Re: Bounds checking, Optimization techniques and undefined behavior

From	Andy Walker <anw@cuboid.co.uk>
Newsgroups	comp.compilers
Subject	Re: Bounds checking, Optimization techniques and undefined behavior
Date	2019-05-06 14:40 +0100
Organization	Not very much
Message-ID	<19-05-039@comp.compilers> (permalink)
References	(10 earlier) <19-05-020@comp.compilers> <19-05-024@comp.compilers> <19-05-025@comp.compilers> <19-05-028@comp.compilers> <19-05-032@comp.compilers>

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On 06/05/2019 01:15, our esteemed moderator wrote:
> [In the struct { int a,b,c,d; } S example it is my understanding that &S and &S.a
> have to be the same,

	Not "the same", as they have different types;  but yes, they must
compare equal.

>		       the four ints have to be in the order declared,

	Yes, and they must be "sequentially allocated".  Whether that means
the same as "contiguously allocated" [like array members] is not entirely
clear, as the C Standard doesn't define these terms.  Structures can contain
padding, in general, but [AFAICT] not in a way that affects this debate.
At least it seems to be the case that &S.a+1 must compare equal to &S.b,
see N1570, section 6.5.9 para 6, and footnote 109;  but I can't quite make
even this case watertight.

>								       and they
> all have the same alignment so it is valid abeit poor form to treat them as
> an array. -John]

	For the purposes of pointer arithmetic [and therefore arrays] and
comparison, non-array objects are treated as arrays with a single element.
So &S+0 and &S+1 are valid, though the latter can't be dereferenced;
likewise &S.a+0 and &S.a+1, and they can be compared with &S.b, etc.  But
I can see no support in the Standard for &S.a+2 being anything other than
UB;  it points neither to "the" element of S.a, nor one past.  That's even
though [or if!] &S.a+1 == &S.b and &S.b+1 == &S.c;  Section 6.5.6, paras
8 and 9.  As George Neuner says, if you want to use S also as an array,
then you should have declared it as a union.

	In Real Life, you will surely get away with such things, and with
much technically undefined pointer arithmetic that temporarily strays
outside an array [without dereference].  It's sanctioned by dubious custom,
but not by the Standard.  [Yes, I've done it myself.  I'm suitably ashamed
and am hereby rapped on the knuckles.]

--
Andy Walker,
Nottingham.

