Shouldn't we also formally request page alignment for zones with size > PAGE_SIZE?

I am not sure that this usage makes much sense, since at least busdma code wouldn't do it. Anyway, I changed it.

This also needs change to size = roundup2(align), I think.

MIN from param.h?

I believe that an allocation request for 384 bytes with 128 byte alignment will still go to the malloc-384 zone, yes? But, we aren't requiring better than UMA_ALIGN_PTR alignment.

bde@ did not liked the macro AFAIR.

Isn't this going to be slow on architectures without that instruction? What are the actual requirements here? This could at least do with a comment; flsl is a rather obscure function that's non-obvious unless you've seen it before.

A "count leading zeroes" instruction is available on every widely used architecture, including all of the architectures supported by FreeBSD. That said, the last time that I looked at the various cpufunc.h implementations, only 32-bit and 64-bit arm and x86 used the builtins that provide access to these instructions.

The objective is to round up to the next power of 2, because we can only guarantee alignment for power-of-2-sized allocations. I agree that a comment to this effect would help.

Ok, which then confirms my suspicion that this is wrong: if size is a power of two it doubles the allocation size. https://godbolt.org/z/a6dhEq (using the arm64 implementation since it's a simple one in terms of the builtin).

(you want to subtract 1 before passing to flsl, and be careful about 0 - 1 if that's possible)

I think that this sentence would be clearer without "at size".

Isn't the following one line sufficient?

asize = size < align ? align : 1UL << flsl(size - 1);