linux-kernel - RE: [PATCH net v3] net: Force inlining of checksum functions in net/checksum.h

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Date:   Thu, 17 Feb 2022 13:36:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe Leroy' <christophe.leroy@...roup.eu>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Masahiro Yamada" <yamada.masahiro@...ionext.com>
Subject: RE: [PATCH net v3] net: Force inlining of checksum functions in
 net/checksum.h

From: Christophe Leroy
> Sent: 17 February 2022 12:19
> 
> All functions defined as static inline in net/checksum.h are
> meant to be inlined for performance reason.
> 
> But since commit ac7c3e4ff401 ("compiler: enable
> CONFIG_OPTIMIZE_INLINING forcibly") the compiler is allowed to
> uninline functions when it wants.
> 
> Fair enough in the general case, but for tiny performance critical
> checksum helpers that's counter-productive.

There isn't a real justification for allowing the compiler
to 'not inline' functions in that commit.

It rather seems backwards.
The kernel sources don't really have anything marked 'inline'
that shouldn't always be inlined.
If there are any such functions they are few and far between.

I've had enough trouble (elsewhere) getting gcc to inline
static functions that are only called once.
I ended up using 'always_inline'.
(That is 4k of embedded object code that will be too slow
if it ever spills a register to stack.)

	David

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