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

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Date:   Wed, 03 Mar 2021 06:51:02 -0800
From:   Joe Perches <joe@...ches.com>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        kernelnewbies <kernelnewbies@...nelnewbies.org>,
        kernel-janitors <kernel-janitors@...r.kernel.org>,
        cocci <cocci@...teme.lip6.fr>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: linux-kernel janitorial RFP: Mark static arrays as const

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.