RFR: 8315361: C2 SuperWord: refactor out loop analysis into shared auto-vectorization facility VLoopAnalyzer

[Fei Gao]

On Fri, 10 Nov 2023 17:24:22 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> This is a refactoring of `SuperWord`.
> I intend to push it for JDK23, after [this bug fix](https://github.com/openjdk/jdk/pull/14785).
> 
> **Goals**
> 
> 1. Clean up `SuperWord`: disentangle different components, make them more **modular**.
> 2. Make the loop analysis parts a **shared facility**, not just for SuperWord but also the post-loop-vectorizer ([JDK-8308994](https://bugs.openjdk.org/browse/JDK-8308994)).
> 3. It is also a necessary step on my bigger plans for improvement with the C2 Auto-Vectorizer ([see my blog post](https://eme64.github.io/blog/2023/11/03/C2-AutoVectorizer-Improvement-Ideas.html)).
> 4. Improve tracing in the auto-vectorization by making it more systematic.
> 
> **Summary**
> 
> - I wrote a summary of how C2 auto-vectorization with SuperWord works (please read!):
> https://github.com/openjdk/jdk/blob/95fd361e60fc66eb91edad321662e508b2d1bdde/src/hotspot/share/opto/superword.hpp#L32-L177
> - I moved many `Superword` components out to `VLoop` and its subclass `VLoopAnalyzer`. The idea is that any vectorizer can use these facilities in the future. They are therefore made more modular, which should hopefully make future changes easier. These components are:
>   - Checking the pre-conditions for vectorization (e.g. no unwanted ctrl-flow).
>     - `VLoop::check_preconditions_helper` replaces code from old `SuperWord::transform_loop`.
>   - Running all submodules of `VLoopAnalyzer`: `VLoopAnalyzer::analyze_helper`. Replaces analysis part of `SuperWord::SLP_extract`.
>   - Finding and marking reductions -> `VLoopReductions`
>   - Detecting memory slices -> `VLoopMemorySlices`
>   - Analyzing the body -> `VLoopBody`  (renamed `in_bb` -> `in_body`)
>   - Determining vector element types, and functions to determine the `vector_width` of a node -> `VLoopTypes`
>   - Constructing the dependence graph -> `VLoopDependenceGraph`. Replaces old `DepGraph` with all its components.
> - New: CompileCommand option `TraceAutovectorization`
>   - Run with `-XX:CompileCommand=traceAutovectorization,*::*,help` to get a usage description.
>   - Replaced all printing with flags `TraceSuperWord` (and `Verbose`) and of `VectorizeDebug`.
>   - The advantage of a CompileCommand is that tracing can be applied selectively for only a limited set of java classes / methods.
>   - It uses tags, which are more readable than the `VectorizeDebug` bit-flags. These tags can be used for all parts of the vectorizer, but one can also target SuperWord specifically.
>   - I systematically added tracing at every point where vector...

src/hotspot/share/opto/superword.hpp line 282:

> 280:   bool is_trace_superword_adjacent_memops() const {
> 281:     return vla().is_trace_superword_adjacent_memops();
> 282:   }

How about redefining it as:

  bool is_trace_superword_adjacent_memops() const {
    return TraceSuperWord || vla().is_trace_tag_active(TraceAutovectorizationTag::TAG_SW_ADJACENT_MEMOPS);
  }


And add a consulting interface in `class VLoop`:

  bool is_trace_tag_active(TraceAutovectorizationTag tag) const {
    return _trace_tags.at(tag);
  }


Thus, we don't have to involve any `SuperWord` specific words or options in shared facility.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16620#discussion_r1416920701