Re: [Xen-devel] [PATCH 10/17] vmx: nest: VMExit handler in L2

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 11:35 +0100 on 21 May (1274441723), Qing He wrote:
> > I understand that.  It just seems inefficient to bundle them all
> > together into one clause of the switch statement and then scan an array
> > looking for which one you've hit.  Wouldn't it be better to give each
> > one its own clause and then use goto (!) or similar to jump to the
> > common code?
> 
> Ok, I'll change it to switch clauses, does it mean to be more friendly to
> the compiler?

No, it's just faster; I don't think GCC can optimize out a while loop,
even scanning a static array with a known limited set of possible inputs
(though i would be delighted to hear otherwise).

Just to be clear, I'm talking about replacing this kind of logic

switch (x) {
case a: 
case b:
case c: 
  for (i = o; i < 3 ; i++)
     if ( x == array[i] ) 
         /* do case-specific thing */
  /* do common case */ 
}

with this equivalent:

switch (x) {
case a: 
     /* do a-specific thing */
     goto common;
case b:
     /* do b-specific thing */
     goto common;
case c: 
     /* do c-specific thing */
     goto common;
common:
  /* do common case */ 
}


Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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