C2: "failed: non-reduction loop contains reduction nodes" assert failure

Similarly to JDK-8279622, a node marked as a reduction is hoisted out of its original reduction loop into an outer loop that is not marked as a reduction. The assertion introduced in JDK-8279622 catches this inconsistency and fails. Unlike JDK-8279622 though, the inconsistent reduction marking in this case is not fatal, in that it would not lead to a miscompilation should the assertion be disabled. This is because the inconsistently marked loop(s) contains safepoints, which inhibits SLP vectorization.

HOW TO REPRODUCE

$ javac Test.java FuzzerUtils.java
$ java Test

# Internal Error (/home/roland/jdk-jdk/src/hotspot/share/opto/superword.cpp:113), pid=4077494, tid=4077507
# assert(!lpt->has_reduction_nodes() || cl->is_reduction_loop()) failed: non-reduction loop contains reduction nodes
#
# JRE version: OpenJDK Runtime Environment (19.0) (fastdebug build 19-internal-adhoc.roland.jdk-jdk)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 19-internal-adhoc.roland.jdk-jdk, mixed mode, sharing, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x1a3cb04] SuperWord::transform_loop(IdealLoopTree*, bool)+0x424

FAILURE ANALYSIS

The node corresponding to (acc += j) is marked as a reduction together with its (inner) loop:

  int i = 0, acc = 0;
  do {
      int j = 0;
      do { // inner loop marked as a reduction
          if (b) {
              acc += j; // node marked as a reduction
          }
          j++;
      } while (j < 5);
      i++;
  } while (i < 100);
  return acc;

After unrolling and constant-folding the inner loop (b is always true), the node formerly corresponding to (acc += j), and now corresponding to (acc += 10), is hoisted into the outer loop, which is not marked as a reduction. This creates an inconsistent state that is later caught by the failing assertion:

  int i = 0, acc = 0;
  do {
      acc += 10; // node marked as a reduction (inconsistent, outer loop is not marked as a reduction)
      i++;
  } while (i < 100);
  return acc;