linux-kernel - Re: [PATCH: 1/1] sh4: avoid spurious gcc warning

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Date:   Mon, 23 Jan 2023 17:11:13 +0100
From:   Jakub Jelinek <jakub@...hat.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "'Michael.Karcher'" <Michael.Karcher@...berlin.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Rich Felker <dalias@...c.org>,
        Yoshinori Sato <ysato@...rs.osdn.me>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        "jakub@....gnu.org" <jakub@....gnu.org>
Subject: Re: [PATCH: 1/1] sh4: avoid spurious gcc warning

On Mon, Jan 23, 2023 at 04:06:27PM +0000, David Laight wrote:
> From: Michael.Karcher
> > Sent: 22 January 2023 00:15
> > 
> > Prevent sizeof-pointer-div warning in SH4 intc macros
> > 
> > Gcc warns about the pattern sizeof(void*)/sizeof(void), as it looks like
> > the abuse of a pattern to calculate the array size. This pattern appears
> > in the unevaluated part of the ternary operator in _INTC_ARRAY if the
> > parameter is NULL.
> > 
> > The replacement uses an alternate approach to return 0 in case of NULL
> > which does not generate the pattern sizeof(void*)/sizeof(void), but still
> > emits the warning if _INTC_ARRAY is called with a nonarray parameter.
> > 
> > This patch is required for successful compilation with -Werror enabled.
> > 
> > The idea to use _Generic for type distinction is taken from Comment #7
> > in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108483 by Jakub Jelinek
> > 
> > Signed-off-by: Michael Karcher <kernel@...rcher.dialup.fu-berlin.de>
> > ---
> > 
> > diff --git a/include/linux/sh_intc.h b/include/linux/sh_intc.h
> > index c255273b0281..d7a7ffb60a34 100644
> > --- a/include/linux/sh_intc.h
> > +++ b/include/linux/sh_intc.h
> > @@ -97,7 +97,7 @@ struct intc_hw_desc {
> >       unsigned int nr_subgroups;
> >   };
> > 
> > -#define _INTC_ARRAY(a) a, __same_type(a, NULL) ? 0 : sizeof(a)/sizeof(*a)
> 
> FWIW it is (currently) enough to add 0 to the top or bottom
> of the division.

If you don't want the warning at all, sure.  But if you want the compiler
to warn if you use the macro on a (non-void *) pointer rather than array,
what has been posted is needed.

	Jakub