RFR: 8325451: Missed elimination of assertion predicates [v2]

[Joshua Cao]

> For this example:
> 
> 
> public void blackhole(int[] arr) {
>   for (int i = 0; i < arr.length; ++i) {
>     blackhole(arr[i]);
>   }
> }
> 
> 
> We generate predicates for the lower/upper bound. The lower bound predicate gets optimized away, but the associated assertion predicate remains in the code.
> 
> The issue is that we iterate over list `C->template_assertion_predicate_opaq_node`, and we delete items from it in `_igvn.replace_node(n, n->in(2))`. I was surprised that this actually removes an item from the predicates list. Here is the trace:
> 
> 
> #0  Compile::remove_template_assertion_predicate_opaq (this=0x7fffb5d5a7e0, n=0x7fff84097eb0) at /local/home/joshcao/pea/jdk/src/hotspot/share/opto/compile.hpp:776
> #1  0x00007ffff5313c53 in Compile::remove_useless_node (this=0x7fffb5d5a7e0, dead=0x7fff84097eb0) at /local/home/joshcao/pea/jdk/src/hotspot/share/opto/compile.cpp:395
> #2  0x00007ffff5cebb95 in PhaseIterGVN::remove_globally_dead_node (this=0x7fffb5d584e0, dead=0x7fff84097eb0) at /local/home/joshcao/pea/jdk/src/hotspot/share/opto/phaseX.cpp:1380
> #3  0x00007ffff52241ce in PhaseIterGVN::remove_dead_node (this=0x7fffb5d584e0, dead=0x7fff84097eb0) at /local/home/joshcao/pea/jdk/src/hotspot/share/opto/phaseX.hpp:527
> #4  0x00007ffff5cebfc7 in PhaseIterGVN::subsume_node (this=0x7fffb5d584e0, old=0x7fff84097eb0, nn=0x7fff8408fb80) at /local/home/joshcao/pea/jdk/src/hotspot/share/opto/phaseX.cpp:1429
> #5  0x00007ffff50045bf in PhaseIterGVN::replace_node (this=0x7fffb5d584e0, old=0x7fff84097eb0, nn=0x7fff8408fb80) at /local/home/joshcao/pea/jdk/src/hotspot/share/opto/phaseX.hpp:539
> #6  0x00007ffff5aec060 in PhaseIdealLoop::eliminate_useless_template_assertion_predicates (this=0x7fffb5d58120, useful_predicates=...) at /local/home/joshcao/pea/jdk/src/hotspot/share/opto/loopnode.cpp:4366
> 
> 
> We can fix this by iterating in reverse over the linked list.
> 
> Caveat: this patch only affects the first test `TestPredicateBasic::basic()`. Adding the other tests for completeness and can potentially be improved in other work.

Joshua Cao 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 three additional commits since the last revision:

 - rename n->opaque4
 - Merge branch 'master' into linkedlist
 - 8325451: Missed elimination of assertion predicates

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17767/files
  - new: https://git.openjdk.org/jdk/pull/17767/files/ac2b3291..7af9b738

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17767&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17767&range=00-01

  Stats: 2637 lines in 233 files changed: 1795 ins; 340 del; 502 mod
  Patch: https://git.openjdk.org/jdk/pull/17767.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17767/head:pull/17767

PR: https://git.openjdk.org/jdk/pull/17767