RFR: 8290892: C2: Intrinsify Reference.reachabilityFence [v27]

[Vladimir Ivanov]

On Mon, 8 Dec 2025 09:23:55 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   IR test cases
>
> test/hotspot/jtreg/compiler/c2/TestReachabilityFence.java line 200:
> 
>> 198:     @IR(counts = {IRNode.REACHABILITY_FENCE, "2"}, phase = CompilePhase.AFTER_LOOP_OPTS)
>> 199:     @IR(counts = {IRNode.REACHABILITY_FENCE, "0"}, phase = CompilePhase.EXPAND_REACHABILITY_FENCES)
>> 200:     @IR(counts = {IRNode.REACHABILITY_FENCE, "1"}, phase = CompilePhase.FINAL_CODE)
> 
> Can you add a small comment here, why we go from 2 -> 0 -> 1 ? Is it because we eliminate one of the two RF? Which one is supposed to be eliminated?

There's one referent and one interfering safepoint. So, both RFs are transformed into a single reachability edge and it is expanded into a single RF at the end.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25315#discussion_r2738903535