RFR: 8371464: C2: assert(no_dead_loop) failed: dead loop detected [v2]

[Roland Westrelin]

> 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...

Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - review
 - Merge branch 'master' into JDK-8371464
 - test
 - fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28554/files
  - new: https://git.openjdk.org/jdk/pull/28554/files/052b7a46..825c9dd5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28554&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28554&range=00-01

  Stats: 19717 lines in 576 files changed: 12800 ins; 3715 del; 3202 mod
  Patch: https://git.openjdk.org/jdk/pull/28554.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28554/head:pull/28554

PR: https://git.openjdk.org/jdk/pull/28554