GiST "choose subtree" support function to inline penalty

I'm looking at PostGIS geometry GiST index build times and try to optimize withing the current GiST framework. The function that shows a lot on my flame graphs is penalty. 

I spent weekend rewriting PostGIS penalty to be as fast as possible. 

(FYI https://github.com/postgis/postgis/pull/425/files) 

However I cannot get any meaningfully faster build time. Even when I strip it to "just return edge extension" index build time is the same.

Is there a way to inline the penalty into above "choose subtree" loop somehow? That would also let us stop bit-fiddling floats to simulate a more complex choosing scheme.

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