[Bug c/36614] New: incorrect "warning: array subscript is above array bounds"

["gnu at rothwell dot id dot au" <gcc-bugzilla at gcc dot gnu dot org>]

In some circumstances gcc 4.3.1 and later give the above warning incorrectly.
I have tried this with:

gcc (Debian 4.3.1-2) 4.3.1 on x86_64 (This is the compiler in current Debian
unstable)

gcc (GCC) 4.4.0 20080624 (experimental) [trunk revision 137057] on PowerPC -
this was built with "--disable-bootstrap --enable-languages=c")

The .i version of my test program is as follows:

# 1 "arrtest.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "arrtest.c"

struct pt_regs {
 unsigned long gpr[32];
 unsigned long nip;
 unsigned long msr;
 unsigned long orig_gpr3;
 unsigned long ctr;
 unsigned long link;
 unsigned long xer;
 unsigned long ccr;
 unsigned long softe;
 unsigned long trap;
 unsigned long dar;
 unsigned long dsisr;
 unsigned long result;
};

void restore_sigcontext(struct pt_regs *regs)
{
 unsigned long *gregs = (unsigned long *)regs;
 int i;


 for (i = 31; i <= 43; i++)
  gregs[i] = 1;


 for (i = 32; i <= 43; i++)
  gregs[i] = 1;


 for (i = 31; i <= 60; i++)
  gregs[i] = 1;
}

I compiled this with "gcc -Wall -O2 -c arrtest.c" and get the warning for the
seond "gregs[i] = 1;" but not either of the others.  This is a cut down from a
file in the current Linux kernel PowerPC arch code.

Compiler output:
arrtest.c: In function â??restore_sigcontextâ??:
arrtest.c:29: warning: array subscript is above array bounds


-- 
           Summary: incorrect "warning: array subscript is above array
                    bounds"
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gnu at rothwell dot id dot au


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36614