Re: On Undefined Behavior

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: On Undefined Behavior
Date	2026-01-02 10:31 +0100
Organization	A noiseless patient Spider
Message-ID	<10j8399$9grh$1@dont-email.me> (permalink)
References	<10j6qdt$3q9n4$1@dont-email.me>

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On 01/01/2026 22:54, highcrew wrote:
> Hello,
> 
> While I consider myself reasonably good as C programmer, I still
> have difficulties in understanding undefined behavior.
> I wonder if anyone in this NG could help me.
> 
> Let's take an example.  There's plenty here:
> https://en.cppreference.com/w/c/language/behavior.html
> So let's focus on https://godbolt.org/z/48bn19Tsb
> 
> For the lazy, I report it here:
> 
>    int table[4] = {0};
>    int exists_in_table(int v)
>    {
>        // return true in one of the first 4 iterations
>        // or UB due to out-of-bounds access
>        for (int i = 0; i <= 4; i++) {
>            if (table[i] == v) return 1;
>        }
>        return 0;
>    }
> 
> This is compiled (with no warning whatsoever) into:
> 
>    exists_in_table:
>            mov     eax, 1
>            ret
>    table:
>            .zero   16
> 
> 
> Well, this is *obviously* wrong. And sure, so is the original code,
> but I find it hard to think that the compiler isn't able to notice it,
> given that it is even "exploiting" it to produce very efficient code.
> 

Since the original code is wrong, it doesn't make sense to ask if the 
generated object code is right or wrong.  This principle was understood 
from the days of mechanical computers: 
<https://www.brainyquote.com/quotes/charles_babbage_141832>

However, it /does/ make sense to ask whether the compiler could have 
been more helpful in pointing out your mistake - and clearly, in theory 
at least, the answer is yes.

Compilers first parse the source code into an internal format.  Then 
they run through this in a series of passes, detecting errors and 
generating warnings, manipulating the code for optimisation, generating 
new types of internal formats, and so on.  For modern compilers there 
will be scores of passes.  Ideally, the ordering of these passes is such 
that the pass that figures out the size of "table" and the range of the 
loop variable will come before the pass that issues out-of-bounds 
warnings, and that will come before the pass that figures out that the 
"return 0;" cannot be reached without passing through UB and thus the 
only possible return value for the function is 1.  For many optimisation 
passes, once the compiler has figured out a way to simplify the code, 
some aspects (such as paths leading up to UB that have been eliminated) 
are lost, and the compiler can't see the problem (and therefore can't 
issue a diagnostic) later on.

Unfortunately here, the passes are not ordered ideally for catching this 
source code bug.  I think it is inevitable that such situations will 
arise - no matter how the passes are ordered, you are going to be able 
to find samples where there is a bug that can easily be spotted by a 
human reader but which is not diagnosed by the compiler.  Change the 
pass order to one that will catch this situation, and another bug will 
now go undiagnosed.  Compiler writers do strive to catch what they can, 
but they must also balance compile times, compiler complexity, and of 
course other demands on their own development time.  Sometimes that 
means things can appear simple and obvious to the user, but would 
require unwarranted effort to implement in the compiler.

I had a little look in the gcc bugzilla, but could not find any issue 
that directly matches this case.  So I think it is worthwhile if you 
file it in as a gcc bug.  (It is not technically a "bug", but it is 
definitely an "opportunity for improvement".)  If the gcc passes make it 
hard to implement as a normal warning, it may still be possible to add 
it to the "-fanalyzer" passes.

> I understand the formalism: the resulting assembly is formally
> "correct", in that UB implies that anything can happen.
> Yet I can't think of any situation where the resulting assembly
> could be considered sensible.  The compiled function will
> basically return 1 for any input, and the final program will be
> buggy.
> 
> Wouldn't it be more sensible to have a compilation error, or
> at least a warning?  The compiler will be happy even with -Wall -Wextra
> -Werror.
> 
> There's plenty of documentation, articles and presentations that
> explain how this can make very efficient code... but nothing
> will answer this question: do I really want to be efficiently
> wrong?

