Re: [RFC][PATCH 3/5] refcount: Improve out-of-line code-gen

[Linus Torvalds]

On Thu, Dec 9, 2021 at 12:33 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> For some obscure raisin it causes this: [snip]

Looks like the code generation at one point thought that it needs to
get the return value from the slow-path function call, and by the time
it had optimized that away it had already ended up with enough
temporaries that it needed to spill things into %rbx.

That looks like a fairly "internal to gcc" thing, but I think dropping
that patch is indeed the right thing to do.

When you made it do

        return refcount_warn_saturate(r, ..);

it now means that those inline functions don't obviously always return
'false' any more, so the compiler did end up needing a variable for
the return value at one point, and it really was a much more complex
decision. After it has inlined that top-level function, the return
value is not visible to the compiler because it doesn't see that
refcount_warn_saturate() always returns false.

The fact that quite often that whole refcount_warn_saturate() call
ended up then being in a cold part of the code, and could then be
optimized to a tailcall if that was the last thing generated, doesn't
end up helping in the general case, and instead only hurts: the
compiler doesn't see what the return value is for the warning case, so
it might end up doing other things in the place that the function was
inlined into.

I think the original patch was likely a mis-optimization triggered by
your test-case being a function that *only* did that
refcount_add_not_zero(), and returned the value. It didn't actually
have other code that then depended on the return value of it.

              Linus