RFR: 8289943: Simplify some object allocation merges

[Xin Liu]

On Tue, 7 Jun 2022 23:24:02 GMT, Cesar Soares <duke 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 original Phi node should be merging Allocate nodes in all inputs.~~
> - 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:
> - Linux. fastdebug -> hotspot_all, renaissance, dacapo

src/hotspot/share/opto/c2_globals.hpp line 474:

> 472:           " register allocation.")                                          \
> 473:                                                                             \
> 474:   product(bool, ReduceAllocationMerges, false,                              \

we need enable it anyway.

src/hotspot/share/opto/callnode.cpp line 1704:

> 1702: }
> 1703: 
> 1704: bool ReducedAllocationMergeNode::register_use(Node* n) {

I think register_use/addp() don't fail in runtime.  it will simplify your code.

src/hotspot/share/opto/callnode.hpp line 1031:

> 1029: // In some cases a reference to the whole merged object is needed and
> 1030: // we handle that by creating an SafePointScalarObjectNode.
> 1031: class ReducedAllocationMergeNode : public TypeNode {

Is that possible to make it just subclass of PhiNode?  if so, you wouldn't need to touch cfgnode.[hpp/cpp]

src/hotspot/share/opto/escape.cpp line 647:

> 645:   _igvn->_worklist.push(ram);
> 646: 
> 647: //  if (n->_idx == 257 && ram->_idx == 1239) {

some mysterious code? :)

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