Calculating array sizes in C - was: Re: Build regressions/improvements in v6.2-rc1

[Geert Uytterhoeven]

Hi Adrian,

On Tue, Jan 17, 2023 at 5:42 PM John Paul Adrian Glaubitz
<glaubitz at physik.fu-berlin.de> wrote:
> On 1/6/23 16:17, Geert Uytterhoeven wrote:
> >> I'm not seeing this one, but I am getting this one instead:
> >>
> >> In file included from ./arch/sh/include/asm/hw_irq.h:6,
> >>                    from ./include/linux/irq.h:596,
> >>                    from ./include/asm-generic/hardirq.h:17,
> >>                    from ./arch/sh/include/asm/hardirq.h:9,
> >>                    from ./include/linux/hardirq.h:11,
> >>                    from ./include/linux/interrupt.h:11,
> >>                    from ./include/linux/serial_core.h:13,
> >>                    from ./include/linux/serial_sci.h:6,
> >>                    from arch/sh/kernel/cpu/sh2/setup-sh7619.c:11:
> >> ./include/linux/sh_intc.h:100:63: error: division 'sizeof (void *) / sizeof (void)' does not compute the number of array elements [-Werror=sizeof-pointer-div]
> >>     100 | #define _INTC_ARRAY(a) a, __same_type(a, NULL) ? 0 : sizeof(a)/sizeof(*a)
> >>         |                                                               ^
> >> ./include/linux/sh_intc.h:105:31: note: in expansion of macro '_INTC_ARRAY'
> >>     105 |         _INTC_ARRAY(vectors), _INTC_ARRAY(groups),      \
> >>         |                               ^~~~~~~~~~~
> >
> > The easiest fix for the latter is to disable CONFIG_WERROR.
> > Unfortunately I don't know a simple solution to get rid of the warning.
>
> I did some research and it seems that what the macro _INT_ARRAY() does with "sizeof(a)/sizeof(*a)"
> is a commonly used way to calculate array sizes and the kernel has even its own macro for that
> called ARRAY_SIZE() which Linus asks people to use here [1].
>
> So, I replaced _INTC_ARRAY() with ARRAY_SIZE() (see below), however the kernel's own ARRAY_SIZE()
> macro triggers the same compiler warning. I'm CC'ing Michael Karcher who has more knowledge on
> writing proper C code than me and maybe an idea how to fix this warning.
>
> Thanks,
> Adrian
>
> > [1] https://lkml.org/lkml/2015/9/3/428
>
> diff --git a/include/linux/sh_intc.h b/include/linux/sh_intc.h
> index c255273b0281..07a187686a84 100644
> --- a/include/linux/sh_intc.h
> +++ b/include/linux/sh_intc.h
> @@ -97,14 +97,12 @@ struct intc_hw_desc {
>          unsigned int nr_subgroups;
>   };
>
> -#define _INTC_ARRAY(a) a, __same_type(a, NULL) ? 0 : sizeof(a)/sizeof(*a)
> -
>   #define INTC_HW_DESC(vectors, groups, mask_regs,       \
>                       prio_regs, sense_regs, ack_regs)   \
>   {                                                      \
> -       _INTC_ARRAY(vectors), _INTC_ARRAY(groups),      \
> -       _INTC_ARRAY(mask_regs), _INTC_ARRAY(prio_regs), \
> -       _INTC_ARRAY(sense_regs), _INTC_ARRAY(ack_regs), \
> +       ARRAY_SIZE(vectors), ARRAY_SIZE(groups),        \
> +       ARRAY_SIZE(mask_regs), ARRAY_SIZE(prio_regs),   \
> +       ARRAY_SIZE(sense_regs), ARRAY_SIZE(ack_regs),   \
>   }

The issue is that some of the parameters are not arrays, but
NULL. E.g.:

arch/sh/kernel/cpu/sh2/setup-sh7619.c:static
DECLARE_INTC_DESC(intc_desc, "sh7619", vectors, NULL,
arch/sh/kernel/cpu/sh2/setup-sh7619.c-                   NULL,
prio_registers, NULL);
--

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds