On 2019-Dec-26, John Naylor wrote: > In commit ab5b4e2f9ed, we optimized AllocSetFreeIndex() using a lookup > table. At the time, using CLZ was rejected because compiler/platform > support was not widespread enough to justify it. For other reasons, we > recently added bitutils.h which uses __builtin_clz() where available, > so it makes sense to revisit this. I modified the test in [1] (C files > attached), using two separate functions to test CLZ versus the > open-coded algorithm of pg_leftmost_one_pos32(). > > These are typical results on a recent Intel platform: > > HEAD 5.55s > clz 4.51s > open-coded 9.67s
I can confirm these results on my Intel laptop. I ran it with a repetition of 20, averages of 4 runs: clz 1,614 bitutils 3,714 current 2,088 (stddevs are under 0,031). -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
