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

[Benoît Maillard]

On Wed, 19 Nov 2025 12:54:57 GMT, Quan Anh Mai <qamai 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)
>> - [x] 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.

Thank you for your review @merykitty. Regarding your comment, I filed [JDK-8373397](https://bugs.openjdk.org/browse/JDK-8373397).

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PR Comment: https://git.openjdk.org/valhalla/pull/1742#issuecomment-3636053503