RFR: 8373495: C2: Aggressively fold loads from objects that have not escaped

[Quan Anh Mai]

On Sat, 13 Dec 2025 03:51:32 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> Hi,
>> 
>> This patch is an alternative to #28764 but it does the analysis during IGVN instead.
>> 
>> Please take a look and leave your thoughts, thanks a lot.
>
> Very nice! I definitely prefer the approach here to #28764.
> 
> I see that the unit test stays the same and there's an adjustment in some other test, so I assume this version is functionally more powerful than #28764 version.
> 
> Have you had a chance to measure how much it affects compilation speed compared to #28764?
> 
> (The code is dense and hard to reason about, so some polishing/refactoring to make it more readable. Also, please, think about verification checks.)

@iwanowww Thanks for your comment. I have added a lot more comments to explain in detail the steps of `MemNode::find_previous_store`. I have also made a small modification: instead of traversing the outputs of the control nodes from the call to the allocation, we traverse the outputs of the nodes that may alias `base` instead. This has some benefits:

- It is likely cheaper. This is because there are often few nodes that may alias `base`, while there may be numerous control nodes from the call to the allocation. The number of nodes that directly use a pointer is also less than the number of nodes that directly use a random control node.
- It is more conservative. This is because we can limit the type of the outputs of a pointer and be conservative with everything else, while exhaustively checking if a random use of a random control node makes `base` escape seems hard.

I have also added some verification that if a step determines that `base` does not escape, then the following steps must not determine otherwise.

For the runtime cost, I don't see a noticeable difference compared to master.

For the unit test, compared to the previous PR, I have removed the `failOn = LoadI` from the tests that involve loops. But I think improving load folding on `Phi` can be another PR. For the change in `TestZGCEffectiveBarrierElision`, it is because I decided to add `Blackhole` to the list of nodes that do not escape an object, not sure if it is necessary, though. However, I managed to change the test so the load is not elided.

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PR Comment: https://git.openjdk.org/jdk/pull/28812#issuecomment-3649201130