RFR: 8313672: C2: PhaseCCP does not correctly track analysis dependencies involving shift, convert, and mask

Daniel Lundén duke at openjdk.org
Tue Oct 31 07:19:40 UTC 2023


This changeset fixes an issue where the `PhaseCCP` analysis stopped before fixpoint due to `Value`-updates for `AndL` nodes involving shift, convert, and mask (`MulNode::AndIL_shift_and_mask_is_always_zero`). Such updates did not previously repopulate the `PhaseCCP` worklist correctly.

Changes:
- Fix the issue by adding the missing pattern to `PhaseCCP::push_and`.
- Update the `assert` error message that reported the issue to better describe the problem. The `PhaseCCP` analysis is optimistic, and the previous error message seemed to indicate the opposite ("Missed optimization opportunity"). If the assert triggers, it could be that the analysis result leads to unsound transformations later on.

Result after `PhaseCCP` analysis (without fix, note `long:0` for node 116):
![before](https://github.com/openjdk/jdk/assets/4222397/3f2343da-8573-464d-9352-a147d0faeab8)

Result after `PhaseCCP` analysis (with fix, note `long` for node 116):
![after](https://github.com/openjdk/jdk/assets/4222397/6bf44964-cff8-49db-9825-4a4667f4e769)

### Testing (on all Oracle-supported platforms)
- `tier1`
- HotSpot parts of `tier2` and `tier3`

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

Commit messages:
 - Change to more accurate assert message
 - Fix issue

Changes: https://git.openjdk.org/jdk/pull/16429/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16429&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313672
  Stats: 10 lines in 1 file changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16429.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16429/head:pull/16429

PR: https://git.openjdk.org/jdk/pull/16429


More information about the hotspot-compiler-dev mailing list