On Fri, Dec 27, 2019 at 9:54 AM Tom Lane <t...@sss.pgh.pa.us> wrote: > > Anyway, getting back to the presented patch, I find myself a bit > dissatisfied with it because it seems like it's leaving something > on the table. Specifically, looking at the generated assembly > code on a couple of architectures, the setup logic generated by > > tsize = (size - 1) >> ALLOC_MINBITS; > > looks like it costs as much or more as the clz proper. I'm not > sure we can get rid of the subtract-one step,
As I understand it, the desired outcome is ceil(log2(size)), which can be computed by clz(size - 1) + 1. > but couldn't the > right shift be elided in favor of changing the constant we > subtract clz's result from? Shifting off those bits separately > made sense in the old implementation, but assuming that CLZ is > more or less constant-time, it doesn't with CLZ. That makes sense -- I'll look into doing that. -- John Naylor https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
