linux-kernel - Re: linux-kernel janitorial RFP: Mark static arrays as const

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Message-ID: <alpine.DEB.2.22.394.2103072011480.2930@hadrien>
Date:   Sun, 7 Mar 2021 20:14:11 +0100 (CET)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Joe Perches <joe@...ches.com>
cc:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        kernelnewbies <kernelnewbies@...nelnewbies.org>,
        kernel-janitors <kernel-janitors@...r.kernel.org>,
        cocci <cocci@...teme.lip6.fr>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: linux-kernel janitorial RFP: Mark static arrays as const



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