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

[Quan Anh Mai]

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.

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Commit messages:
 - Aggressively fold loads from objects that have not escaped

Changes: https://git.openjdk.org/jdk/pull/28764/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28764&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373495
  Stats: 680 lines in 6 files changed: 680 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28764.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28764/head:pull/28764

PR: https://git.openjdk.org/jdk/pull/28764