RE: [PATCH] mm: change "char *bdi_unknown_name" to "char bdi_unknown_name[]"

[David Laight]

From: Andrew Morton
> Sent: 12 May 2022 21:01
> 
> On Thu, 12 May 2022 16:26:37 +0800 liqiong <liqiong@xxxxxxxxxxxx> wrote:
> 
> > "char bdi_unknown_nam[]" string form declares a single variable.
> > It is better then "char *bdi_unknown_name" which creates two
> > variables.
> >
> > ...
> >
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -20,7 +20,7 @@ struct backing_dev_info noop_backing_dev_info;
> >  EXPORT_SYMBOL_GPL(noop_backing_dev_info);
> >
> >  static struct class *bdi_class;
> > -static const char *bdi_unknown_name = "(unknown)";
> > +static const char bdi_unknown_name[] = "(unknown)";
> >
> 
> heh, fun patch.  We actually do this quite a lot.
> 
> 	grep -r "^[a-z].*char \*[a-z].*= \"" .
> 
> is a pathetic pattern which catches a lot of them.
> 
> 
> However.  I expected your patch to shrink the kernel a bit, but it has
> the opposite effect:
> 
> hp2:/usr/src/25> size mm/backing-dev.o
>    text	   data	    bss	    dec	    hex	filename
>   21288	   9396	   3808	  34492	   86bc	mm/backing-dev.o-before
>   21300	   9428	   3808	  34536	   86e8	mm/backing-dev.o-after
> 
> Even .data became larger.  I didn't investigate why.

The linker can merge replicated strings
(ie data in .rodata.str1.n sections)
but I don't think the compiler puts variables into that section.

So if you have:
static const char *const foo_xxx = "foo";
in multiple source/object files you get lots of pointers
but only one string.
OTOH with:
static const char foo_xxx[] = "foo";
you get lots of copies of the string.
Which is smaller depends on the number of variables and the length
of the string.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)