[lworld] RFR: 8354694: [lworld] Refactor AssertUnsetFields to Early Larval Frame

[Chen Liang]

On Wed, 16 Apr 2025 21:48:19 GMT, Matias Saavedra Silva <matsaave at openjdk.org> wrote:

> This patch adds refactors so the runtime aligns with the new strict field initialization spec (https://cr.openjdk.org/~dlsmith/jep401/jep401-20250409/specs/strict-fields-jvms.html). The stackmap table entry `AssertUnsetFields` is now referred to as `Early_Larval` and it follows a new format where it is a frame that encompasses a base frame.

We might need a switch condition based on the preview status.

src/hotspot/share/classfile/stackMapTable.cpp line 298:

> 296:     _parsed_frame_count++;
> 297:   }
> 298: 

If we have no early_larval wrapping, and there is no more `uninitializedThis` in locals, we implicitly set unset fields to empty. See last sentence in section 4.7.4.

So for control flow like:


if (check) {} else {}
this.strictField = xxx;
super();
if (check1) {} else {}

The frame at first then-else merge has `strictField` unset, but the frame at 2nd else branch off must not be wrapped in early_larval, and verifier must accordingly clear the unset fields.

src/hotspot/share/classfile/stackMapTable.cpp line 358:

> 356:   u2 offset_delta = _stream->get_u2(CHECK_NULL);
> 357: 
> 358:   if (frame_type < EARLY_LARVAL) {

Here this can just be
Suggestion:

  if (frame_type < SAME_LOCALS_1_STACK_ITEM_EXTENDED) {

the case where it is `== EARLY_LARVAL` is already handled earlier, and you don't want this to accidentally handle newer frame types that nest other frames.

src/hotspot/share/classfile/verifier.cpp line 494:

> 492:       break;
> 493:     case STRICT_FIELDS_MISMATCH:
> 494:     ss->print("Current frame's strict instance fields not compatible with stackmap.");

Suggestion:

      ss->print("Current frame's strict instance fields not compatible with stackmap.");

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

PR Review: https://git.openjdk.org/valhalla/pull/1435#pullrequestreview-2774630303
PR Review Comment: https://git.openjdk.org/valhalla/pull/1435#discussion_r2048270388
PR Review Comment: https://git.openjdk.org/valhalla/pull/1435#discussion_r2048268059
PR Review Comment: https://git.openjdk.org/valhalla/pull/1435#discussion_r2048275741