RFR: 8345067: C2: enable implicit null checks for ZGC reads [v3]

[Fei Yang]

On Fri, 16 May 2025 08:02:16 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> Two small nits/questions, but otherwise ready from my side :)
>
>> Two small nits/questions, but otherwise ready from my side :)
> 
> Thanks again for reviewing @eme64, I have addressed your questions now. And thanks also for your review @vnkozlov.
> 
> @stefank @fisk @xmas92 @jsikstro may I get a review from the GC side?
> 
> @RealFYang @TheRealMDoerr note that this PR also introduces implicit null check support for ZGC loads in RISC-V and PPC, but I cannot test it beyond GHA. May I ask you to test the changes on your respective platforms? (or let me know if you prefer to add the support in separate PRs).

@robcasloz : Hi, Thanks for the ping! I performed tier1-3 tests on linux-riscv64 platform, result is good.
The new test `test/hotspot/jtreg/compiler/gcbarriers/TestImplicitNullChecks.java` also pass when running with G1 and ZGC using fastdebug build.

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PR Comment: https://git.openjdk.org/jdk/pull/25066#issuecomment-2889518370