Re: use CLZ instruction in AllocSetFreeIndex()

On 2019-Dec-27, Tom Lane wrote:

> This kind of leads me to wonder if we don't need to expend more
> effort on the non-CLZ version of pg_leftmost_one_pos32; it seems
> like it shouldn't be losing this badly to the only-slightly-
> improved logic that's currently in AllocSetFreeIndex.  On the
> other hand, the buildfarm thinks that __builtin_clz is essentially
> universal these days --- the only active non-MSVC critter that
> reports not having it is anole.  So maybe it's not worth sweating
> over that.  Perhaps what we really ought to be working on is
> finding MSVC equivalents for __builtin_clz and friends.

Apparently clz() can be written using _BitScanReverse(), per
https://stackoverflow.com/a/20468180
https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanreverse-bitscanreverse64?view=vs-2015

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Álvaro Herrera                https://www.2ndQuadrant.com/
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