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
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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