RFR: 8318446: C2: optimize stores into primitive arrays by combining values into larger store [v11]

[Vladimir Kozlov]

On Mon, 26 Feb 2024 13:52:54 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Yes, this could be the issue if we don't check that memory accesses offset overlaps.
>> 
>> There was issue we recently fixed [#16015](https://github.com/openjdk/jdk/pull/16015) where we did not take into account wider memory access to array.
>
> @vnkozlov Hmm, ok. You seem to say that it is mostly a question of bugs that exist elsewhere. But my change here would not really change things then, since we could always trigger those things with Unsafe anyway, right?
> 
> Or is there something intrinsically wrong about using **raw store**?
> 
> Is there any alternative for me here? @TobiHartmann suggested I could use CPU barriers.

Adding `MemBarCPUOrder` after this store is good suggestion. But hte need for it depends how you replace all merged stores with new wide store and attach following loads. Looking on changes and it seems you allow this optimization for sequential fields of normal object and not just array (I don't see check for AryPtrType). Such accesses will have separate memory slices (aliases). If you attach all memory edges of following related Load nodes to new wide Store we may be fine because loads will not pass the store. If not, we have a problem. It needs to be verified. Also looking on `find_previous_store()` and `can_see_stored_value()` they seem have checks which may prevent Loads from looking above such Store.

We need more testing for such cases. I see only 2 test methods which do that: test300R and test300a.
Also all since all init values the same it is difficult to see if Load node get correct value.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16245#discussion_r1503180064