Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex:implement adaptive spinning

[Ingo Molnar]

* Nick Piggin <npiggin@xxxxxxx> wrote:

> > > it seems like a nice opt-in thing that can be used where the aliases 
> > > are verified and the code is particularly performance critical...
> > 
> > Yes. I think we could use it in the kernel, although I'm not sure how 
> > many cases we would ever find where we really care.
> 
> Yeah, we don't tend to do a lot of intensive data processing, so it is 
> normally the cache misses that hurt most as you noted earlier.
> 
> Some places it might be appropriate, though. It might be nice if it can 
> bring code size down too...

I checked, its size effects were miniscule [0.17%] on the x86 defconfig 
kernel and it seems to be a clear loss in total cost as there would be an 
ongoing maintenance cost of this weird new variant of C that language 
lawyers legislated out of thin air and which departs so significantly from 
time-tested C coding concepts and practices.

We'd have to work around aliasing warnings of the compiler again and again 
with no upside and in fact i'd argue that the resulting code is _less_ 
clean.

The lack of data processing complexity in the kernel is not a surprise: 
the kernel is really just a conduit/abstractor between hw and apps, and 
rarely generates genuinely new information. (In fact it can be generally 
considered a broken system call concept if such data processing _has_ to 
be conducted somewhere in the kernel.)

( Notable exceptions would be the crypto code and the RAID5 [XOR checksum]
  and RAID6 [polinomial checksums] code - but those tend to be seriously
  hand-optimized already, with the most critical bits written in assembly. )

	Ingo
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