RFR: 8325451: Missed elimination of assertion predicates

[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.

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

Commit messages:
 - 8325451: Missed elimination of assertion predicates

Changes: https://git.openjdk.org/jdk/pull/17767/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17767&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325451
  Stats: 102 lines in 2 files changed: 97 ins; 0 del; 5 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