[Bug middle-end/54544] Option -Wuninitialized does not work as documented with volatile

["jimfr06 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54544

Zakhar <jimfr06 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |

--- Comment #3 from Zakhar <jimfr06 at gmail dot com> 2012-09-11 16:41:43 UTC ---
On second thoughs, I reopen the issue!

WHY:
---
The 'correction' of the code above is a contrary to what the function intended
to do and thus breaks its logic: the declaration in the second version of the
code are inconsistent with what the function was intented to. That is because,
I suppose, the message gcc delivers and its correction are not OK with what
really happens.



FURTHER EXPLANATIONS:
---------------------
You are right Andrew, the variable 'bar' is indeed (first version of the code)
a 'pointer to a volatile memory location' (location of an int).
*And that was intended so*.
The code is a simplification of a 'memory protection algorithm' where the int
that is pointed represents the count of threads using a given piece of memory.
This count being accessed from several threads, is indeed volatile as no thread
can assume the value didn't change from last time it read/wrote the value.

(atomic instructions where removed from the code example).

The pointer itself is not at all volatile. Anyway, once it is passed to the
function, it sits on the stack where its value should be unchanged as long a
the function lives, and same goes for the 'bar' automatic variable. I'm not
doing crazy threaded things on variables on the stack of this function!

I was confused by the warning message saying:
'bar' may be used uninitialized in this function

... because the message is indeed confusing. 'bar' is NOT used uninitialized
(as demonstrated) but the *content pointed by 'bar'* could be. I must confess
that I didn't look from far enough to interpret the message this way.



HYPOTHESIS:
-----------
So could it be that gcc saw that, and warns incorrectly on 'bar' instead of
'*bar'?


If so, yes, because the function receives a pointer to a memory location, the
function itself cannot know whether the location pointed to was initialized or
not. 'bar' gets the same address ( bar = p ) thus, indeed, the location pointed
by 'bar' could be un-initialized.
This could also be coherent with the fact that when we change the function to
static, the warning disappears. Being static, all the callers have to be on the
same source, thus the compiler can check if the callers initialize properly the
content memory pointed to.


But then, shouldn't the message be:
'*bar' may be used uninitialized in this function.


And that would indeed be correct, because '*bar' being the memory pointed to by
bar, could indeed be un-initialized (if the caller didn't initialize it).

And thus, the compiler would do a good job signaling that, as '*bar' (memory
which bar points to) is declared as volatile, but it is NOT an automatic
variable (the pointer is, not the memory pointed to).


My hypothesis is probably wrong, because if gcc warned about un-initialized
memory pointed to, it would have to warn on that:

/*01*/ int
/*02*/ foo( p )
/*03*/   int *p;
/*04*/ {
/*05*/   int foobar;
/*06*/
/*07*/   foobar= *p;
/*08*/
/*09*/   return foobar + 2;
/*10*/ }

(we don't know if '*p' has ever been initialized).
And this short code snipet produces no warning at all, which is fine because a
lot of code do that (passing variable 'by reference'), and it is perfectly
correct not to warn.



CONCLUSION:
----------
As previously concluded, it is not strictly a 'bug' as the documentation
perfectly states that in some case the dectection is broken, but I assume this
issue can go in the general thread "better wuninitialized".

- either gcc saw that '*bar' is uninitialized and should report that (and not
report the pointer instead)
- or gcc saw 'bar' (the pointer) to be unitialized, and that is a test case
where it can do better work, as we can demonstrate it IS initialized.




VERSION 3 of the code:
---------------------
Of course, NOT to break my program logic, the correct declaration of the
variable should be:
/*09*/   volatile int * volatile bar;

The 1st 'volatile' because indeed the memory we point to MUST be declared as
volatile.
The 2nd 'volatile' to suppress the 'false positive' detection on the pointer.



QUESTION @Andrew:
----------------

Should I post something on the general thread "better wuninitialized" (unless
my deductions are wrong again), or do you attach the use case directly from
this bug report?