Re: int a = a

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

[how to indicate a variable not being used is okay]
[some quoted text rearranged]

Unless I'm missing something, `(void)x` also has undefined beahvior
if x is uninitialized,

Right.  Using (void)&x is better.

though it's very likely to do nothing in practice.

Unless x is volatile qualified, in which there must be an access
to x in the generated code.

The behavior [of int a = a;] is undefined.  In C11 and later
(N1570 6.3.2.1p2):
>
    Except when [...] an lvalue that does not have array type is
    converted to the value stored in the designated object (and is
    no longer an lvalue); this is called lvalue conversion.
    [...]
    If the lvalue designates an object of automatic storage
    duration that could have been declared with the register
    storage class (never had its address taken), and that object
    is uninitialized (not declared with an initializer and no
    assignment to it has been performed prior to use), the
    behavior is undefined.

Long digression follows.
>
The "could have been declared with the register storage class"
seems quite odd.  And in fact it is quite odd.

I don't have the same reaction.  The point of this phrase is that
undefined behavior occurs only for variables that don't have
their address taken.  The phrase used describes that nicely.
Any questions related to "registerness" can be ignored, because
'register' in C really has nothing to do with hardware registers,
despite the name.

It's tempting to assume that `int n = n;` did not have undefined
behavior prior to C11, or that accessing an automatic object whose
address has not been taken does not have undefined behavior even
in C11 or later, but it's not that simple.
>
In C90, the non-normative Annex G (renamed to Annex J in later
editions) says:
>
    The behavior in the following circumstances is undefined:
    [...]
    - The value of an uninitialized object that has automatic storage
      duration is used before a value is assigned (6.5.7).
>
6.5.7 discusses initialization, and says that "If an object that
has automatic storage duration is not initialized explicitly, its
value is indeterminate", and C90's definition of "undefined behavior"
explicitly refers to use of indeterminately valued objects, though
it's not 100% clear that using an indeterminate value *always*
has undefined behavior.
>
So in C90, `int n = n;` explicitly had undefined behavior, even if
all possible bit representations for an object of type int correspond
to valid values (C90 didn't mention "trap representations").
>
C99 added a definition for "indeterminate value":  "either an
unspecified value or a trap representation", and drops the mention
of indeterminate values in the definition of "undefined behavior".
It dropped the reference to uninitialized objects in Annex G/J.
I believe that in C99, `int n = n;` is well defined *if* int
has no trap representations, or if the representation stored in
the memory occupied by n happens not to be a trap representation.
If int has trap representations, and that memory happens to contain
such a representation, the behavior is undefined.
>
I found a discussion in comp.std.c from 2023, subject "Does reading
an uninitialized object have undefined behavior?".
>
The discontinued IA-64/Itanium processor had something called
"NaT", "Not a Thing".  NaT representations exist only in CPU
registers, not in memory.  (Imagine an extra bit for each register
indicating whether the register contains a "thing".)  A NaT allows
for representations that act like C trap representations (called
non-value representations in C23) even for types with no trap
representations (for example where all 2**N possible representations
correspond to valid values) -- but again, only in CPU registers.
>
https://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_338.htm
>
So the "could have been declared with the register storage class"
wording was added in C11 specifically to cater to the IA64.  This
change would have been superfluous in C90, where the behavior was
undefined anyway, but is a semantically significant change between
C99 and C11.  (If some future CPU has something like NaT that can
be stored in memory, the wording might need to be updated yet again.)
>
My takeaway is that if it requires this much research to determine
whether accessing the value of an uninitialized object has undefined
behavior (in which circumstances and which edition of the standard),
I'll just avoid doing so altogether.  I'll initialize objects
when they're defined whenever practical.  If it's not practical
for some reason, I won't initialize it with some dummy value;  I'll
leave it uninitialized so the compiler has a chance to warn me if
I accidentally use it before assigning a value to it.

I think you are overthinking the question.  In cases where it's
important to give an initial value to a variable, and can be done
so at the point of its declaration, use an initializer;  otherwise
don't.  We don't have to read several different C standards, or
even only one, to reach that conclusion.  If someone wants to know
exactly which border cases are safe and which cases are not, then
reading the relevant version(s) of the C standard is needed, but
in most situations it isn't.  It's important for the C standard to
be precise about what it prescribes, but as far as initialization
goes it's easy to write code that doesn't need that level of
detail.  Compiler writers need to know such things;  in the
particular case of when and where to initialize, most developers
don't.