Re: [PATCH v1] KVM: s390: use switch vs jump table in interrupt.c

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 8 Feb 2018 10:53:00 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 02/08/2018 09:14 AM, Cornelia Huck wrote:
> > On Wed, 7 Feb 2018 19:28:04 +0100
> > Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> >   
> >> I see a minimal regression for uperf 1byte ping pong between two guests (~3%)
> >> Probably because the old code first handled IO interrupts and then did the remaing
> >> stuff.  Not sure if its worth to keep the old io_ioirq hack.  
> > 
> > Hm, that confuses me a bit. We search the pending bit map, which should
> > give us the irq with the highest priority, and the switch/case still
> > starts out with I/O interrupts.  
> 
> gcc does not obey the order of the case statements. It uses several heuristics depending
> on the size and others. So gcc might fall back to jump tables for large switches, or
> it uses bisecting or it might even split the search into a jump table and several
> relative branches if there are strange distributions. Quite often the default 
> case is evaulated first.

But should we really try to optimize something that may change with a
different compiler anyway? The important thing is the priority in the
bitmap.