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