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Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-04-25 21:58 +0200
  Re: Optimization techniques Kaz Kylheku <847-115-0292@kylheku.com> - 2019-04-26 00:18 +0000
    Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-04-28 23:49 +0200
      Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-04-29 00:31 +0100
        Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-04-29 17:08 +0200
          Re: Optimization techniques and undefined behavior Christian Gollwitzer <auriocus@gmx.de> - 2019-04-29 18:10 +0200
            Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-04-30 14:46 +0200
              Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-01 13:53 +0100
                Re: Optimization techniques and undefined behavior Andy Walker <anw@cuboid.co.uk> - 2019-05-02 11:29 +0100
                Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-03 00:48 +0100
                Re: Optimization techniques and undefined behavior Martin Ward <martin@gkc.org.uk> - 2019-05-03 10:52 +0100
                Re: Optimization techniques and undefined behavior George Neuner <gneuner2@comcast.net> - 2019-05-04 17:44 -0400
                Re: Bounds checking, Optimization techniques and undefined behavior George Neuner <gneuner2@comcast.net> - 2019-05-05 17:10 -0400
                Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-06 08:14 +0200
                Re: Optimization techniques and undefined behavior Gene Wirchenko <genew@telus.net> - 2019-05-11 22:25 -0700
                Re: not a lot of memory, was Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-03 12:45 +0100
                Re: Optimization techniques and undefined behavior Andy Walker <anw@cuboid.co.uk> - 2019-05-03 13:29 +0100
                Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-03 23:10 +0100
                Re: Optimization techniques and undefined behavior Andy Walker <anw@cuboid.co.uk> - 2019-05-04 10:45 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-05 11:14 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2019-05-05 20:44 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2019-05-06 10:15 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-07 11:04 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior "Nuno Lopes" <nuno.lopes@ist.utl.pt> - 2019-05-07 22:38 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-08 01:14 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-08 09:55 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior "Derek M. Jones" <derek@_NOSPAM_knosof.co.uk> - 2019-05-08 19:08 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior Andy Walker <anw@cuboid.co.uk> - 2019-05-08 01:42 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-08 10:16 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-09 01:15 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-09 21:56 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-08 10:03 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-09 09:19 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior Kaz Kylheku <847-115-0292@kylheku.com> - 2019-05-10 03:38 +0000
                Re: Bounds checking, Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-08 14:37 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior Christopher F Clark <christopher.f.clark@compiler-resources.com> - 2019-05-06 05:05 -0400
                Re: Bounds checking, Optimization techniques and undefined behavior George Neuner <gneuner2@comcast.net> - 2019-05-05 17:38 -0400
                Re: Bounds checking, Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-06 13:07 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-07 14:01 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior Andy Walker <anw@cuboid.co.uk> - 2019-05-06 01:15 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior Andy Walker <anw@cuboid.co.uk> - 2019-05-06 14:40 +0100
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-07 15:05 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-08 10:18 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior Jan Ziak <0xe2.0x9a.0x9b@gmail.com> - 2019-05-06 05:39 -0700
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-07 15:42 +0200
                Re: Bounds checking, Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-06 16:32 +0200
                Re: Optimization techniques and undefined behavior George Neuner <gneuner2@comcast.net> - 2019-05-04 17:59 -0400
                Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-02 16:51 +0200
                Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-02 20:04 +0100
                Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-03 17:23 +0200
                Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-03 21:10 +0100
                Re: Optimization techniques and undefined behavior Martin Ward <martin@gkc.org.uk> - 2019-05-06 13:25 +0100
                Re: Optimization techniques and undefined behavior "Derek M. Jones" <derek@_NOSPAM_knosof.co.uk> - 2019-05-06 16:32 +0100
                Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-07 16:03 +0200
                Re: Optimization techniques and undefined behavior Martin Ward <martin@gkc.org.uk> - 2019-05-08 13:16 +0100
                Re: Optimization techniques and undefined behavior George Neuner <gneuner2@comcast.net> - 2019-05-08 15:13 -0400
                Re: Optimization techniques and undefined behavior "Robin Vowels" <robin51@dodo.com.au> - 2019-05-07 01:22 +1000
                Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-07 16:05 +0200
                Re: Optimization techniques and undefined behavior Christian Gollwitzer <auriocus@gmx.de> - 2019-05-02 22:22 +0200
          Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-04-29 18:15 +0100
            Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-04-30 15:48 +0200
              Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-01 12:40 +0100
                Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-02 17:27 +0200
                Re: Optimization techniques and undefined behavior Bart <bc@freeuk.com> - 2019-05-02 18:59 +0100
                Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-07 16:16 +0200
              Re: Optimization techniques and undefined behavior Martin Ward <martin@gkc.org.uk> - 2019-05-02 14:54 +0100
      Re: Optimization techniques and runtime checks Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2019-04-29 22:36 +0200
        Re: Optimization techniques and runtime checks David Brown <david.brown@hesbynett.no> - 2019-05-07 16:29 +0200
          Re: Optimization techniques and runtime checks Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2019-05-08 02:27 +0200
            Re: Optimization techniques and runtime checks David Brown <david.brown@hesbynett.no> - 2019-05-08 10:31 +0200
              Re: Optimization techniques and runtime checks Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2019-05-08 22:50 +0200
              Re: Optimization techniques and runtime checks "Robin Vowels" <robin51@dodo.com.au> - 2019-05-11 19:26 +1000
              Re: Optimization techniques and runtime checks Gene Wirchenko <genew@telus.net> - 2019-05-11 22:43 -0700
                Re: Optimization techniques and runtime checks David Brown <david.brown@hesbynett.no> - 2019-05-12 20:17 +0200
          Re: Optimization techniques and runtime checks Bart <bc@freeuk.com> - 2019-05-08 14:58 +0100
            Re: Optimization techniques and runtime checks David Brown <david.brown@hesbynett.no> - 2019-05-08 23:02 +0200
              Re: Optimization techniques and runtime checks Bart <bc@freeuk.com> - 2019-05-09 18:28 +0100
                Re: Optimization techniques and runtime checks David Brown <david.brown@hesbynett.no> - 2019-05-09 22:07 +0200
      Re: Optimization techniques Gene Wirchenko <genew@telus.net> - 2019-04-30 18:24 -0700
        Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-05-01 09:20 +0200
          Re: Optimization techniques Kaz Kylheku <847-115-0292@kylheku.com> - 2019-05-02 17:40 +0000
          Re: Optimization techniques and error detection Gene Wirchenko <genew@telus.net> - 2019-05-03 10:16 -0700
          Re: Optimization techniques "Robin Vowels" <robin51@dodo.com.au> - 2019-05-07 01:42 +1000
  Re: Optimization techniques Kaz Kylheku <847-115-0292@kylheku.com> - 2019-04-26 02:26 +0000
    Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-04-29 00:12 +0200
      Re: Optimization techniques and undefined behavior Kaz Kylheku <847-115-0292@kylheku.com> - 2019-05-02 17:18 +0000
        Re: Optimization techniques and undefined behavior David Brown <david.brown@hesbynett.no> - 2019-05-07 16:38 +0200

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