[jdk17u-dev] RFR: 8296545: C2 Blackholes should allow load optimizations

Wed Apr 19 13:43:01 UTC 2023

Unclean backport to improve Blackhole performance and match it to released JDK 20.
The backport is unclean because there are two little conflicts:
  1. The context conflict in `MemBarNode::make`, where the `MemBarStoreStore` was moved to another place. I resolved the conflict by hand: accepted only the `Blackhole` case removal.
  2. IR Framework needs `MUL_I` constant for test to work.

Additional testing:
  - [x] macos-aarch64-server-fastdebug, `compiler/irTests/blackhole` (includes new test)
  - [ ] Eyeballing perfasm outputs with/without this patch

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Commit messages:
 - Backport eab0ada3a16a432fdfd1f0b8fceca149c725451b

Changes: https://git.openjdk.org/jdk17u-dev/pull/1273/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1273&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296545
  Stats: 211 lines in 8 files changed: 168 ins; 42 del; 1 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1273.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1273/head:pull/1273

PR: https://git.openjdk.org/jdk17u-dev/pull/1273