Re: [PATCH v3 2/2] minmax: clamp more efficiently by avoiding extra comparison

Kees Cook
2022-09-26T18:30:21Z

On Mon, Sep 26, 2022 at 03:34:35PM +0200, Jason A. Donenfeld wrote:
> [...]
> In this case, we actually gain a branch, unfortunately, because the
> compiler's replacement axioms no longer as cleanly apply.
>
> So all and all, this change is a bit of a mixed bag.

I'm on the fence -- I think the new macro is a more correct way to
describe the operation, though on the other hand, the old way provides a
simple way to compose the bounds checks.

I suspect we should probably optimize for _performance_, not code size,
so if the new branch is actually visible via cycle counts in "perf"
output, probably we shouldn't use this patch, and instead add a comment
about why it is defined the way it is.

-Kees

--
Kees Cook