RFR: 8317572: C2 SuperWord: refactor/improve TraceSuperWord, replace VectorizeDebugOption with TraceAutoVectorization

[Emanuel Peter]

On Fri, 26 Jan 2024 15:09:43 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> Subtask of https://github.com/openjdk/jdk/pull/16620
>> 
>> I got approval to remove VectorizeDebugOption: [JDK-8320668](https://bugs.openjdk.org/browse/JDK-8320668)
>> 
>> I want a more general flag for AutoVectorization, that can trace different components of AutoVectorization.
>> It should be a CompileCommand, so that it can select which methods it traces for.
>> 
>> TraceSuperWord should still look similar, and select a subset of the TraceAutoVectorization components (those for SuperWord), but still apply to all classes/methods.
>> 
>> With more refactoring later in [JDK-8315361](https://bugs.openjdk.org/browse/JDK-8315361), this flag should become more usable and interpretable. Especially, the idea is that different components of the `VLoop / VLoopAnalyzer` can have tracing enabled / disabled.
>> 
>> **How to use the flag:**
>> Get "help", i.e. see all available tags:
>> `./java -Xcomp -XX:CompileCommand=TraceAutoVectorization,*::*,help --version`
>> 
>> See "rejections" (i.e. failures where we don't vectorize) and successes (using TraceNewVectors):
>> `./java -Xcomp -XX:CompileCommand=TraceAutoVectorization,*::*,SW_REJECTIONS -XX:+TraceNewVectors --version`
>> The results are currently underwhealming. I will have to track many more failures, and I will do that with the bigger refactoring, when I move around the code and require error code returning everywhere, and then I can use that error code for printing.
>
> src/hotspot/share/opto/superword.cpp line 925:
> 
>> 923: void SuperWord::mem_slice_preds(Node* start, Node* stop, GrowableArray<Node*> &preds) {
>> 924:   assert(preds.length() == 0, "start empty");
>> 925:   Node* n = start;
> 
> There is still a usage of `TraceSuperWord` on L927. Should this also be replaced?

It will be removed with https://github.com/openjdk/jdk/pull/17585 anyway ;)

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

PR Review Comment: https://git.openjdk.org/jdk/pull/17586#discussion_r1467800698