[Bug c++/41847] New: warning: array subscript is above array bounds

[Bug c++/41847] New: warning: array subscript is above array bounds

[caolanm at redhat dot com]

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#  gcc -Wall -O2 -c demo.cxx
demo.cxx: In function ‘void isInvertible()’:
demo.cxx:12: warning: array subscript is above array bounds

gcc --version
gcc (GCC) 4.4.2 20091019 (Red Hat 4.4.2-5)

I've the same results with...
gcc (GCC) 4.4.2 20091019 (Red Hat 4.4.2-5) (gcc-4.4.2-5.el6.s390x)
gcc (GCC) 4.4.1 20090725 (Red Hat 4.4.1-3) (gcc-4.4.1-3.s390x)
gcc (GCC) 4.4.0 20090506 (Red Hat 4.4.0-4) (gcc-4.4.0-4.s390x)

Possibly a duplicate of #35392
so its not a very recent thing.

I only get this warning (and related runtime failure) with s390x. Equivalent
x86_64/x86/ppc/ppc64/arm gcc's appear fine


-- 
           Summary: warning: array subscript is above array bounds
           Product: gcc
           Version: 4.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: caolanm at redhat dot com
 GCC build triplet: s390x-ibm-linux
  GCC host triplet: s390x-ibm-linux
GCC target triplet: s390x-ibm-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41847

[Bug c++/41847] warning: array subscript is above array bounds

[caolanm at redhat dot com]

------- Comment #1 from caolanm at redhat dot com  2009-10-27 14:47 -------
Created an attachment (id=18916)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18916&action=view)
standalone demo

gcc -Wall -O2 -c demo.cxx


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41847

[Bug c++/41847] warning: array subscript is above array bounds

[jakub at gcc dot gnu dot org]

------- Comment #2 from jakub at gcc dot gnu dot org  2009-10-27 16:33 -------
Reproduceable on today's trunk as well (s390x-linux target).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41847

[Bug c++/41847] warning: array subscript is above array bounds

[jakub at gcc dot gnu dot org]

------- Comment #3 from jakub at gcc dot gnu dot org  2009-10-27 16:53 -------
The issue is that for some strange reason VRP sees the loop iterator (which is
[0, 4) ) as VARYING.  Then the fDum = get(b, b) is called when
b is not 3, and get uses the return maLine[nRow].get(nColumn); for nRow < 3.
So, VRP figures that in return mpLine->get(nColumn); nColumn (== b) is
necessarily [4, 65535] and reports out of bounds access on something that will
really never be executed.
If VRP figured that b is [0, 4), it would have been able to tell that the bb's
are unreachable and not report diagnostics in that case.

I wonder what related runtime failure you are talking about though, this to me
looks like a false positive on something that is never executed (although not
optimized out at least until *.optimized dump).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41847

[Bug c++/41847] warning: array subscript is above array bounds

[caolanm at redhat dot com]

------- Comment #4 from caolanm at redhat dot com  2009-10-27 20:17 -------
At runtime of the original version of one of these loops (or a similar one, I
decided for ease to boil down to the warning case for this bug) goes way past
the RowSpan limits. If this one is cosmetic, then I've got another more
difficult to extract one to do.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41847

[Bug c++/41847] warning: array subscript is above array bounds

[caolanm at redhat dot com]

------- Comment #5 from caolanm at redhat dot com  2009-10-31 17:59 -------
bug 41892 now logged for the concrete runtime problem I'm encountering.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41847