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

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Date:   Sun, 07 Mar 2021 21:38:49 -0800
From:   Joe Perches <joe@...ches.com>
To:     Julia Lawall <julia.lawall@...ia.fr>
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 Sun, 2021-03-07 at 20:14 +0100, Julia Lawall wrote:
> 
> 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.

You really need consider using defconfig so whatever object code
does not have tracing/debugging support.

For instance, this code with defconfig and analog joystick:

Original:

$ size drivers/input/joystick/analog.o
   text	   data	    bss	    dec	    hex	filename
   8115	    261	    224	   8600	   2198	drivers/input/joystick/analog.o

with const:

$ size drivers/input/joystick/analog.o
   text	   data	    bss	    dec	    hex	filename
   8179	    201	    224	   8604	   219c	drivers/input/joystick/analog.o