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From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/60165] New: "may be used uninitialized" warning with -O3 but not with -O2
Date: Thu, 13 Feb 2014 03:17:00 -0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 4.9.0
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60165

            Bug ID: 60165
           Summary: "may be used uninitialized" warning with -O3 but not
                    with -O2
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net

With:

gcc (Debian 20140111-1) 4.9.0 20140111 (experimental) [trunk revision 206552]

I get the following inconsistency in the warnings:

ypig% cat out.i
int a, b;
int fn2 (int, int);
int fn1 (int *p1)
{
    if (fn2 (a, 0))
        *p1 = b;
    int c;
    fn1 (&c);
    return c;
}
ypig% gcc-snapshot -c -Wall -Werror=maybe-uninitialized -O2 out.i
ypig% gcc-snapshot -c -Wall -Werror=maybe-uninitialized -O3 out.i
out.i: In function 'fn1':
out.i:9:5: error: 'c' may be used uninitialized in this function
[-Werror=maybe-uninitialized]
     return c;
     ^
cc1: some warnings being treated as errors
ypig% 

I don't know whether this is regarded as normal, but this looks strange.

Note: I got this problem when compiling round_prec.c from the GNU MPFR trunk. I
generated the preprocessed file with -E, then used creduce on the following
script:

#!/bin/sh
{
  gcc-snapshot -c -Wall -Werror=maybe-uninitialized -O2 out.i && \
  ! gcc-snapshot -c -Wall -Werror=maybe-uninitialized -O3 out.i
} >/dev/null 2>&1

to generate the simple testcase (and fixed the declarations to avoid additional
warnings -- I think I should have used -Werror in the script to avoid them in
the first place).