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

    On Tue, Jan 17, 2023 at 5:42 PM John Paul Adrian Glaubitz
    <glaubitz@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@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

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