RFR: 8373495: C2: Aggressively fold loads from objects that have not escaped [v4]

[Vladimir Kozlov]

On Thu, 11 Dec 2025 13:38:24 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:
> 
>   store values need normalizing

src/hotspot/share/opto/phaseloadfolding.cpp line 133:

> 131: //       int x = o.value;
> 132: //   In this case, even if the load x = o.value is declared after the store of o to p that allows o
> 133: //   to escape, it is valid for the load to actually happen before the store. As a result, we can

I don't think it is correct. If `p` is external other thread can modify its fields concurrently.
Or are you saying that if `p` is external we will always have memory barrier?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28764#discussion_r2611684108