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

From: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org,
	linux-arm-kernel@lists.infradead.org,
	linux-media@vger.kernel.org, linux-wireless@vger.kernel.org,
	linux-mips@vger.kernel.org, linux-sh@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	linuxppc-dev@lists.ozlabs.org, kasan-dev@googlegroups.com,
	linux-xtensa@linux-xtensa.org,
	Michael Karcher <kernel@mkarcher.dialup.fu-berlin.de>
Subject: Calculating array sizes in C - was: Re: Build regressions/improvements in v6.2-rc1
Date: Tue, 17 Jan 2023 17:42:37 +0100	[thread overview]
Message-ID: <3800eaa8-a4da-b2f0-da31-6627176cb92e@physik.fu-berlin.de> (raw)
In-Reply-To: <CAMuHMdXNJveXHeS=g-aHbnxtyACxq1wCeaTg8LbpYqJTCqk86g@mail.gmail.com>

Hi Geert!

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),   \
  }
  
  struct intc_desc {

-- 
  .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
   `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913

