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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/11/13 14:46, George Dunlap wrote:
> 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

Ok - I will leave this queued up ready for 4.5 to open for general
changes again.

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