Re: [Xen-devel] [PATCH 0/2] Improvements with noreturn

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 11/25/2013 02:02 PM, Andrew Cooper wrote:
> On 25/11/13 11:47, George Dunlap wrote:
> > On Mon, Nov 25, 2013 at 10:25 AM, Andrew Cooper
> > <andrew.cooper3@xxxxxxxxxx> wrote:
> > > Make better use of noreturn.  It allows optimising compilers to produce more
> > > efficient code.
> > >
> > > George:
> > >    I request that this is included for 4.4 - It is no functional change, but
> > >    quite a nice improvement in terms of code size.
> > No functional change *if compilers are correct*.  If they're not, for
> > any reason, it will be very difficult to actually catch between now
> > and the release.
> >
> > I'm not inclined to think that a reduction of 6k is a big enough
> > improvement to take the risk at this point.
> >
> >   -George
> Patch 1 is literally just textual replacement, cleaning up its current
> uses in the codebase.
>
> Patch 2 applies its use to more functions in the codebase.
>
> While I agree that it is "No functional change if compilers are
> correct", this kind of paranoia could be applied to any and every patch
> we consider taking.  Anyway, Linux uses it far more than we do at the
> moment.
But what we're talking about here isn't just changing normal code flow 
or what-not; we're talking about a fairly niche optimization being 
applied to functions were it wasn't applied before.  I would be willing 
to bet that "noreturn" has two or three orders of magnitude less usage 
and testing than anything in the "normal" C standard; that's why I 
consider it to be a higher risk than a patch which changes code in a 
"normal" way.
Now sure, it's a fairly tiny risk, but the benefit is pretty tiny too.  
At the moment I don't see a good reason this can't wait until 4.5.
 -George

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