Hi, On 2019-02-14 15:47:13 -0300, Alvaro Herrera wrote: > On 2019-Feb-14, Tom Lane wrote: > > > Some further thoughts here ... > > > > Does the "lzcnt" runtime probe actually do anything useful? > > On the x86_64 compilers I tried (gcc 8.2.1 and 4.4.7), __builtin_clz > > and __builtin_ctz compile to sequences involving bsrq and bsfq > > regardless of -mpopcnt. It's fairly hard to see how lzcnt would > > buy anything over those sequences even if there were zero overhead > > involved in using it. > > Hah, I just realized you have to add -mlzcnt in order for these builtins > to use the lzcnt instructions. It goes from something like > > bsrq %rax, %rax > xorq $63, %rax
I'm confused how this is a general count leading zero operation? Did you use constants or something that allowed ot infer a range in the test? If so the compiler probably did some optimizations allowing it to do the above. > to > lzcntq %rax, %rax > > Significant? If I understand Agner's tables correctly, then no, this isn't faster than the two instructions above. Greetings, Andres Freund
