RFR: 8289943: Simplify some object allocation merges [v10]

[Vladimir Kozlov]

On Wed, 28 Sep 2022 00:32:30 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:
> 
>   Addressing PR feedback. Added new constraint for case of merging SR and NSR allocations.

DaCapo 9.12 I think.
`DaCapo-xalan-large` showed -36% and -27% in 2 runs. Flags: `--size large --iterations 20 xalan`
`DaCapo-lusearch-large` showed -19% and -114% in 2 runs. Flags: `--size large --iterations 20 lusearch-fix`

@ericcaspole said that Dacapo results are not stable.

On MacOS M1 there is no difference in results (insignificant <0.5%).

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