Re: [PATCH] ext3_clear_inode(): avoid kfree(NULL)

[Arjan van de Ven]

On Sat, 2006-06-24 at 08:33 -0400, Steven Rostedt wrote:
> On Sat, 24 Jun 2006, Arjan van de Ven wrote:
> 
> > On Sat, 2006-06-24 at 08:20 -0400, Steven Rostedt wrote:
> > > On Sat, 24 Jun 2006, Arjan van de Ven wrote:
> > >
> > > >
> > > > >
> > > > > Because at that callsite, NULL is the common case.  We avoid a do-nothing
> > > > > function call most of the time.  It's a nano-optimisation.
> > > >
> > > > but a function call is basically free, while an if () is not... even
> > > > with unlikely()...
> > > >
> > > > sounds like a misoptimization to me.
> > > >
> > >
> > > How is a function call free when an if is not?
> >
> > in general, a function call is 100% predictable without any real control
> > flow dependencies for the processor, and thus there is no real issue in
> > the execution pipeline. An if is a conditional branch, which breaks up
> > the execution pipeline if mispredicted...
> 
> But doesn't the unlikely help the prediction? 

nope none at all, at least not on x86/x86-64.
(in fact there is no way to help the prediction on those architectures
that actually works)

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