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From: Julia Lawall <julia.lawall@inria.fr>
To: Joe Perches <joe@perches.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	kernelnewbies <kernelnewbies@kernelnewbies.org>,
	kernel-janitors <kernel-janitors@vger.kernel.org>,
	cocci <cocci@systeme.lip6.fr>,
	Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Subject: Re: [Cocci] linux-kernel janitorial RFP: Mark static arrays as const
Date: Sun, 7 Mar 2021 20:14:11 +0100 (CET)
Message-ID: <alpine.DEB.2.22.394.2103072011480.2930@hadrien> (raw)
In-Reply-To: <a186c9d063663ac6de66db944d1925146393bec5.camel@perches.com>


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On Wed, 3 Mar 2021, Joe Perches wrote:

> On Wed, 2021-03-03 at 10:41 +0100, Rasmus Villemoes wrote:
> > On 02/03/2021 18.42, Joe Perches wrote:
> > > Here is a possible opportunity to reduce data usage in the kernel.
> > >
> > > $ git grep -P -n '^static\s+(?!const|struct)(?:\w+\s+){1,3}\w+\s*\[\s*\]' drivers/ | \
> > >   grep -v __initdata | \
> > >   wc -l
> > > 3250
> > >
> > > Meaning there are ~3000 declarations of arrays with what appears to be
> > > file static const content that are not marked const.
> > >
> > > So there are many static arrays that could be marked const to move the
> > > compiled object code from data to text minimizing the total amount of
> > > exposed r/w data.
> >
> > You can add const if you like, but it will rarely change the generated
> > code. gcc is already smart enough to take a static array whose contents
> > are provably never modified within the TU and put it in .rodata:
>
> At least some or perhaps even most of the time, true, but the gcc compiler
> from v5 through at least v10 seems inconsistent about when it does the
> appropriate conversion.
>
> See the example I posted:
> https://lore.kernel.org/lkml/6b8b250a06a98ce42120a14824531a8641f5e8aa.camel@perches.com/
>
> It was a randomly chosen source file conversion btw, I had no prior
> knowledge of whether the text/data use would change.
>
> I'm unsure about clang consistently moving static but provably const arrays
> from data to text.  I rarely use clang.  At least for v11 it seems to be
> better though.  I didn't try 10.1.

I tried the relevnt drivers in drivers/input/joystick.  I got only one
driver that changed with gcc 9.3, which was
drivers/input/joystick/analog.c.  It actually got larger:

original:

   text    data     bss     dec     hex filename
  22607   10560     320   33487    82cf drivers/input/joystick/analog.o

after adding const:

   text    data     bss     dec     hex filename
  22728   10816     320   33864    8448 drivers/input/joystick/analog.o

This was the only case where bss was not 0, but I don't know if there is a
connection.

julia

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Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-02 17:42 Joe Perches
2021-03-02 21:41 ` Julia Lawall
2021-03-03  2:47   ` Joe Perches
2021-03-02 22:18 ` Bernd Petrovitsch
2021-03-03  8:36   ` Julia Lawall
2021-03-04  8:34   ` Andy Shevchenko
2021-03-03  9:41 ` Rasmus Villemoes
2021-03-03 14:51   ` Joe Perches
2021-03-07 19:14     ` Julia Lawall [this message]
2021-03-08  5:38       ` Joe Perches
2021-03-08  6:54         ` Julia Lawall
2021-03-03 17:06 ` Mansour Moufid
2021-03-03 17:21   ` Julia Lawall

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