From: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
To: Joe Perches <joe@perches.com>,
kernelnewbies <kernelnewbies@kernelnewbies.org>,
kernel-janitors <kernel-janitors@vger.kernel.org>,
cocci <cocci@systeme.lip6.fr>
Cc: LKML <linux-kernel@vger.kernel.org>
Subject: Re: [Cocci] linux-kernel janitorial RFP: Mark static arrays as const
Date: Wed, 3 Mar 2021 10:41:58 +0100 [thread overview]
Message-ID: <a15e5c4d-a60f-14b9-90e5-4e600771aa9d@prevas.dk> (raw)
In-Reply-To: <053b06c47f08631675c295b5c893b90be4248347.camel@perches.com>
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:
static int x = 7;
static int y[2] = {13, 19};
static int p(int a, const int *foo)
{
return a + *foo;
}
int q(int a)
{
int b = p(a, &x);
return p(b, &y[b & 1]);
}
$ nm c.o
0000000000000000 T q
0000000000000000 r y
$ size c.o
text data bss dec hex filename
111 0 0 111 6f c.o
So x gets optimized away completely, but y isn't so easy to get rid of -
nevertheless, it's never modified and the address doesn't escape the TU,
so gcc treats is as if it was declared const.
I think you'll see the same if you try adding the const on a few of your
real-life examples. (Of course, the control flow may be so convoluted
that gcc isn't able to infer the constness, so I'm not saying it will
never make a difference - only that you shouldn't expect too much.)
Rasmus
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next prev parent reply other threads:[~2021-03-03 10:08 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-02 17:42 [Cocci] linux-kernel janitorial RFP: Mark static arrays as const 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 [this message]
2021-03-03 14:51 ` Joe Perches
2021-03-07 19:14 ` Julia Lawall
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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