Hi, On 2026-03-22 13:43:43 -0400, Greg Burd wrote: > On Sat, Mar 21, 2026, at 10:14 PM, John Naylor wrote: > > On Sat, Mar 21, 2026 at 11:56 PM Greg Burd <[email protected]> wrote: > >> Attached is a small patch that enables hardware popcount on RISC-V when > >> available and also sets the arch flag to 'rv64gc_zbb' flag when > >> appropriate. > > > > I have to ask what the point is -- isn't that like putting a 4-inch > > exhaust tip on a go-kart? > The point is to go fast, right? And to look cool (with awesome 4-inch exhaust > tips) if possible! ;-P > > gburd@rv:~/ws/postgres$ gcc -O2 -o popcnt-wo-zbb riscv-popcnt.c > gburd@rv:~/ws/postgres$ gcc -O2 -march=rv64gc_zbb -o popcnt-zbb riscv-popcnt.c > gburd@rv:~/ws/postgres$ ./popcnt-wo-zbb && ./popcnt-zbb > sw popcount: 0.196 sec ( 510.08 MB/s) > hw popcount: 0.293 sec ( 341.48 MB/s) > > diff: 0.67x > match: 406261900 bits counted > sw popcount: 0.182 sec ( 548.86 MB/s) > hw popcount: 0.044 sec ( 2279.89 MB/s) > > diff: 4.15x > match: 406261900 bits counted > > But my first email/patch was incomplete/rushed, I should have followed the > pattern used for similar ARM-specific logic. v2 attached along with a test > program.
Sure, but what PG workloads are actually affected to a meaningful degree by this? And are those, on riscv, actually most bottlenecked by popcount performance? I'm also pretty doubtful all the effort to e.g. add AVX 512 popcount was spent all that effectively - hard to believe there's any real world workloads where that gain is worth the squeeze. At least for aarch64 and x86-64 there's real world use of those platforms, making niche-y perf improvements somewhat worthwhile. Whereas there's afaict not yet a whole lot of riscv production adoption. Once you add CPU dispatch to the cost it gets a heck of a lot less clearly worthwhile. You need heuristics to decide when the dispatch cost is worth it and even then it's going to slow down your non-worthwhile case somewhat. That's one of the things that make's riscv's decision to put so many crucial features into optional extensions so annoying for people that write non-embedded software. - Andres
