RFR: 8347515: C2: assert(!success || (C->macro_count() == (old_macro_count - 1))) failed: elimination must have deleted one node from macro list [v4]

[duke]

On Wed, 7 May 2025 11:21:45 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.
>
> Saranya Natarajan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   addressing review comments

@sarannat 
Your change (at version e8bcbc9b07ff2310d747e112b692910158bf5432) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk/pull/24890#issuecomment-2862187928