RFR: 8333258: C2: high memory usage in PhaseCFG::insert_anti_dependences() [v2]

[Vladimir Kozlov]

On Tue, 25 Jun 2024 13:15:47 GMT, Roland Westrelin <roland 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()`.
>
> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - review
>  - Merge branch 'master' into JDK-8333258
>  - whitespaces
>  - tests & fix

src/hotspot/share/opto/gcm.cpp line 585:

> 583:     if (worklist_mem.at(j-1) != mem) {
> 584:       // We're done with the uses of mem
> 585:       return false;

Should this be `continue;` instead to make sure we scan all memory nodes?

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PR Review Comment: https://git.openjdk.org/jdk/pull/19791#discussion_r1652858312