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)
