On Mon, 16 Jun 2014, Andrew Morton wrote:
> 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".
>
On linux-next, allyesconfig has a 0.0001% savings as a result of the
patch, but I'd be worried about the extra temp variable it allocates on
the stack that is evident in the mm/slab.c disassembly unless all cases
can be audited to show that we're not potentially deep.
text data bss dec hex filename
108573045 23488016 51580928 183641989 af22785
vmlinux.before
108572908 23488016 51580928 183641852 af226fc
vmlinux.after
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