This PR addresses yet another missed optimization in PhaseIterGVN. The way this optimization is triggered is a bit different this time though, and the notification is missing in Node::has_special_unique_user.

Analysis

The affected optimization is the transformation of MoveX2Y (LoadX mem) into LoadY mem. This is implemented in MoveNode::Ideal. The optimization is as follows:

// Fold reinterpret cast into memory operation:
//    MoveX2Y (LoadX mem) => LoadY mem
LoadNode* ld = in(1)->isa_Load();
if (ld != nullptr && (ld->outcnt() == 1)) { // replace only
  const Type* rt = bottom_type();
  if (ld->has_reinterpret_variant(rt)) {
    if (phase->C->post_loop_opts_phase()) {
      return ld->convert_to_reinterpret_load(*phase, rt);
    } else {
      // attempt the transformation once loop opts are over
      phase->C->record_for_post_loop_opts_igvn(this);
    }
  }
}

The optimization is triggered only if the input is a LoadNode and the MoveNode is its only user. This is a relatively unusual pattern.

The bug was found by the fuzzer. At some point during IGVN, we have the following subgraph:

CountedLoop    LoadL
          \     /  \
            Phi    MoveL2D

In RegionNode::Ideal, we end up calling set_req_X on the Phi node to delete the edge from the Phi node to LoadL. As a result, the LoadL node only has one user left, and the MoveNode::Ideal gets triggered at the next verification pass.

Proposed Solution

Add this particular case to Node::has_special_unique_user, which gets called by Node::set_req_X.

Summary of changes

This PR brings the following changes:

• Detect the optimization pattern in Node::has_special_unique_user.
• Add new test TestMissingOptMoveX2YLoadX.java, initially obtained from the fuzzer and then heavily reduced, both with the usual tools and manually. I tried to get a reproducer for each of the Move nodes, but I was only able to get one for MoveL2D.

Testing

• https://github.com/benoitmaillard/jdk/actions?query=branch%3AJDK-8371674
• tier1-4, plus some internal testing

Thank you for reviewing!

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8371674: C2 fails with Missed optimization opportunity in PhaseIterGVN for MoveL2D (Bug - P4)

Reviewing