RFR: 8302652: [SuperWord] Reduction should happen after loop, when possible [v2]

[Pengfei Li]

On Mon, 8 May 2023 07:12:46 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> src/hotspot/share/opto/loopopts.cpp line 4305:
>> 
>>> 4303: 
>>> 4304:     // Turn the scalar phi into a vector phi.
>>> 4305:     _igvn.rehash_node_delayed(phi);
>> 
>> Is it possible to setup the vector phi first, and then replace all reduction nodes by vector accumulators via calling `_igvn.replace_node()`? Current code here looks a bit wordy.
>
> If I set up the phi first, then where do I keep the `init` value? I have to make sure it has a use throughout the transformation, other wise it may think it is `dead`.

Why can the `init` node be dead? I think it should be ok as long as `init` node is connected well at the end of this transformation function. Registering new nodes just puts nodes into igvn worklist and no actual igvn is performed in the middle.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13056#discussion_r1187134726