RFR: 8347515: C2: assert(!success || (C->macro_count() == (old_macro_count - 1))) failed: elimination must have deleted one node from macro list

[Emanuel Peter]

On Fri, 25 Apr 2025 21:56:10 GMT, Saranya Natarajan <duke at openjdk.org> wrote:

> Issue: The assertion failure , `assert(!success || (C->macro_count() == (old_macro_count - 1))) failed: elimination must have deleted one node from macro list`, occurs when [loop striping mining  ](https://bugs.openjdk.org/browse/JDK-8186027)may create a [MaxL](https://bugs.openjdk.org/browse/JDK-8324655)  after macro expansion.
> 
> Analysis : Before the macro nodes are expanded in` expand_macro_nodes`, there is a process where nodes from the macro list are eliminated. This also includes elimination of  any `OuterStripMinedLoop` node in the macro list. The bug occurs due to the refining of the strip mined loop in `adjust_strip_mined_loop` function just before it is eliminated. In this case, a` MaxL` node is added to the macro list in `adjust_strip_mined_loop`. 
> 
> Fix: The fix involves performing the refining of the strip mined loop before elimination process. More specifically, moving the `adjust_strip_mined_loop` function outside the elimination loop.
> 
> Improvement:  The process of eliminating macro nodes by calling `eliminate_macro_nodes` and performing additional Opaque and LoopLimit nodes elimination in ` expand_macro_nodes` is unintuitive as suggested in [JDK-8325478 ](https://bugs.openjdk.org/browse/JDK-8325478) and the current fix should be moved along with the other elimination code.

@sarannat The fix looks reasonable to me, and looking forward to the follow-up JDK-8325478  :)

It looks like the JBS issue has some `Reduced.java`, which is from a fuzzer `Test.java`, correct? Can you attach that one as a JTREG test here? Or is there already a very good and minimal reproducer in the repository?

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PR Comment: https://git.openjdk.org/jdk/pull/24890#issuecomment-2839499146