cred: group_is_supplementary(): Use bsearch()

[olce]

Summary

This makes that function use a more efficient version of binary search instead,
and removes one more hand-rolled binary search code from the tree (and the
kernel binary).

[emaste]

sys/kern/kern_prot.c

842

somewhat of a bikeshed comment but an explicit "if (a < b) / else if (b < a) / else" is more immediately clear to me, and a quick check on compilerexplorer showed Clang generated identical code. That said, I'm fine either way and perhaps "(a > b) - (a < b)" is idiomatic.

[olce]

sys/kern/kern_prot.c

842

Yes, (a > b) - (a < b) is the usual idiom for compare functions to avoid tests.

I've tested on Compiler Explorer with latest LLVM 17, 18, 19, and GCC 12, 13 and 14, with -O and -O2, with alternatives: a > b ? 1 : -(a < b) and a full chain of if testing first for a > b and then a < b. Switching to any alternative makes GCC produce test and jump instructions. clang is better, but always generates conditional moves, whereas with the idiom it even doesn't need to (and just uses SETA).

I agree with you it's less readable than just a test. Changing it makes a difference in the produced assembly, albeit probably undetectable in practice in the current context. I'm fine either way.

[emaste]

sys/kern/kern_prot.c

842

I did a brief survey of existing code in the tree and find several existing examples of both forms, so let's leave it as is.

[mhorne]

LGTM

sys/kern/kern_prot.c

839–842

For me this is slightly preferable when trying to pick apart what the return calculation is doing.

Plus, one style nit (return braces).

[emaste]

LGTM assuming @mhorne's comments are addressed

[olce]

Suggested modifications staged in my tree.

[olce]

Rename to group_is_supplementary().

[olce]

Re-validate to make it easier to see where we stand on this series (@mhorne and @emaste validated, and the last comments were addressed).