RFR: 8289943: Simplify some object allocation merges [v11]

[Vladimir Kozlov]

On Wed, 5 Oct 2022 21:43:15 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

>> Hi there, can I please get some feedback on this approach to simplify object allocation merges in order to promote Scalar Replacement of the objects involved in the merge?
>> 
>> The basic idea for this [approach was discussed in this thread](https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2022-April/055189.html) and it consists of: 
>> 1) Identify Phi nodes that merge object allocations and replace them with a new IR node called ReducedAllocationMergeNode (RAM node).
>> 2) Scalar Replace the incoming allocations to the RAM node.
>> 3) Scalar Replace the RAM node itself.
>> 
>> There are a few conditions for doing the replacement of the Phi by a RAM node though - Although I plan to work on removing them in subsequent PRs:
>> 
>> - The only supported users of the original Phi are AddP->Load, SafePoints/Traps, DecodeN.
>> 
>> These are the critical parts of the implementation and I'd appreciate it very much if you could tell me if what I implemented isn't violating any C2 IR constraints:
>> 
>> - The way I identify/use the memory edges that will be used to find the last stored values to the merged object fields.
>> - The way I check if there is an incoming Allocate node to the original Phi node.
>> - The way I check if there is no store to the merged objects after they are merged.
>> 
>> Testing:
>> - Windows/Linux/MAC fastdebug/release 
>>   - hotspot_all
>>   - tier1
>>   - Renaissance
>>   - dacapo
>>   - new IR-based tests
>
> Cesar Soares Lucas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address PR feedback. Fix test & one bug. Set RAM parameter to true by default.

The only is issue is that experimental flags usually have `false` default value. Alternatively you can make it diagnostic which we use for C2 optimizations flags.

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PR: https://git.openjdk.org/jdk/pull/9073