On Sat, May 7, 2016 at 4:26 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > David Rowley <david.row...@2ndquadrant.com> writes: >> I'd like to see us using those functions, when they're available and >> falling back on the array when they're not. Sounds like that would >> just be a new configure test. Perhaps a good home for some shared code >> would be numutils.c. > > Meh --- numutils.c is about numbers. Maybe "bitutils.c"? > > Another point here is that there might easily be a use-case for such > functionality in frontend code, so I'd lean towards putting the support in > src/common if we do this.
I played around with this a bit on the weekend (see rough sketch attached). The trouble with __builtin_popcount and the POPCNT instruction is that you only get them if you ask for -msse4.2 or -mpopcnt, and then you get an illegal instruction trap instead of sensible fallback behaviour on ancient hardware, so no packager would be able to do that. So I guess we'd have to use a runtime test like we do for CRC32 hardware support (the test may actually be identical since both depend on the SSE4.2 instruction set) and maybe inline assembler rather the GCC builtin, and that seems a bit over the top unless someone can show that it's worth it. My aim with this thread was mainly reducing code duplication and needless code: perhaps at least the other ideas in the attached sketch, namely using ffs instead of the rightmost_one_pos table loop and consolidation of popcount into a reusable API (without trying to get hardware support) could be worth polishing for the next CF? Annoyingly, it seems Windows doesn't have POSIX/SUSv2 ffs, though apparently it can reach that instruction with MSVC intrinsic _BitScanReverse or MingW __builtin_ffs. -- Thomas Munro http://www.enterprisedb.com
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