[Bug c/66618] New: Failure to diagnose non-constant initializer for static object with -O1

[Bug c/66618] New: Failure to diagnose non-constant initializer for static object with -O1

[Keith.S.Thompson at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

            Bug ID: 66618
           Summary: Failure to diagnose non-constant initializer for
                    static object with -O1
           Product: gcc
           Version: 4.8.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Keith.S.Thompson at gmail dot com
  Target Milestone: ---

$ uname -a
Linux m5 3.13.0-37-generic #64-Ubuntu SMP Mon Sep 22 21:28:38 UTC 2014 x86_64
x86_64 x86_64 GNU/Linux
$ gcc --version | head -n 1
gcc (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4
$ cat bug.c
int main(void) {
    const int not_a_constant = 0;
    static int n = not_a_constant; /* constraint violation */
    return n;                      /* avoid unused variable warning */
}
$ gcc -O0 -c -std=c11 -pedantic-errors -Wall -Wextra bug.c
bug.c: In function ‘main’:
bug.c:3:5: error: initializer element is not constant
     static int n = not_a_constant; /* constraint violation */
     ^
$ gcc -O1 -c -std=c11 -pedantic-errors -Wall -Wextra bug.c
$ 

In the test program "bug.c", the object "not_a_constant" is defined as "const",
but its name is not a constant expression. The initializer for a static
expression must be a constant expression; using a non-constant expression is a
constraint violation, requiring a diagnostic (N1570 6.7.9p4).

With "-O0", gcc correctly diagnoses this. I see the same problem with "-O1",
"-O2", and "-O3", and with "-std=c90", "-std=c99", and "-std=c11".

This is based on a question posted to Stack Overflow:
http://stackoverflow.com/q/30962512/827263
by user "meet". Grzegorz Szpetkowski gets the credit for realizing that the
"-O" option triggers the bug.
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf578

--- Comment #15 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
Is this something which could also have caused wrong-code issues in
other circumstances, which we just didn't happen to find until
this was exposed by Andre's patch and the test case?

In other words, should we backport Mikael's patch?

[Bug c/66618] Failure to diagnose non-constant initializer for static object with -O1

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-06-22
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.  What's going on here is that digest_init has
 6630   inside_init = c_fully_fold (inside_init, require_constant,
&maybe_const);
 6631   inside_init = decl_constant_value_for_optimization (inside_init);
and that decl_constant_value_for_optimization when optimizing is able to turn a
"const int" into an integer constant.  And an integer constant is fine as an
initializer thus we don't error.

[Bug c/66618] Failure to diagnose non-constant initializer for static object with -O1

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
Although diagnosing this probably makes sense, it's not required by the 
standard ("An implementation may accept other forms of constant 
expressions." - and this expression doesn't contain "assignment, 
increment, decrement, function-call, or comma operators", so isn't 
required by the Constraints for constant expressions not to be one).

[Bug c/66618] Failure to diagnose non-constant initializer for static object with -O1

[Keith.S.Thompson at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

--- Comment #3 from Keith Thompson <Keith.S.Thompson at gmail dot com> ---
It would be easier to argue that gcc accepts "other forms of constant 
expressions" if (a) those other forms were documented and (b) they were
accepted at all optimization levels.

Admittedly the standard doesn't seem to require such documentation (which I
find a little surprising), and you could argue that "gcc -O0" and "gcc -O1" are
two different implementations.

[Bug c/66618] Failure to diagnose non-constant initializer for static object with -O1

[alexhenrie24 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

Alex Henrie <alexhenrie24 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alexhenrie24 at gmail dot com

--- Comment #6 from Alex Henrie <alexhenrie24 at gmail dot com> ---
This wasn't fixed properly, or it was broken again before the release of GCC
8.1: In GCC 8.1 and later no warning at all is issued for the example program,
even at -O0.

[Bug c/66618] Failure to diagnose non-constant initializer for static object with -O1

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Alex Henrie from comment #6)
> This wasn't fixed properly, or it was broken again before the release of GCC
> 8.1: In GCC 8.1 and later no warning at all is issued for the example
> program, even at -O0.

The fix was to change it to be a valid constant expression ....

[Bug c/66618] Failure to diagnose non-constant initializer for static object with -O1

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #7)
> (In reply to Alex Henrie from comment #6)
> > This wasn't fixed properly, or it was broken again before the release of GCC
> > 8.1: In GCC 8.1 and later no warning at all is issued for the example
> > program, even at -O0.
> 
> The fix was to change it to be a valid constant expression ....

At -O0 even.

[Bug c/66618] Failure to diagnose non-constant initializer for static object with -O1

[alexhenrie24 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

--- Comment #9 from Alex Henrie <alexhenrie24 at gmail dot com> ---
(In reply to Andrew Pinski from comment #7)
> (In reply to Alex Henrie from comment #6)
> > This wasn't fixed properly, or it was broken again before the release of GCC
> > 8.1: In GCC 8.1 and later no warning at all is issued for the example
> > program, even at -O0.
> 
> The fix was to change it to be a valid constant expression ....
Oh, okay. When I read Comment 2 I thought that a -Wpedantic warning was
intended. If the intention was to have no warning ever then everything is fine.