RFR: 8367389: C2 SuperWord: refactor VTransform to model the whole loop instead of just the basic block [v4]

[Roland Westrelin]

On Fri, 19 Sep 2025 05:56:20 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> I'm working on cost-modeling, and am integrating some smaller changes from this proof-of-concept PR:
>> https://github.com/openjdk/jdk/pull/20964
>> [See plan overfiew.](https://bugs.openjdk.org/browse/JDK-8340093)
>> 
>> This is a pure refactoring - no change in behaviour. I'm presenting it like this because it will make reviews easier.
>> 
>> ------------------------------
>> 
>> **Goals**
>> - VTransform models **all nodes in the loop**, not just the basic block (enables later VTransform::optimize, like moving reductions out of the loop)
>> - Remove `_nodes` from the vector vtnodes.
>> 
>> **Details**
>> - Remove: `AUTO_VECTORIZATION2_AFTER_REORDER`, `apply_memops_reordering_with_schedule`, `print_memops_schedule`.
>>   - Instead of reordering the scalar memops, we create the new memory graph during `VTransform::apply`. That is why the `VTransformApplyState` now needs to track the memory states.
>> - Refactor `VLoopMemorySlices`: map not just memory slices with phis (have stores in loop), but also those with only loads (no phi).
>> - Create vtnodes for all nodes in the loop (not just the basic block), as well as inputs (already) and outputs (new). Mapping also the output nodes means during `apply`, we naturally connect the uses after the loop to their inputs from the loop (which may be new nodes after the transformation).
>> - `_mem_ref_for_main_loop_alignment` -> `_vpointer_for_main_loop_alignment`. Instead of tracking the memory node to later have access to its `VPointer`, we take it directly. That removes one more use of `_nodes` for vector vtnodes.
>> 
>> I also made a lot of annotations in the code below, for easier review.
>> 
>> **Suggested order for review**
>> - Removal of `VTransformGraph::apply_memops_reordering_with_schedule` -> sets up need to build memory graph on the fly.
>> - Old and new code for `VLoopMemorySlices` -> we now also track load-only slices.
>> - `build_scalar_vtnodes_for_non_packed_nodes`, `build_inputs_for_scalar_vtnodes`, `build_uses_after_loop`, `apply_vtn_inputs_to_node` (use in `apply`), `apply_backedge`, `fix_memory_state_uses_after_loop`
>> - `VTransformApplyState`: how it now tracks the memory state.
>> - `VTransformVectorNode` -> removal of `_nodes` (Big Win!)
>> - Then look at all the other details.
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update src/hotspot/share/opto/vtransform.cpp
>   
>   Co-authored-by: Galder Zamarreño <galder at ibm.com>

Looks reasonable to me.

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Marked as reviewed by roland (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/27208#pullrequestreview-3285338935