On Fri, Dec 27, 2019 at 01:29:47PM -0300, Alvaro Herrera wrote: > 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
There's also various flavors of lczntN for N in (16,32,64) on (relatively) modern architectures. https://docs.microsoft.com/en-us/cpp/intrinsics/lzcnt16-lzcnt-lzcnt64?redirectedfrom=MSDN&view=vs-2019 Best, David. -- David Fetter <david(at)fetter(dot)org> http://fetter.org/ Phone: +1 415 235 3778 Remember to vote! Consider donating to Postgres: http://www.postgresql.org/about/donate
