On Wed, 12 Jan 2022 15:48:20 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org> wrote:
> When looking at larger benchmarks, I noted a discrepancy between the > performance of off-heap segments and on-heap segments. Looking at the > assembly for the `MemorySegment::asSlice` method I could see some additional > barriers in the off-heap case, but could not initially make sense of them. > Vlad pointed me at G1 (in fact no such barrier was emitted when using a > different GC, such as the serial GC, or ZGC), and later Erik narrowed the > problem down to a failure in a C2 optimization to remove barriers around > initializing stores. This problem was caused by a synthetic cast added by > javac to a value (the base object) that initialized the newly created memory > segment slice. Because of that case, C2 missed the store as an "initializing" > one, and inserted additional barriers. This patch should make performance of > on-heap segments a lot more reliable, especially when slicing is involved. This pull request has now been integrated. Changeset: c6b02755 Author: Maurizio Cimadamore <mcimadam...@openjdk.org> URL: https://git.openjdk.java.net/jdk18/commit/c6b027559c6e055b1475ada4001ef483b1a12d24 Stats: 292 lines in 3 files changed: 263 ins; 0 del; 29 mod 8279930: Synthetic cast causes generation of store barriers when using heap segments Reviewed-by: psandoz ------------- PR: https://git.openjdk.java.net/jdk18/pull/97
