[Bug middle-end/41952] New: False uninitialized variable warning

[Bug middle-end/41952] New: False uninitialized variable warning

[xinliangli at gmail dot com]

Compiling the following example with -Wuninitialized, 4.4 gcc will emit false
warning on uninitialized variable. In fact the definition is guarded by a
condition which is always true (coming from the assignment operator) for
locals. The solution is to fold the predicate (see proposed patch).

On 4.5, due to vsym related changes, the false positive warning issue is masked
(but false negative one occurs -- see a different PR). While this patch can be
used to fix the 4.4 issue, it is also applicable to 4.5 as a very safe
optimization. Is it ok to get it in at this stage?


#ifndef WORK_AROUND_GCC_4_4_0_BROKENNESS
#define WORK_AROUND_GCC_4_4_0_BROKENNESS 0
#endif

struct ExtentsBase {
 ExtentsBase() : startx_(), endx_() { }
 ExtentsBase(const ExtentsBase &b) {
  *this = b;
 }

 const ExtentsBase & operator=(const ExtentsBase &b) {
  if (this != &b || WORK_AROUND_GCC_4_4_0_BROKENNESS) {
    startx_ = b.startx_;
  }
  return *this;
 }

 int startx_;
  int endx_;
};

int f(const ExtentsBase &e1) {
 ExtentsBase my_extents = e1;
 return my_extents.startx_;
}

====================

self2.cc: In function 'int f(const ExtentsBase&)':
self2.cc:25: warning: 'my_extents' may be used uninitialized in this function


-- 
           Summary: False uninitialized variable warning
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: xinliangli at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41952

[Bug middle-end/41952] False uninitialized variable warning

[xinliangli at gmail dot com]

------- Comment #1 from xinliangli at gmail dot com  2009-11-05 18:18 -------
Created an attachment (id=18975)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18975&action=view)
proposed patch

Bootstrapped and regression tested on x86-64 linux with gcc 4.5


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41952

[Bug middle-end/41952] False uninitialized variable warning

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2009-11-06 10:01 -------
Patches should go to gcc-patches, you need to add the testcase and you
should use auto_var_in_fn_p instead of !is_global_var.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |diagnostic, missed-
                   |                            |optimization
   Last reconfirmed|0000-00-00 00:00:00         |2009-11-06 10:01:05
               date|                            |
            Summary|False uninitialized variable|False uninitialized variable
                   |warning                     |warning


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41952

[Bug middle-end/41952] False uninitialized variable warning

[xinliangli at gmail dot com]

------- Comment #3 from xinliangli at gmail dot com  2009-11-06 18:11 -------
(In reply to comment #2)
> Patches should go to gcc-patches, you need to add the testcase and you
> should use auto_var_in_fn_p instead of !is_global_var.
> 

auto_var_in_fn_p uses DECL_CONTEXT --- is it guaranteed to always point to the
enclosing functions even for inner block scoped locals (it seems so now).


David


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41952

[Bug middle-end/41952] False uninitialized variable warning

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2009-11-06 19:06 -------
Yes, the predicate is implemented correctly.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41952

[Bug middle-end/41952] patch to improve Wuninitialized

[hjl dot tools at gmail dot com]

------- Comment #5 from hjl dot tools at gmail dot com  2010-04-29 20:40 -------
Fixed by revision 158567:

http://gcc.gnu.org/ml/gcc-cvs/2010-04/msg00673.html


-- 

hjl dot tools at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.6.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41952