RFR: 8345067: C2: enable implicit null checks for ZGC reads

Tue May 13 08:55:55 UTC 2025

On Tue, 13 May 2025 06:08:38 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> I think I need to have a look at the GC barrier code myself, to see which things are constant and which things can be mutated (possibly by another thread). What code / documentation do you recommend I look at?

Regarding code, I recommend you starting [here](https://github.com/openjdk/jdk/blob/522c7b446fef17a8400bc589c55b161e939770cc/src/hotspot/cpu/x86/gc/z/z_x86_64.ad#L126-L129) and following `z_load_barrier`. The slow barrier path is generated in a stub [here](https://github.com/openjdk/jdk/blob/522c7b446fef17a8400bc589c55b161e939770cc/src/hotspot/cpu/x86/gc/z/zBarrierSetAssembler_x86.cpp#L1217-L1235).

Regarding documentation, you might have a look at the [TOPLAS paper](https://dl.acm.org/doi/full/10.1145/3538532) (which is unfortunately a bit outdated because it only covers non-generational ZGC, but might still offer some intuition that is valid for the latest ZGC version, in particular regarding concurrent relocation and load barriers), the [Generational ZGC JEP](https://openjdk.org/jeps/439), or one of the numerous presentations available on YouTube (e.g. I found the overview in https://www.youtube.com/watch?v=YyXjC68l8mw&t=864s pretty useful).

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

PR Comment: https://git.openjdk.org/jdk/pull/25066#issuecomment-2875604527