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Re: [PATCH 04/15] hw/ppc/spapr_rtas: Restrict variables scope to single


From: David Gibson
Subject: Re: [PATCH 04/15] hw/ppc/spapr_rtas: Restrict variables scope to single switch case
Date: Mon, 13 Jan 2020 17:16:48 +1000

On Fri, Jan 10, 2020 at 10:50:55AM +0100, Greg Kurz wrote:
> On Fri, 10 Jan 2020 10:34:07 +0100
> Philippe Mathieu-Daudé <address@hidden> wrote:
> 
> > On 1/9/20 6:43 PM, Greg Kurz wrote:
> > > On Thu,  9 Jan 2020 16:21:22 +0100
> > > Philippe Mathieu-Daudé <address@hidden> wrote:
> > > 
> > >> We only access these variables in RTAS_SYSPARM_SPLPAR_CHARACTERISTICS
> > >> case, restrict their scope to avoid unnecessary initialization.
> > >>
> > > 
> > > I guess a decent compiler can be smart enough detect that the 
> > > initialization
> > > isn't needed outside of the RTAS_SYSPARM_SPLPAR_CHARACTERISTICS branch...
> > > Anyway, reducing scope isn't bad. The only hitch I could see is that some
> > > people do prefer to have all variables declared upfront, but there's a 
> > > nested
> > > param_val variable already so I guess it's okay.
> > 
> > I don't want to outsmart compilers :)
> > 
> > The MACHINE() macro is not a simple cast, it does object introspection 
> > with OBJECT_CHECK(), thus is not free. Since 
> 
> Sure, I understand the motivation in avoiding an unneeded call
> to calling object_dynamic_cast_assert().
> 
> > object_dynamic_cast_assert() argument is not const, I'm not sure the 
> > compiler can remove the call.
> > 
> 
> Not remove the call, but delay it to the branch that uses it,
> ie. parameter == RTAS_SYSPARM_SPLPAR_CHARACTERISTICS.

I think any performance consideration here is a red herring.  This
particular RTAS call is a handful-of-times-per-boot thing, and only
AFAIK used by AIX guests.

I'm in favour of the change on the grounds of code locality and
readability.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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