RFR: 8361699: C2: assert(can_reduce_phi(n->as_Phi())) failed: Sanity: previous reducible Phi is no longer reducible before SUT.

[Emanuel Peter]

On Wed, 3 Sep 2025 00:53:59 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

> Please, review this patch to fix issue that may occur when reducing allocation merge.
> 
> As the assert message describe, the problem is a `Phi` considered reducible during one invocation of  `adjust_scalar_replaceable_state` turned out to be later non-reducible. This situation can happen if a subsequent invocation of the same method causes all inputs to the phi to be NSR; therefore there is no point in reducing the Phi. It can also happen during the propagation of NSR state done by `find_scalar_replaceable_allocs`. 
> 
> The change in `revisit_reducible_phi_status` is just a clean-up.
> The real fix is in `find_scalar_replaceable_allocs`.
> 
> Tested on Linux x64/Aarch64 release/fastdebug with JTREG tier1-3.

Just a drive-by comment.

You also have a broken title ;)

src/hotspot/share/opto/escape.cpp line 3078:

> 3076:     Node* phi = reducible_merges.at(i);
> 3077: 
> 3078:     if (!can_reduce_phi(phi->as_Phi())) {

You say this is a pure cleanup? There are some slight differences in the code though, right?
This method call checks `PhaseMacroExpand::can_eliminate_allocation`, and has a side effect with `ptn->set_scalar_replaceable(false)`.

Just pointing it out, not a EA expert.

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PR Review: https://git.openjdk.org/jdk/pull/27063#pullrequestreview-3188687569
PR Review Comment: https://git.openjdk.org/jdk/pull/27063#discussion_r2324618370