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

[Saranya Natarajan]

On Tue, 29 Apr 2025 16:18:45 GMT, Emanuel Peter <epeter 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?

@eme64 : I will attach a version of the `Reduced.java` to the JTREG test in the next commit.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/24890#issuecomment-2841323530