RFR: 8308994: C2: Re-implement experimental post loop vectorization [v2]

[Emanuel Peter]

On Mon, 3 Jul 2023 14:57:50 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Could you elaborate how to do such reservation in C2? Just allocation with some larger sizes at the beginning? Or any other examples to refer?
>
> I think what I have seen people do is just to `map` a high enough index value with `nullptr`. A bit hacky, but it ensures that the arrays underneath get grown sufficiently immediately. It would be nice to have some kind of `reserve` methods... but I don't think we have that.

Also: Use the `VectorSet` instead of the `Unique_Node_List` if you can. It uses less space

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1251002744