On Mon, 16 Jun 2014 16:25:15 -0700 (PDT) David Rientjes <rient...@google.com>
wrote:
> On Mon, 16 Jun 2014, Andrew Morton wrote:
>
> > > It appears that gcc is better at optimising a double call to min
> > > and max rather than open coded min3 and max3. This can be observed
> > > here:
> > >
> > > ...
> > >
> > > Furthermore, after ___make allmodconfig && make bzImage modules___ this
> > > is the
> > > comparison of image and modules sizes:
> > >
> > > # Without this patch applied
> > > $ ls -l arch/x86/boot/bzImage **/*.ko |awk '{size += $5} END {print
> > > size}'
> > > 350715800
> > >
> > > # With this patch applied
> > > $ ls -l arch/x86/boot/bzImage **/*.ko |awk '{size += $5} END {print
> > > size}'
> > > 349856528
> >
> > We saved nearly a megabyte by optimising min3(), max3() and clamp()?
> >
> > I'm counting a grand total of 182 callsites for those macros. So the
> > saving is 4700 bytes per invokation? I don't believe it...
> >
>
> I was checking just the instances of min3() in mm/ and gcc ends up
> inlining transfer_objects() in mm/slab.c as a result of this change and
> increases its text size:
>
> text data bss dec hex filename
> 28369 21559 4 49932 c30c slab.o.before
> 28399 21559 4 49962 c32a slab.o.after
Maybe that's a good thing in disguise: gcc said "hey this thing is now
small enough to inline it".
> It also seems to use one additional temp variable of type typeof(x) on the
> stack, so I do think the old version was superior.
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