RFR: 8347515: C2: assert(!success || (C->macro_count() == (old_macro_count - 1))) failed: elimination must have deleted one node from macro list [v5]

[Tobias Hartmann]

On Thu, 8 May 2025 09:47:11 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:
> 
>   removing redundant comment

Looks good to me otherwise.

src/hotspot/share/opto/macro.cpp line 2354:

> 2352: 
> 2353: //------------------refine_strip_mined_loop_macro_node-------------------
> 2354: // Perform refining of strip mined loop node in the macro nodes list.

Suggestion:

// Perform refining of strip mined loop nodes in the macro nodes list.

src/hotspot/share/opto/macro.cpp line 2355:

> 2353: //------------------refine_strip_mined_loop_macro_node-------------------
> 2354: // Perform refining of strip mined loop node in the macro nodes list.
> 2355: void PhaseMacroExpand::refine_strip_mined_loop_macro_node() {

Suggestion:

void PhaseMacroExpand::refine_strip_mined_loop_macro_nodes() {

src/hotspot/share/opto/macro.cpp line 2471:

> 2469: //  Returns true if a failure occurred.
> 2470: bool PhaseMacroExpand::expand_macro_nodes() {
> 2471:   refine_strip_mined_loop_macro_node();

Suggestion:

  refine_strip_mined_loop_macro_nodes();

src/hotspot/share/opto/macro.hpp line 206:

> 204:   }
> 205: 
> 206:   void refine_strip_mined_loop_macro_node();

Suggestion:

  void refine_strip_mined_loop_macro_nodes();

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

Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24890#pullrequestreview-2824859469
PR Review Comment: https://git.openjdk.org/jdk/pull/24890#discussion_r2079556192
PR Review Comment: https://git.openjdk.org/jdk/pull/24890#discussion_r2079556743
PR Review Comment: https://git.openjdk.org/jdk/pull/24890#discussion_r2079556616
PR Review Comment: https://git.openjdk.org/jdk/pull/24890#discussion_r2079557080