[lworld] RFR: 8228361: [lworld] Optimize the substitutability check in C2 [v2]

[Manuel Hässig]

On Thu, 18 Dec 2025 12:16:48 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> C2 will now remove the slow call to `ValueObjectMethods::isSubstitutable(Alt)` whenever it's able to determine the layout of one of the operands. It will then emit code to directly compare the fields.
>> 
>> This patch also contains an intrinsic for `_getFieldMap` that will be used by the new core-libs implementation of the substitutability check ([JDK-8370450](https://bugs.openjdk.org/browse/JDK-8370450)) that's used by the interpreter / C1 and as a slow path in C2.
>> 
>> When browsing code, I marked a few rough edges in unrelated code for follow-up cleanups with the corresponding bug numbers.
>> 
>> Testing: tier1-tier6 + valhalla-comp-stress
>> 
>> Thanks,
>> Tobias
>
> Tobias Hartmann has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 21 commits:
> 
>  - Merge branch 'lworld' into JDK-8228361
>  - More refactoring, better comments
>  - More comments, removing workaround
>  - Merge
>  - v5
>  - Refactoring v4
>  - Refactoring v3
>  - Refactoring, more checks, new test
>  - Merge branch 'lworld' into JDK-8228362
>  - All tests pass
>  - ... and 11 more: https://git.openjdk.org/valhalla/compare/3c41c2aa...4f8556c7

src/hotspot/share/opto/inlinetypenode.cpp line 659:

> 657:     assert(is_reference_type(bt), "must be");
> 658:     cmp = igvn->register_new_node_with_optimizer(new CmpPNode(val1, val2));
> 659:   }

The following might be a bit cleaner and self-explanatory.
Suggestion:

  if (is_single_word_type(bt)) {
    if (bt == T_FLOAT) {
      val1 = igvn->register_new_node_with_optimizer(new MoveF2INode(val1));
      val2 = igvn->register_new_node_with_optimizer(new MoveF2INode(val2));
    }
    cmp = igvn->register_new_node_with_optimizer(new CmpINode(val1, val2));
  } else if (is_double_word_type(bt)) {
    if (bt == T_DOUBLE) {
      val1 = igvn->register_new_node_with_optimizer(new MoveD2LNode(val1));
      val2 = igvn->register_new_node_with_optimizer(new MoveD2LNode(val2));
    }
    cmp = igvn->register_new_node_with_optimizer(new CmpLNode(val1, val2));
  } else {
    assert(is_reference_type(bt), "must be");
    cmp = igvn->register_new_node_with_optimizer(new CmpPNode(val1, val2));
  }

 Note, this still needs an appropriate implementation for `is_single_word_type()` in `gobal_definitions.hpp`:
```c++
inline bool is_integral_type(BasicType t) {
  return is_subword_type(t) || t == T_INT || t == T_FLOAT;
}

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

PR Review Comment: https://git.openjdk.org/valhalla/pull/1823#discussion_r2631186288