RFR: 8370519: C2: Hit MemLimit when running with +VerifyLoopOptimizations [v3]

[Roland Westrelin]

On Thu, 11 Dec 2025 12:12:02 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> They are allocated in the thread's resource area. So there's no leak and while for `_loop_or_ctrl` and `_body` there were issues that were solved by moving them to the compile arena, there hasn't been any so far with other data structures such as `_idom`. So, sure, we could pro actively move them to the new arena but do we gain anything from doing that?
>
> Yes, I think we would! Keeping `_idom` on the thread resource area means we often cannot use `ResourceMark` for other data structures, if possibly `_idom` is modified in the same scope.
> 
> @iwanowww could not place `ResourceMark`s in some of this current PR:
> https://github.com/openjdk/jdk/pull/25315
> 
> For example:
> 
> +bool PhaseIdealLoop::optimize_reachability_fences() {
> +  Compile::TracePhase tp(_t_reachability_optimize);
> +
> +  assert(OptimizeReachabilityFences, "required");
> +
> +  // ResourceMark rm; // NB! not safe because insert_rf may trigger _idom reallocation
> +  Unique_Node_List redundant_rfs;
> +  GrowableArray<Pair<Node*,Node*>> worklist;

Ok. Done in new commit for `_idom` and `_dom_depth`

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PR Review Comment: https://git.openjdk.org/jdk/pull/28581#discussion_r2610504368