RFR: 8333258: C2: high memory usage in PhaseCFG::insert_anti_dependences()

[Emanuel Peter]

On Thu, 20 Jun 2024 12:59:07 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> In a debug build, `PhaseCFG::insert_anti_dependences()` is called
>> twice for a single node: once for actual processing, once for
>> verification.
>> 
>> In TestAntiDependenciesHighMemUsage, the test has a `Region` that
>> merges 337 incoming path. It also has one `Phi` per memory slice that
>> are stored to: 1000 `Phi` nodes. Each `Phi` node has 337 inputs that
>> are identical except for one. The common input is the memory state on
>> method entry. The test has 60 `Load` that needs to be processed for
>> anti dependences. All `Load` share the same memory input: the memory
>> state on method entry. For each `Load`, all `Phi` nodes are pushed 336
>> times on the work lists for anti dependence processing because all of
>> them appear multiple times as uses of each `Load`s memory state: `Phi`s
>> are pushed 336 000 on 2 work lists. Memory is not reclaimed on exit
>> from `PhaseCFG::insert_anti_dependences()` so memory usage grows as
>> `Load` nodes are processed:
>> 
>> 336000 * 2 work lists * 60 loads * 8 bytes pointer = 322 MB. 
>> 
>> The fix I propose for this is to not push `Phi` nodes more than once
>> when they have the same inputs multiple times.
>> 
>> In TestAntiDependenciesHighMemUsage2, the test has 4000 loads. For
>> each of them, when processed for anti dependences, all 4000 loads are
>> pushed on the work lists because they share the same memory
>> input. Then when they are popped from the work list, they are
>> discarded because only stores are of interest:
>> 
>> 4000 loads processed * 4000 loads pushed * 2 work lists * 8 bytes pointer = 256 MB. 
>> 
>> The fix I propose for this is to test before pushing on the work list
>> whether a node is a store or not.
>> 
>> Finally, I propose adding a `ResourceMark` so memory doesn't
>> accumulate over calls to `PhaseCFG::insert_anti_dependences()`.
>
> src/hotspot/share/opto/gcm.cpp line 673:
> 
>> 671:     Node* store = worklist_store.pop();
>> 672:     uint op = store->Opcode();
>> 673:     assert(!store->needs_anti_dependence_check(), "only stores");
> 
> I find the "only stores" comment a bit confusing. Because `store` can also be a `MergeMem`, right? Any maybe other things. Honestly, the name `store` seems generally confusing to me, since it is not at all clear that this is really a `StoreNode`.

Probably a better name would have been `def` and `use`. Maybe you can at least add a comment for explanation. Or do a refactoring separately, if you feel like it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19791#discussion_r1647539260