RFR: 8373495: C2: Aggressively fold loads from objects that have not escaped [v5]

[Vladimir Ivanov]

On Fri, 12 Dec 2025 02:30:22 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> The current escape analysis mechanism is all-or-nothing: either the object does not escape, or it does. If the object escapes, we lose the ability to analyse the values of its fields completely, even if the object only escapes at return.
>> 
>> This PR tries to find the escape status of an object at a load, and if it is decided that the object has not escaped there, we can try folding the load aggressively, ignoring calls and memory barriers to find a corresponding store that the load observes.
>> 
>> For the runtime cost, this phase runs very fast, around 5 - 7% the runtime of EA, and about 0.5% the total runtime of C2.
>> 
>> Please take a look and leave your thoughts, thanks a lot.
>
> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   more detailed explanations

Interesting idea, Quan!

Why can't the same be done as part of `MemNode::can_see_stored_value()`? Based on your reasoning in the comments, if the base escapes, the walk over memory graph happening there should encounter it as well. (But you need to ensure it climbs up to the Allocation node to be sure.)

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

PR Comment: https://git.openjdk.org/jdk/pull/28764#issuecomment-3644664178