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-xtensa@linux-xtensa.org, Arnd Bergmann <arnd@arndb.de>,
linux-sh@vger.kernel.org, linux-wireless@vger.kernel.org,
linux-mips@vger.kernel.org, amd-gfx@lists.freedesktop.org,
linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
Michael Karcher <kernel@mkarcher.dialup.fu-berlin.de>,
linux-f2fs-devel@lists.sourceforge.net,
linuxppc-dev@lists.ozlabs.org,
linux-arm-kernel@lists.infradead.org,
linux-media@vger.kernel.org
Subject: Re: [f2fs-dev] 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
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next prev parent reply other threads:[~2023-01-20 4:28 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAHk-=wgf929uGOVpiWALPyC7pv_9KbwB2EAvQ3C4woshZZ5zqQ@mail.gmail.com>
[not found] ` <20221227082932.798359-1-geert@linux-m68k.org>
[not found] ` <alpine.DEB.2.22.394.2212270933530.311423@ramsan.of.borg>
2023-01-06 15:10 ` [f2fs-dev] Build regressions/improvements in v6.2-rc1 John Paul Adrian Glaubitz
2023-01-06 15:17 ` Geert Uytterhoeven
2023-01-06 15:18 ` Geert Uytterhoeven
2023-01-17 16:42 ` [f2fs-dev] Calculating array sizes in C - was: " John Paul Adrian Glaubitz
2023-01-17 17:01 ` Geert Uytterhoeven
2023-01-17 17:06 ` John Paul Adrian Glaubitz
2023-01-17 20:05 ` Geert Uytterhoeven
2023-01-17 20:37 ` John Paul Adrian Glaubitz
2023-01-19 22:11 ` Michael.Karcher
2023-01-20 3:31 ` Rob Landley [this message]
2023-01-20 10:53 ` Segher Boessenkool
2023-01-20 11:42 ` David Laight
2023-01-20 18:29 ` Michael.Karcher
2023-01-20 8:49 ` John Paul Adrian Glaubitz
[not found] ` <def16c9b-7bb1-a454-0896-b063a9e85964@fu-berlin.de>
2023-01-21 21:26 ` John Paul Adrian Glaubitz
2023-01-06 15:39 ` [f2fs-dev] " Alex Deucher
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