* Christoph Lameter <c...@linux.com> wrote:
> On Mon, 14 Sep 2015, Austin S Hemmelgarn wrote:
>
> >I can comment at least a little about the -Os aspect (although not I'm no
> >expert on this in particular). In general, for _most_ use cases, a kernel
> >compiled with CONFIG_CC_OPTIMIZE_FOR_SIZE will run slower than one compiled
> >without it. On rare occasion though, it may actually run faster, the only
> >cases I've seen where this happens are specialized uses that are very memory
> >pressure dependent and run almost entirely in userspace with almost no
> >syscalls (for example math related stuff operating on _very, very big_ (as
> >in,
> >>1 trillion elements) multidimensional matrices, with complex memory
> >constraints), and even then it's usually a miniscule improvement in
> >performance (generally less than 1%, which can of course be significant
> >depending on how long it takes before the improvement).
>
> Cache footprint depends on size which has a significant impact on
> performance. In our experience the kernel (and any other code) is
> generally faster if optimized for size.
Unfortunately, GCC overdoes -Os generating outright silly code, which makes the
result generally slower - despite the reduced instruction count and reduced
cache
footprint.
We've recently applied patches to the x86 tree that give us a good chunk of the
size savings that -Os brings:
52648e83c9a6 x86: Pack loops tightly as well
be6cb02779ca x86: Align jump targets to 1-byte boundaries
these two shave about 5% off from the typical distro kernel's size. That's
still
way off the 15%-20% that -Os can muster, but another ~10% are possible by not
aligning functions to byte boundaries (instead of the default 16 bytes).
So about 70% of the -Os size win is from simple and pure alignment relaxation,
not
from any deeper compiler optimizations.
So LLVM could emulate most of the good effects of -Os by only compressing the
various alignment parameters - and this would be a pretty safe optimization as
well.
Thanks,
Ingo
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