You are asking if you want the generated code to be efficiently wrong or 
inefficiently wrong?

> 
> I mean, yes I would find the problem, thanks to my 100% coverage
> unit testing, but couldn't the compiler give me a hint?

You can also make use of run-time sanitizers that are ideal for catching 
this sort of thing (albeit with an inevitable speed overhead).

<https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html>

> 
> Could someone drive me into this reasoning? I know there is a lot of
> thinking behind it, yet everything seems to me very incorrect!
> I'm in deep cognitive dissonance here! :) Help!
> 

Basically, gcc is a great tool, but not it is not perfect.  Reporting 
such issues can help improve it, but there are no guarantees.

Thread

On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-01 22:54 +0100
  Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-02 00:26 +0200
    Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-01 23:57 +0100
    Re: On Undefined Behavior Kaz Kylheku <046-301-5902@kylheku.com> - 2026-01-02 22:56 +0000
      Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-03 20:48 +0200
      Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-04 14:38 +0100
        Re: On Undefined Behaviour Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-04 21:42 +0000
          Re: On Undefined Behaviour candycanearter07 <candycanearter07@candycanearter07.nomail.afraid> - 2026-01-07 06:40 +0000
        Re: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-04 16:58 -0500
          Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-05 08:49 +0100
  Re: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-01 17:49 -0500
  Re: On Undefined Behavior antispam@fricas.org (Waldek Hebisch) - 2026-01-02 05:53 +0000
    Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-02 17:38 +0100
      Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-03 13:30 +0100
  Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-02 10:31 +0100
    Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-02 17:51 +0100
      Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-03 13:42 +0100
        Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-03 14:42 +0100
          Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-03 17:51 +0100
            Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-04 00:20 +0100
  Re: On Undefined Behavior Kaz Kylheku <046-301-5902@kylheku.com> - 2026-01-02 22:52 +0000
    Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-03 23:47 +0100
      Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-04 12:58 +0100
  Re: On Undefined Behavior Andrey Tarasevich <noone@noone.net> - 2026-01-03 07:53 -0800
    Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-04 00:15 +0100
      NULL dereference in embedded [was: On Undefined Behavior] highcrew <high.crew3868@fastmail.com> - 2026-01-04 00:25 +0100
        Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-03 18:59 -0500
          Re: NULL dereference in embedded [was: On Undefined Behavior] scott@slp53.sl.home (Scott Lurndal) - 2026-01-04 15:51 +0000
            Re: NULL dereference in embedded [was: On Undefined Behavior] David Brown <david.brown@hesbynett.no> - 2026-01-05 08:55 +0100
        Re: NULL dereference in embedded [was: On Undefined Behavior] Andrey Tarasevich <noone@noone.net> - 2026-01-03 17:24 -0800
          Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-04 02:19 +0000
            Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-03 21:31 -0500
              Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-04 04:52 +0000
                Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-04 13:00 -0500
                Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-04 21:22 +0000
                Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-04 16:53 -0500
                Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-05 00:16 +0000
                Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-05 06:41 -0500
                Re: NULL dereference in embedded [was: On Undefined Behavior] David Brown <david.brown@hesbynett.no> - 2026-01-05 09:07 +0100
            Re: NULL dereference in embedded [was: On Undefined Behavior] scott@slp53.sl.home (Scott Lurndal) - 2026-01-04 15:56 +0000
          Re: NULL dereference in embedded [was: On Undefined Behavior] Andrey Tarasevich <noone@noone.net> - 2026-01-03 18:44 -0800
        Re: NULL dereference in embedded [was: On Undefined Behavior] David Brown <david.brown@hesbynett.no> - 2026-01-04 17:16 +0100
          Re: NULL dereference in embedded [was: On Undefined Behavior] antispam@fricas.org (Waldek Hebisch) - 2026-01-06 13:08 +0000
            Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-06 21:59 +0000
            Re: NULL dereference in embedded [was: On Undefined Behavior] Andrey Tarasevich <noone@noone.net> - 2026-01-07 20:48 -0800
              Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-08 23:56 +0000
  Re: On Undefined Behavior Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-03 23:14 +0000
  Re: On Undefined Behavior "Paul J. Lucas" <paul@lucasmail.org> - 2026-01-03 17:10 -0800
    Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-04 12:51 +0100
      Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-05 15:39 +0100
