Re: [PATCH] asm-generic: Force inlining of get_order() to work around gcc10 poor decision

[Christophe Leroy]

Le 19/10/2020 à 06:55, Joel Stanley a écrit :
> On Sat, 17 Oct 2020 at 15:55, Christophe Leroy
> <christophe.leroy@xxxxxxxxxx> wrote:
> > When building mpc885_ads_defconfig with gcc 10.1,
> > the function get_order() appears 50 times in vmlinux:
> >
> > [linux]# ppc-linux-objdump -x vmlinux | grep get_order | wc -l
> > 50
> >
> > [linux]# size vmlinux
> >     text    data     bss     dec     hex filename
> > 3842620  675624  135160 4653404  47015c vmlinux
> >
> > In the old days, marking a function 'static inline' was forcing
> > GCC to inline, but since commit ac7c3e4ff401 ("compiler: enable
> > CONFIG_OPTIMIZE_INLINING forcibly") GCC may decide to not inline
> > a function.
> >
> > It looks like GCC 10 is taking poor decisions on this.
> >
> > get_order() compiles into the following tiny function,
> > occupying 20 bytes of text.
> >
> > 0000007c <get_order>:
> >    7c:   38 63 ff ff     addi    r3,r3,-1
> >    80:   54 63 a3 3e     rlwinm  r3,r3,20,12,31
> >    84:   7c 63 00 34     cntlzw  r3,r3
> >    88:   20 63 00 20     subfic  r3,r3,32
> >    8c:   4e 80 00 20     blr
> >
> > By forcing get_order() to be __always_inline, the size of text is
> > reduced by 1940 bytes, that is almost twice the space occupied by
> > 50 times get_order()
> >
> > [linux-powerpc]# size vmlinux
> >     text    data     bss     dec     hex filename
> > 3840680  675588  135176 4651444  46f9b4 vmlinux
>
> I see similar results with GCC 10.2 building for arm32. There are 143
> instances of get_order with aspeed_g5_defconfig.
>
> Before:
>   9071838 2630138  186468 11888444         b5673c vmlinux
> After:
>   9069886 2630126  186468 11886480         b55f90 vmlinux
>
> 1952 bytes smaller with your patch applied. Did you raise this with
> anyone from GCC?

Yes I did, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445

For the time being, it's at a standstill.

Christophe

> Reviewed-by: Joel Stanley <joel@xxxxxxxxx>
>
>
>
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> >   include/asm-generic/getorder.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/asm-generic/getorder.h b/include/asm-generic/getorder.h
> > index e9f20b813a69..f2979e3a96b6 100644
> > --- a/include/asm-generic/getorder.h
> > +++ b/include/asm-generic/getorder.h
> > @@ -26,7 +26,7 @@
> >    *
> >    * The result is undefined if the size is 0.
> >    */
> > -static inline __attribute_const__ int get_order(unsigned long size)
> > +static __always_inline __attribute_const__ int get_order(unsigned long size)
> >   {
> >          if (__builtin_constant_p(size)) {
> >                  if (!size)
> > --
> > 2.25.0