some of the flags you list here? Just so that we have wider coverage, especially in combination with other flags that can be passed from the outside.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20087#discussion_r1671963361