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

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Date:   Wed, 12 May 2021 16:43:33 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>
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()



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 ?

Christophe