On 2019-Feb-15, Alvaro Herrera wrote: > On 2019-Feb-15, Kyotaro HORIGUCHI wrote:
> > And even worse lzcntx is accidentially "fallback"s to bsrx on > > unsupported CPUs, which leads to bogus results. > > __builtin_clzll(8) = 3, which should be 60. > > I'm not sure I understand this point. Are you saying that if we use > -mlzcnt to compile, then the compiler builtin is broken in platforms > that don't support the lzcnt instruction? That's horrible. Let's stay > away from -mlzcnt then. 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! I think David's code was correct because it was testing CPUID for instruction support before using the specialized code (well, except that he forgot to add the right compiler option to *enable* the LZCNT/TZCNT instructions in the first place); but given subsequent discussion that the instruction is not worth it anyway, we might as well ignore it. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