        Re: On Undefined Behavior "Paul J. Lucas" <paul@lucasmail.org> - 2026-01-06 18:08 -0800
          Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-07 11:25 +0100
          Re: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-07 06:31 -0500
          Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-07 14:10 +0200
  Re: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-09 01:42 -0800
    Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-09 14:36 +0200
      Re: On Undefined Behavior Kaz Kylheku <046-301-5902@kylheku.com> - 2026-01-09 20:14 +0000
        Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-10 18:19 +0200
        Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-10 18:41 +0200
          Re: On Undefined Behavior Kaz Kylheku <046-301-5902@kylheku.com> - 2026-01-13 23:31 +0000
            Re: On Undefined Behavior antispam@fricas.org (Waldek Hebisch) - 2026-01-14 03:57 +0000
            Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-14 10:47 +0200
            Re: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-14 14:49 +0000
        Re: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-10 17:08 +0000
      Re: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-11 11:48 -0800
        Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-11 22:52 +0200
          Re: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-11 22:53 -0800
            Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 11:44 +0200
              Re: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-12 20:29 -0500
          Re: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-02-03 05:29 -0800
  Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-09 15:54 +0200
    Re: On Undefined Behavior wij <wyniijj5@gmail.com> - 2026-01-10 00:08 +0800
  UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 16:28 +0200
    Re: UB or not UB? was: On Undefined Behavior bart <bc@freeuk.com> - 2026-01-12 15:58 +0000
      Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 20:08 +0200
        Re: UB or not UB? was: On Undefined Behavior scott@slp53.sl.home (Scott Lurndal) - 2026-01-12 20:02 +0000
          Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-12 21:09 -0500
            Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-13 11:31 +0200
              Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:21 -0500
        Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:19 -0500
          Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-14 09:35 +0100
            Re: UB or not UB? was: On Undefined Behavior antispam@fricas.org (Waldek Hebisch) - 2026-01-14 17:23 +0000
              Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-14 12:53 -0800
            Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-14 14:43 -0800
              Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 11:45 +0100
                Re: UB or not UB? was: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-15 06:16 -0500
                Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-15 04:04 -0800
                Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 13:56 +0100
                Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-02-03 05:34 -0800
                Re: UB or not UB? was: On Undefined Behavior scott@slp53.sl.home (Scott Lurndal) - 2026-01-15 15:10 +0000
                Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 16:23 +0100
      Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-13 21:54 +0000
        Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 21:58 -0500
          Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-13 22:02 -0800
            Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-14 14:24 +0000
              Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-14 16:48 +0200
    Re: UB or not UB? was: On Undefined Behavior Andrey Tarasevich <noone@noone.net> - 2026-01-12 08:03 -0800
      Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 19:36 +0200
        Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-12 12:03 -0800
          Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 22:41 +0200
            Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-13 09:12 +0100
            Re: UB or not UB? was: On Undefined Behavior pa@see.signature.invalid (Pierre Asselin) - 2026-01-13 20:19 +0000
            Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:20 -0500
            Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-02-03 21:53 -0800
          Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-13 23:53 +0000
            Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-14 08:06 +0000
        Re: UB or not UB? was: On Undefined Behavior Andrey Tarasevich <noone@noone.net> - 2026-01-13 08:11 -0800
          Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:10 -0500
          Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-01 22:53 -0800
        Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:20 -0500
    Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-12 20:35 -0500
      Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-13 11:07 +0200
  Re: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-13 20:37 +0000

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