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

From: Rob Landley <rob@landley.net>
To: "Michael.Karcher" <Michael.Karcher@fu-berlin.de>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	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>,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: Calculating array sizes in C - was: Re: Build regressions/improvements in v6.2-rc1
Date: Thu, 19 Jan 2023 21:31:21 -0600	[thread overview]
Message-ID: <0f51dac4-836b-0ff2-38c6-5521745c1c88@landley.net> (raw)
In-Reply-To: <1732342f-49fe-c20e-b877-bc0a340e1a50@fu-berlin.de>

On 1/19/23 16:11, Michael.Karcher wrote:
> Isn't this supposed to be caught by this check:
>>>>
>>>>          a, __same_type(a, NULL)
>>>>
>>>> ?
>>>
>>> Yeah, but gcc thinks it is smarter than us...
>>> Probably it drops the test, assuming UB cannot happen.
>> Hmm, sounds like a GGC bug to me then. Not sure how to fix this then.
> 
> 
> I don't see a clear bug at this point. We are talking about the C expression
> 
>    __same_type((void*)0, (void*)0)? 0 : sizeof((void*)0)/sizeof(*((void*0))

*(void*) is type "void" which does not have a size.

The problem is gcc "optimizing out" an earlier type check, the same way it
"optimizes out" checks for signed integer math overflowing, or "optimizes out" a
comparison to pointers from two different local variables from different
function calls trying to calculate the amount of stack used, or "optimizes out"
using char *x = (char *)1; as a flag value and then doing "if (!(x-1)) because
it can "never happen"...
> I suggest to file a bug against gcc complaining about a "spurious 
> warning", and using "-Werror -Wno-error-sizeof-pointer-div" until gcc is 
> adapted to not emit the warning about the pointer division if the result 
> is not used.

Remember when gcc got rewritten in c++ starting in 2007?

Historically the main marketing push of C++ was that it contains the whole of C
and therefore MUST be just as good a language, the same way a mud pie contains
an entire glass of water and therefore MUST be just as good a beverage. Anything
C can do that C++ _can't_ do is seen as an existential threat by C++ developers.
They've worked dilligently to "fix" C not being a giant pile of "undefined
behavior" the way C++ is for 15 years now.

I have... opinions on this.

> Regards,
>    Michael Karcher

Rob