RFR: 8371464: C2: assert(no_dead_loop) failed: dead loop detected

[Damon Fenacci]

On Fri, 28 Nov 2025 10:50:44 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> Crash occurs because a `MergeMem` node references itself:
> 
> 
>  608  MergeMem  === _ 1 608 1 1 1 1 1 1 1 1 1 1 878  [[ 877 878 608 420 597 ]]  { - - - - - - - - - - N878:java/lang/Throwable (java/io/Serializable)+20 * [narrow] }  Memory: @BotPTR *+bot, idx=Bot; !orig=[524] !jvms: TestDeadLoopAtMergeMem::test @ bci:94 (line 62)
>  ```
> 
> Before IGVN, that part of the stream is:
> 
> 
>  522  Region  === 522 604 521  [[ 522 538 523 524 525 526 527 528 529 530 531 ]] #reducible  !jvms: TestDeadLoopAtMergeMem::test @ bci:75 (line 59)
>  524  Phi  === 522 608 464  [[ 588 581 564 546 564 559 ]]  #memory  Memory: @BotPTR *+bot, idx=Bot; !jvms: TestDeadLoopAtMergeMem::test @ bci:75 (line 59)
>  
>  538  If  === 522 535  [[ 539 540 ]] P=0.999000, C=-1.000000 !jvms: TestDeadLoopAtMergeMem::test @ bci:79 (line 59)
>  539  IfTrue  === 538  [[ 553 547 ]] #1 !jvms: TestDeadLoopAtMergeMem::test @ bci:79 (line 59)
>  540  IfFalse  === 538  [[ 548 546 ]] #0 !jvms: TestDeadLoopAtMergeMem::test @ bci:79 (line 59)
>  553  If  === 539 535  [[ 554 555 ]] P=0.999000, C=-1.000000 !jvms: TestDeadLoopAtMergeMem::test @ bci:82 (line 59)
>  554  IfTrue  === 553  [[ 562 560 ]] #1 !jvms: TestDeadLoopAtMergeMem::test @ bci:82 (line 59)
>  555  IfFalse  === 553  [[ 548 559 ]] #0 !jvms: TestDeadLoopAtMergeMem::test @ bci:82 (line 59)
>  
>  548  Region  === 548 _ 540 555  [[ 548 562 561 563 564 565 566 567 568 569 570 571 572 573 574 575 576 ]] #reducible  !jvms: TestDeadLoopAtMergeMem::test @ bci:88 (line 60)
>  564  Phi  === 548 _ 524 524  [[ 581 ]]  #memory  Memory: @BotPTR *+bot, idx=Bot; !jvms: TestDeadLoopAtMergeMem::test @ bci:85 (line 61)
> 
> 562  Region  === 562 548 554  [[ 562 600 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 596 ]] #reducible  !jvms: TestDeadLoopAtMergeMem::test @ bci:90 (line 62)
> 581  Phi  === 562 564 524  [[ 420 597 610 608 ]]  #memory  Memory: @BotPTR *+bot, idx=Bot; !jvms: TestDeadLoopAtMergeMem::test @ bci:90 (line 62)
> 
> 608  MergeMem  === _ 1 581 1 1 1 1 1 1 1 1 1 1 588  [[ 524 ]]  { - - - - - - - - - - N588:java/lang/Throwable (java/io/Serializable)+20 * [narrow] }  Memory: @BotPTR *+bot, idx=Bot; !jvms: TestDeadLoopAtMergeMem::test @ bci:94 (line 62)
> 
> 
> 522 is a loop head, 604 is the backedge. The loop becomes unreachable
> during IGVN. The loop body above is transformed to:
> 
> 
>  538  If  === 604 535  [[ 539 540 ]] P=0.999000, C=-1.000000 !jvms: TestDeadLoopAtMergeMem::test @ bci:79 (line 59)
>  539  IfTrue  === 538  [[ 562 547 560 ]] #1 !jvms: TestDeadLoopAtMergeMem::test @ bci:79 (l...

Thanks for fixing this @rwestrel! Barring Christian's testing, the change looks good to me.

src/hotspot/share/opto/cfgnode.cpp line 1404:

> 1402:       Node* other_phi_input = in(j);
> 1403:       if (other_phi_input != nullptr && other_phi_input == merge_mem->base_memory() && !is_data_loop(region, phi_input, igvn)) {
> 1404:         // merge_mem is a successor memory to other_phi_input, and is not pinned inside the diamond, so push it out.

Do you think it might be worth adding an additional reason for `!is_data_loop` in the comment?

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

Marked as reviewed by dfenacci (Committer).

PR Review: https://git.openjdk.org/jdk/pull/28554#pullrequestreview-3523662101
PR Review Comment: https://git.openjdk.org/jdk/pull/28554#discussion_r2576027929