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From: "mark.d.rustad at intel dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/56574] New: False possibly uninitialized variable warning
Date: Fri, 08 Mar 2013 19:10:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56574

             Bug #: 56574
           Summary: False possibly uninitialized variable warning
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mark.d.rustad@intel.com


The following code produces the warning,'value' may be used uninitialized:

int get_value(void);

void show_problem(int flag)
{
    int value;

    if (!flag)
        value = get_value();

    for (;;) {
        if (!flag && value)
            continue;

        if (!flag)
            break;
    }
}

The warning occurs only with -O1 and -O2 optimization. -O0 and -O3 and higher
do not. It also occurs with many versions. I have gotten the same result with
the following versions of gcc:

i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664)
gcc (SUSE Linux) 4.3.4 [gcc-4_3-branch revision 152973]
gcc (GCC) 4.4.6 20110731 (Red Hat 4.4.6-3)
gcc (GCC) 4.7.0 20120507 (Red Hat 4.7.0-5)

Interestingly, the much larger function that originally displayed the problem
does not generate a warning with the 4.7.0 compiler, but does on the others.