RFR: 8343541: C1: Plain memory accesses are emitted with membars with +AlwaysAtomicAccesses [v2]

[Xiaolong Peng]

On Mon, 18 Nov 2024 09:30:54 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> C1 and C2 has different implementations for `+AlwaysAtomicAccesses`, [C2 impl](https://github.com/openjdk/jdk/blob/4a7ce1d7c1bd4b751063b98cf8bedcd27055760b/src/hotspot/share/gc/shared/c2/barrierSetC2.cpp#L410) only guarantees atomicity hence no membars  are emitted for plain memory access, but C1 treats it same as volatile access hence it emits membars.  The change removes the unnecessary membars in C1 for `+AlwaysAtomicAccesses`. 
>> 
>> The test have been verified by very simple JMH benchmarks to measure the latency of reading/writing long/volatile long variable 10000 times, and run with VM option `-XX:TieredStopAtLevel=3 -XX:+UnlockExperimentalVMOptions -XX:+AlwaysAtomicAccesses`: 
>> 
>> Before the fix:
>> 
>> Benchmark                                   Mode  Cnt       Score      Error  Units
>> AlwaysAtomicAccesses.testReadLong           avgt    5   58711.131 ±  716.940  ns/op
>> AlwaysAtomicAccesses.testReadVolatileLong   avgt    5   59014.735 ±  675.354  ns/op
>> AlwaysAtomicAccesses.testWriteLong          avgt    5  115817.978 ±  302.089  ns/op
>> AlwaysAtomicAccesses.testWriteVolatileLong  avgt    5  116317.835 ± 1451.365  ns/op
>> 
>> 
>> After the fix:
>> 
>> Benchmark                                   Mode  Cnt       Score      Error  Units
>> AlwaysAtomicAccesses.testReadLong           avgt    5   49651.527 ±  159.948  ns/op
>> AlwaysAtomicAccesses.testReadVolatileLong   avgt    5   58668.844 ±  316.029  ns/op
>> AlwaysAtomicAccesses.testWriteLong          avgt    5   23008.361 ±   10.947  ns/op
>> AlwaysAtomicAccesses.testWriteVolatileLong  avgt    5  116440.017 ± 1240.832  ns/op
>
> Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   renaming

Thanks all for the review!

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PR Comment: https://git.openjdk.org/jdk/pull/22191#issuecomment-2486793350