I don't know why __builtin_ffs for RISC-V on GCC calls ffs rather than __ffssi2 for its libcall. The latter would seem cleaner to me.

In D54967#1256889, @mhorne wrote:

Fine by me.

A little more context: there were previously a handful of places in the kernel which implemented binary search instead of using the slow ffs/fls variants from our libkern. In 2023, @dougm and I cleaned these up, under the assumption that the compiler builtins would always provide an equally optimal implementation, if not better. See also: D40698, D40703, D40704, D40705, ffaea08319.

So this assumption is violated by this particular combination of riscv built by GCC, and the hidden consequence is that it will have a performance penalty in a couple areas, as compared to clang which picks up the compiler-rt versions of these functions. I do not think this is anything to fret over, but I want to note this as being quite obscure, and likely to be forgotten in the future.

Clang doesn't pick up the compiler-rt versions, it just inlines these things instead of emitting libcalls. GCC tends to fall back to libcalls more often than clang. When I previously tried to build the riscv64 world with GCC it emitted a bunch of libcalls for atomics that are provided by libatomic.a in GCC (and which don't have an equivalent in compiler_rt), but GCC seems to have fixed that and now does direct code-generation instead. My primary goal with building the base system with GCC is getting the extra warning coverage as GCC finds things clang doesn't (and vice versa). I haven't tried to actually boot a kernel/world built with GCC in quite a long time (though I do kind of wish someone was testing that as well).

If we cared about the performance of these, we could certainly make them less dumb following something similar to the ctz* implementations in compiler_rt.