RFR: 8289943: Simplify some object allocation merges [v13]

[Vladimir Ivanov]

On Thu, 6 Oct 2022 16:50:28 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:
> 
>   Fix x86 tests.

src/hotspot/share/opto/escape.cpp line 522:

> 520:           if (child->is_Store()) {
> 521:             assert(child->in(0) != NULL || m->in(0) != NULL, "No control for store or AddP.");
> 522:             if (_igvn->is_dominator(merge_phi_region, child->in(0) != NULL ? child->in(0) : m->in(0))) {

`PhaseIterGVN::is_dominator()` is a conservative approximation and can result in false negatives.

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