Re: [PATCH 0/2] Improvements with noreturn

[George Dunlap <george.dunlap@eu.citrix.com>]

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@citrix.com> 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