On 2019-Feb-15, Tom Lane wrote: > Alvaro Herrera <alvhe...@2ndquadrant.com> writes: > > Ah, I understand it now: > > https://stackoverflow.com/questions/25683690/confusion-about-bsr-and-lzcnt/43443701#43443701 > > if you call LZCNT/TZCNT on a CPU that doesn't support it, it won't raise > > SIGILL or anything ... it'll just silently compute the wrong result. > > That's certainly not what I call a fallback! > > Yeah, that's pretty nasty; it means there's no backstop for whether > your choose function gets it right :-(
Hopefully other tests will fail in some visible way, though. My fear is whether we have such systems in buildfarm. > Is POPCNT any better in this respect? I couldn't find how is POPCNT encoded. https://stackoverflow.com/a/28803917/242383 I did find these articles: http://danluu.com/assembly-intrinsics/ https://stackoverflow.com/questions/25078285/replacing-a-32-bit-loop-counter-with-64-bit-introduces-crazy-performance-deviati This suggests that this all a largely pointless exercise at least on Intel and GCC/Clang. It may be better on AMD ... but to get really better performance we'd need to be coding the popcnt calls in assembly rather than using the compiler intrinsics, even with -mpopcnt, because the intrinsics suck. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
