linux-kernel - Re: [PATCH] powerpc: Force inlining of csum

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Date:   Wed, 12 May 2021 13:21:03 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: Force inlining of csum_add()

On Wed, May 12, 2021 at 04:43:33PM +0200, Christophe Leroy wrote:
> Le 12/05/2021 à 16:31, Segher Boessenkool a écrit :
> >On Wed, May 12, 2021 at 02:56:56PM +0200, Christophe Leroy wrote:
> >>Le 11/05/2021 à 12:51, Segher Boessenkool a écrit :
> >>>Something seems to have decided this asm is more expensive than it is.
> >>>That isn't always avoidable -- the compiler cannot look inside asms --
> >>>but it seems it could be improved here.
> >>>
> >>>Do you have (or can make) a self-contained testcase?
> >>
> >>I have not tried, and I fear it might be difficult, because on a kernel
> >>build with dozens of calls to csum_add(), only ip6_tunnel.o exhibits such
> >>an issue.
> >
> >Yeah.  Sometimes you can force some of the decisions, but that usually
> >requires knowing too many GCC internals :-/
> >
> >>>>And there is even one completely unused instance of csum_add().
> >>>
> >>>That is strange, that should never happen.
> >>
> >>It seems that several .o include unused versions of csum_add. After the
> >>final link, one remains (in addition to the used one) in vmlinux.
> >
> >But it is a static function, so it should not end up in any object file
> >where it isn't used.
> 
> Well .... did I dream ?
> 
> Now I only find one extra .o with unused csum_add() : That's 
> net/ipv6/exthdrs.o
> It matches the one found in vmlinux.
> 
> Are you interested in -fdump-tree-einline-all for that one as well ?

Sure.  Hopefully it will show more :-)


Segher