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

[Christophe Leroy]

Le 19/10/2020 à 10:32, Segher Boessenkool a écrit :
> On Mon, Oct 19, 2020 at 07:50:41AM +0200, Christophe Leroy wrote:
> > Le 19/10/2020 à 06:55, Joel Stanley a écrit :
> > > > 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.
>
> > > 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.
>
> The kernel should just use __always_inline if that is what it *wants*;
> that is true here most likely.  GCC could perhaps improve its heuristics
> so that it no longer thinks these functions are often too big for
> inlining (they *are* pretty big, but not after basic optimisations with
> constant integer arguments).

Yes I guess __always_inline is to be added on functions like this defined in headers for exactly 
that, and that's the purpose of this patch.

However I find it odd that get_order() is outlined by GCC even in some object files that don't use 
it at all, for instance in fs/pipe.o

Christophe