[Bug middle-end/20644] bogus uninitialized warning on unused variable

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[h dot b dot furuseth at usit dot uio dot no]

------- Comment #4 from h dot b dot furuseth at usit dot uio dot no  2005-11-02 19:52 -------
I think I'd appreciate that warning when writing portable code:

The warning can be useful if the 1 is replaced with a macro
which may or may not expand to 1, or an enum defined in an #ifdef,
or an implementation-dependent expression like ((char)-1 < 0).

But of course, it depends on how many false positives the warning
tends to give for normal programs.

Maybe there could be a warning option to turn on and off some
warnings that do not apply with the particular #defines and
constants being used.  (And also turn on/off -Wunreachable
for this case.)


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h dot b dot furuseth at usit dot uio dot no changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |h dot b dot furuseth at usit
                   |                            |dot uio dot no


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[law at redhat dot com]

------- Comment #5 from law at redhat dot com  2005-11-08 17:23 -------
Bogus warning no longer issued with GCC 4.1 based compilers.


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law at redhat dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[pinskia at gcc dot gnu dot org]

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------- Comment #6 from pinskia at gcc dot gnu dot org  2005-11-08 17:25 -------
(In reply to comment #5)
> Bogus warning no longer issued with GCC 4.1 based compilers.


Huh:
gcc version 4.1.0 20051106 (experimental)
../t6.c: In function ‘foo’:
../t6.c:13: warning: ‘j’ is used uninitialized in this function


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|FIXED                       |
   Target Milestone|4.1.0                       |---


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[gdr at gcc dot gnu dot org]

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gdr at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-11-26 07:38:31
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[manu at gcc dot gnu dot org]

------- Comment #7 from manu at gcc dot gnu dot org  2007-08-20 14:18 -------
Even simpler testcase:

int foo ()
{
    int i = 0;
    int j;

    if (1 == i)
        return j;

    return 0;
}

This will only be reliably fixed by building a better SSA representation.
Moving the passes around will just solve it by chance (because CCP will assume
that j undefined value is actually 0, and thus remove j). Also, it will silence
many warnings (for the same reason, CCP happily initializing uninitialized
variables)

So instead of:

foo ()
{
  int j;
  int i;
  int D.1280;

<bb 0>:
  [pr20644.c : 3] i_2 = 0;
  [pr20644.c : 6] if ([pr20644.c : 6] i_2 == 1) goto <L0>; else goto <L1>;

<L0>:;
  [pr20644.c : 7] D.1280_6 = j_5;
  [pr20644.c : 7] goto <bb 3> (<L2>);

<L1>:;
  [pr20644.c : 9] D.1280_4 = 0;

  # D.1280_1 = PHI <D.1280_6(1), D.1280_4(2)>;
<L2>:;
  return D.1280_1;

}


We could generate:

foo ()
{
  int j;
  int i;

<bb 0>:
  [pr20644.c : 3] i_2 = 0;
  [pr20644.c : 6] if ([pr20644.c : 6] i_2 == 1) goto <L0>; else goto <L1>;

<L0>:;
  [pr20644.c : 7] goto <bb 3> (<L2>);

<L1>:;
  [pr20644.c : 9] j_6 = 0;

  # j_7 = PHI <j_5(D), j_6(2)>;
<L2>:;
  return j_7;

}


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[manu at gcc dot gnu dot org]

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------- Comment #8 from manu at gcc dot gnu dot org  2007-08-22 17:38 -------
(In reply to comment #6)
> (In reply to comment #5)
> gcc version 4.1.0 20051106 (experimental)
> ../t6.c: In function ‘foo’:
> ../t6.c:13: warning: ‘j’ is used uninitialized in this function
> 

Despite what I said before, for this particular case, we should never give a
"is used" warning if the BB is not executed with 100% probability. Hmm, I'll
check whether we can detect this.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[manu at gcc dot gnu dot org]

------- Comment #9 from manu at gcc dot gnu dot org  2007-08-23 14:17 -------
(In reply to comment #8)
> Despite what I said before, for this particular case, we should never give a
> "is used" warning if the BB is not executed with 100% probability. Hmm, I'll
> check whether we can detect this.

We could avoid the warning by doing the following:

--- gcc/tree-ssa.c      (revision 126606)
+++ gcc/tree-ssa.c      (working copy)
@@ -1302,8 +1334,11 @@
    }
 }

static unsigned int
execute_early_warn_uninitialized (void)
 {
  block_stmt_iterator bsi;
  basic_block bb;

  FOR_EACH_BB (bb)
-    for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
-      {
-       tree context = bsi_stmt (bsi);
-       walk_tree (bsi_stmt_ptr (bsi), warn_uninitialized_var,
-                  context, NULL);
-      }
+    {
+      edge e;
+      edge_iterator ei;
+      FOR_EACH_EDGE (e, ei, bb->preds)
+        if (e->flags & EDGE_FALLTHRU)
+          {
+            for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
+              {
+                tree context = bsi_stmt (bsi);
+                walk_tree (bsi_stmt_ptr (bsi), warn_uninitialized_var,
+                           context, NULL);
+              }
+            break;
+          }
+    }


I think this is the "Right Thing To Do". Otherwise, we are giving "is used"
warnings for BBs that are conditionally executed. On the other hand, we will
miss a few of the correct warnings that we get by chance by not doing the
"Right Thing".

Comments?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[manu at gcc dot gnu dot org]

------- Comment #10 from manu at gcc dot gnu dot org  2008-08-10 18:47 -------
Subject: Bug 20644

Author: manu
Date: Sun Aug 10 18:46:10 2008
New Revision: 138933

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=138933
Log:
2008-08-10  Manuel Lopez-Ibanez  <manu@gcc.gnu.org>

        PR middle-end/20644
        * tree-ssa.c (struct walk_data): Add new flag
        warn_possibly_uninitialized.
        (warn_uninitialized_var): Use it.
        (warn_uninitialized_vars): New.
        (execute_early_warn_uninitialized): Call it.
        (execute_late_warn_uninitialized): Likewise.
testsuite/
        * gcc.dg/uninit-pr20644-O0.c: New.
        * gcc.dg/uninit-pr20644.c: New.

Added:
    trunk/gcc/testsuite/gcc.dg/uninit-pr20644-O0.c
    trunk/gcc/testsuite/gcc.dg/uninit-pr20644.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/tree-ssa.c


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[manu at gcc dot gnu dot org]

------- Comment #11 from manu at gcc dot gnu dot org  2008-08-10 19:48 -------
This is FIXED in GCC 4.4. This may have fixed other uninitialized PRs, so if
you have reported one, please recheck with the a recent 4.4 revision


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.4.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-26 01:13 -------
(In reply to comment #2)
> I can imagine that it may not be straightforward to fix but I can't think of a
> reason why a warning could ever be useful in this case (i.e., when the code is
> provably safe). I could of course be missing something -- could you point me to
> the discussion where this was decided? Thanks!

Actually in 4.0.0 and above, it is an easy fix, just moving around the order around of the passes.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[sebor at roguewave dot com]

------- Additional Comments From sebor at roguewave dot com  2005-03-26 01:08 -------
I can imagine that it may not be straightforward to fix but I can't think of a
reason why a warning could ever be useful in this case (i.e., when the code is
provably safe). I could of course be missing something -- could you point me to
the discussion where this was decided? Thanks!

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644

[Bug middle-end/20644] bogus uninitialized warning on unused variable

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-26 00:43 -------
I think we had decided even though the code is unreachable, we want to warn about this.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end
           Keywords|                            |diagnostic


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20644