[lworld] RFR: 8367623: [lworld] C2: Ideal Optimization on InlineTypeNode should not be carried out after Macro Expansion

[Quan Anh Mai]

On Mon, 17 Nov 2025 16:14:27 GMT, Benoît Maillard <bmaillard at openjdk.org> wrote:

> This PR tightens the conditions under which an optimization from `InlineTypeNode::Ideal` should be carried out. This optimization was initially reported as missed with `-XX:VerifyIterativeGVN`.
> 
> ### Summary
> 
> The original failure appeared with `TestFieldNullMarkers.java` and `-XX:VerifyIterativeGVN=1110`. This test performs various allocations with value classes and other Valhalla features.
> 
> In `InlineTypeNode::Ideal`, we have the following optimization:
> 
> https://github.com/openjdk/valhalla/blob/b1d14c658511cced2a860d9e2741ae89827e25ba/src/hotspot/share/opto/inlinetypenode.cpp#L835-L853
> 
> As explained in the optimization comments, we don't want to use the base oop if it corresponds to the larval oop. This is enforced by the condition `AllocateNode::Ideal_allocation(base) == nullptr`.
> 
> In our case this is exactly what happens, and the optimization is prevented. HOwever, this changes during macro expansion. The allocate node disappears, and `AllocateNode::Ideal_allocation(base) == nullptr` becomes true. As this is not intended, there is no notification mechanism for this case and we end up with the missing optimization assert.
> 
> The conditions for this bug to reproduce are somewhat subtle. When things go well, the larval `Allocate` node is eliminated by `eliminate_allocate_node` before this problematic case shows up. However, there are cases where the `<init>` method is not inlined, and this prevents the removal of the `Allocate` node. It stays until macro expansion, and that is where things go wrong.  
> 
> In the extracted reproducer, the `<init>` method is not inlined because of unloaded signature classes, as `CompileCommand=printinlining` shows:
> 
> 
> CompileCommand: PrintInlining *.* bool PrintInlining = true
>                             @ 5   compiler.valhalla.inlinetypes.TestMissingOptUseBaseOop$MyValue::<init> (10 bytes)   failed to inline: unloaded signature classes
> 
> 
> This could also happen for other reasons though.
> 
> ### Solution
> 
> The solution is to not do this optimization after macro expansion. Adding a `phase->C->allow_macro_nodes()` check ensures that the `AllocateNode::Ideal_allocation(base)` call is relevant in the current phase and that we can use it to check if we are dealing with the larval oop.
> 
> ### Testing
> - [x] [GitHub Actions](https://github.com/benoitmaillard/valhalla/actions?query=branch%3AJDK-8367623)
> - [ ] tier1-4, plus some internal testing

Maybe we should assert that `AllocateNode::Ideal_allocation` can only be called when `C->allow_macro_nodes()` so that the result is meaningful.

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Marked as reviewed by qamai (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/1742#pullrequestreview-3482551688