Re: patch make struct protosw pr_input non-variadic

[Taylor R Campbell <campbell+netbsd-tech-net%mumble.net@localhost>]

   Date: Sun, 15 May 2016 10:42:24 -0400
   From: Thor Lancelot Simon <tls%panix.com@localhost>

   On Fri, May 13, 2016 at 11:14:13PM +0000, Christos Zoulas wrote:
   > >
   > >This patch:
   > >* moves pr_input out of struct protosw and places it into <proto>sw
   > >  specitic structure.
   > >* converts domain::dom_protosw to an array of pointers
   > 
   > Why? What's the point of the extra indirection? You could just stick
   > and & in front of the array element assignment.

   I want to reemphasize the potentially negative performance impact of
   this kind of change.  It turns the branch into a computed branch in a
   way that will defeat branch prediction on many CPUs and thus cause
   stalls.  Don't call through pointers if you don't have to.

It may well have a negative performance impact to change

	(*inetsw[ip_proto[nh]].pr_input)(m, off, nh)

in ip_input to

	(*inetsw[ip_proto[nh]]->pr_input)(m, off, nh)

That's why this patch does *not* do that.  The actual change to
ip_input is only

	(*inetsw[ip_proto[nh]].ippr_input)(m, off, nh)

where we can now give struct ipprotosw::ippr_input a true prototype to
distinguish it from, e.g., the IPv6 code which passes &m and &off
instead of m and off.

The only place the extra indirection is used is in paths that are cold
anyway, where the caller is iterating over all protocols in a domain,
mostly in uipc_domain.c, during kernel initialization, socket
creation, routing table updates, &c.