RFR: 8287087: C2: perform SLP reduction analysis on-demand [v2]

[Roberto Castañeda Lozano]

On Wed, 5 Apr 2023 09:13:20 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> src/hotspot/share/opto/superword.cpp line 539:
>> 
>>> 537:       pred = current;
>>> 538:       current = original_input(current, reduction_input);
>>> 539:     }
>> 
>> If we bookkeep the nodes in the reduction chain path during initial backward traversal we may simplify this checking and also another call to _original_input_ while populating _loop_reductions set on [#L547 ](https://github.com/openjdk/jdk/pull/13120/files#diff-8f29dd005a0f949d108687dabb7379c73dfd85cd782da453509dc9b6cb8c9f81R547)
>
> Thanks, will consider this together with your earlier comments.

I tried out your suggestion but unfortunately, the bookkeeping code (marking/storing candidate nodes and their predecessors in the tentative reduction chain) became more complex than the simplifications it enabled.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13120#discussion_r1166790520