[Bug regression/42145] New: Incorrect "may be used uninitialized warning" for a very specific test case

[Bug regression/42145] New: Incorrect "may be used uninitialized warning" for a very specific test case

[colin at reactos dot org]

I'm compiling the attached test case with "gcc version 4.5.0 20091018
(experimental) [trunk revision 152966] (Ubuntu 20091018-0ubuntu1)".
When using the command line "gcc -Wuninitialized -Os -o test test_gccbug.c",
I'm getting the following incorrect warning:

test_gccbug.c: In function 'main':
test_gccbug.c:17:6: warning: 'ret' may be used uninitialized in this function

Same happens with different builds of GCC 4.4.2, 4.4.1 and 4.3.4.
No such warning occurs with builds of GCC 4.2.4 and 4.1.3.

This problem does not occur if one or more of the following is done:
- You change the optimization level to -O0 or -O1 (-O2 and -O3 still trigger
the bug in this GCC 4.5.0 version, but not always in older versions I've
tested)
- You comment out line 10 of the test case
- You change line 22 of the test case to just 'if (!argc)'


-- 
           Summary: Incorrect "may be used uninitialized warning" for a very
                    specific test case
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: regression
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: colin at reactos dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

[Bug regression/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[colin at reactos dot org]

------- Comment #1 from colin at reactos dot org  2009-11-22 14:43 -------
Created an attachment (id=19084)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=19084&action=view)
The test case


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

[Bug regression/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[colin at reactos dot org]

------- Comment #2 from colin at reactos dot org  2009-11-22 14:44 -------
Created an attachment (id=19085)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=19085&action=view)
The preprocessed test case


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

[Bug regression/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[manu at gcc dot gnu dot org]

------- Comment #3 from manu at gcc dot gnu dot org  2009-12-30 16:59 -------
This would need conditional PHIs, so a duplicate of PR20968.

GCC never detects that ret is always initialized, it doesn't warn because at
low optimization levels we do not warn for PHIs or because CCP (PR18501) just
initializes the variable (probably to 1 or 10). There is some differences
between the dumps of -O1 and -O2 in the way the logical or is transformed int
bitwise-or and how dom1 deals with each of them, but I cannot tell if there is
some missing optimization involved.

*** This bug has been marked as a duplicate of 20968 ***


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145