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

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.

   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.

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

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.


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