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From: "danielmicay at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/67999] Wrong optimization of pointer comparisons
Date: Mon, 19 Oct 2015 08:47:00 -0000 [thread overview]
Message-ID: <bug-67999-4-a5usaMI5dN@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-67999-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67999
--- Comment #10 from Daniel Micay <danielmicay at gmail dot com> ---
(In reply to Florian Weimer from comment #7)
> If this is not a GCC bug and it is the responsibility of allocators not to
> produce huge objects, do we also have to make sure that no object crosses
> the boundary between 0x7fff_ffff and 0x8000_0000? If pointers are treated
> as de-facto signed, this is where signed overflow would occur.
No, that's fine. It's the offsets that are treated as ptrdiff_t. Clang/LLVM
handle it the same way. There's a very important assumption for optimizations
that pointer arithmetic cannot wrap (per the standard) and all offsets are
treated as signed integers. AFAIK, `ptr + size` is equivalent to `ptr +
(ptrdiff_t)size` in both Clang and GCC.
There's documentation on how this is handled in LLVM IR here, specifically the
inbounds marker which is added to all standard C pointer arithmetic:
http://llvm.org/docs/LangRef.html#getelementptr-instruction
I expect GCC works very similarly, but I'm not familiar with the GCC internals.
It's not really a compiler bug because the standard allows object size limits,
but the compiler and standard C library both need to be aware of those limits
and enforce them if they exist. So it's a bug in GCC + glibc or Clang + glibc,
not either of them alone. I think dealing with it in libc is the only full
solution though due to issues like `p - q` and the usage of ssize_t for sizes
in functions like read/write.
next prev parent reply other threads:[~2015-10-19 8:47 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <bug-67999-4@http.gcc.gnu.org/bugzilla/>
2015-10-17 8:03 ` glisse at gcc dot gnu.org
2015-10-17 8:14 ` pinskia at gcc dot gnu.org
2015-10-17 8:35 ` schwab@linux-m68k.org
2015-10-17 12:52 ` ch3root at openwall dot com
2015-10-19 2:30 ` danielmicay at gmail dot com
2015-10-19 5:36 ` bugdal at aerifal dot cx
2015-10-19 8:17 ` fw at gcc dot gnu.org
2015-10-19 8:26 ` fw at gcc dot gnu.org
2015-10-19 8:41 ` danielmicay at gmail dot com
2015-10-19 8:47 ` danielmicay at gmail dot com [this message]
2015-10-19 9:05 ` fw at gcc dot gnu.org
2015-10-19 9:09 ` fw at gcc dot gnu.org
2015-10-19 9:12 ` danielmicay at gmail dot com
2015-10-19 9:26 ` danielmicay at gmail dot com
2015-10-19 9:55 ` danielmicay at gmail dot com
2015-10-19 9:56 ` rguenth at gcc dot gnu.org
2015-10-19 10:12 ` jakub at gcc dot gnu.org
2015-10-21 2:09 ` ch3root at openwall dot com
2015-10-21 2:18 ` ch3root at openwall dot com
2015-10-21 3:21 ` danielmicay at gmail dot com
2015-10-28 0:12 ` joseph at codesourcery dot com
2015-10-28 0:20 ` bugdal at aerifal dot cx
2015-10-28 2:29 ` joseph at codesourcery dot com
2015-10-28 18:26 ` ch3root at openwall dot com
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