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@xxxxxxxxxxx/
>
> 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
