Re: Does reading an uninitialized object have undefined behavior?

From	Spiros Bousbouras <spibou@gmail.com>
Newsgroups	comp.std.c
Subject	Re: Does reading an uninitialized object have undefined behavior?
Date	2023-08-27 08:31 +0000
Organization	To protect and to server
Message-ID	<5+eRe7cp3yQjL4=AX@bongo-ra.co> (permalink)
References	(7 earlier) <864jkz7hrm.fsf@linuxsc.com> <e043af84-3153-4097-9505-666869fcf727n@googlegroups.com> <867cpu5h8w.fsf@linuxsc.com> <a3199783-d8b7-4065-836b-08f647a6808en@googlegroups.com> <868r9xz0ek.fsf@linuxsc.com>

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On Sat, 26 Aug 2023 19:25:55 -0700
Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
> Sometimes people use compiler options
> to turn off, for example, so-called "strict aliasing", and of course
> the C standard allows us to do that.  But compilers aren't required
> to provide such an option, and if they do the option may not do
> exactly what we expect it to do, because there is no standard
> specification for it.  The C standard should define officially
> sanctioned mechanisms -- as for example standard #pragma's -- to
> give standard-defined semantics to certain constructs of undefined
> behavior that resemble, eg, -fno-strict-aliasing.

Surely the starting point for this should be the documentation of the
compilers to specify precisely what  -fno-strict-aliasing  does. If
a consensus emerges out of these precise specifications or C programmers
indicate that they prefer the specification of some particular compiler
then this can become part of the standard. Adding a relevant  #pragma
should be trivial.

> The second problem is basically The Law of Unintended Consequences
> smashing into The Law of Least Astonishment.  As compiler writers
> have gotten more and more clever at exploiting the implications of
> "undefined behavior", we see more and more cases of code that looks
> reasonable being turned into mush by overly clever "optimizing"
> compilers.  There is obviously something wrong with the way this
> trend is going -- ever more clever "optimizations", followed by ever
> more arcane compiler options to work around the problems caused by
> the too-clever compilers.  This problem must be addressed by the C
> standard, for if it is not the ecosystem will transform into a
> confused state that is exactly what the C standard was put in place
> to avoid.  (I do have some ideas about how to address this issue,
> but I want to make sure everyone appreciates the extent of the
> problem before we start talking about solutions.)

Without specific examples , it's impossible to comment on this. Why did
the "reasonable" code have the undefined behaviour ? Could the result
the programmer was aiming for have been achieved with defined behaviour
? For example it has been pointed out on  comp.lang.c  that it's
impossible to write a  malloc()  implementation in conforming C. This is
certainly a weakness which should be addressed with some appropriate
#pragma .

> Before leaving the sub-topic of undefined behavior, let me mention
> two success stories.  The first is 'restrict':  the performance
> implications are local, the choice is under control of the program
> (and programmer), and the default choice is to play safe.  Good
> show.

From my point of view , restrict  is not a success because the
specification of  restrict  is the one part of the C1999 standard I have
given up trying to understand. I understand the underlying idea but the
specifics elude me. I remember many years ago someone asked on this
group about some code involving  restrict  and a member of the standard
committee replied and I found the reply counterintuitive. So I have
decided to not use  restrict  in my own code taking also into account
that I don't need the microoptimisations which  restrict  is intended to
allow. But for all I know , people who do need these optimisations find
the specification of  restrict  in the standard perfectly adequate.

-- 
It is not widely known that the "CPC" in "Amstrad CPC" actually stands
for "cool people club".

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Thread

Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-20 22:16 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 16:33 +0100
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-21 11:56 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 20:54 +0100
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-21 14:26 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 23:39 +0100
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-12 17:00 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-13 23:41 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-15 21:06 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-15 22:40 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 23:13 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-17 07:08 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-18 12:44 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-19 05:04 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-19 01:36 -0700
Re: Does reading an uninitialized object have undefined behavior? Richard Damon <Richard@Damon-Family.org> - 2023-08-19 09:18 -0400
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-19 11:12 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-18 20:20 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-19 05:23 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-18 22:56 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-18 12:52 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-26 19:25 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-27 08:31 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-29 04:35 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-30 19:53 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-30 17:40 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-31 18:18 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-05 05:39 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-05 17:03 -0700
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-09-07 17:09 +0200
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-07 17:19 +0100
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-09-08 23:12 +0200
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-08 22:31 +0100
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-07-22 06:40 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-07-22 06:03 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-25 21:53 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 11:11 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-07-21 17:42 +0000
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-07-24 07:53 +0200
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-25 21:57 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-03 13:13 -0700
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-08-03 15:20 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-05 01:15 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 09:19 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 19:51 +0000
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 20:03 +0000
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-08-16 13:43 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 21:08 +0000

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