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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/55079] [4.8 regression] false positive -Warray-bounds (also seen at -O3 bootstrap)
Date: Wed, 14 Nov 2012 19:31:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D55079

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[4.8 regression] false      |[4.8 regression] false
                   |positive -Warray-bounds     |positive -Warray-bounds
                   |                            |(also seen at -O3
                   |                            |bootstrap)

--- Comment #8 from Jan Hubicka <hubicka at gcc dot gnu.org> 2012-11-14 19:=
31:16 UTC ---
Hi,
at -O3 bootstrap we have false positive on simplify-rtx.c
../../gcc/simplify-rtx.c: In function =E2=80=98rtx_def* simplify_plus_minus=
(rtx_code,
machine_mode, rtx, rtx)=E2=80=99:
../../gcc/simplify-rtx.c:4285:63: warning: array subscript is below array
bounds [-Warray-bounds]
           while (j-- && simplify_plus_minus_op_data_cmp (ops[j].op, save.o=
p));


the siplified testcase is as follows:

int a[8];

void
test(unsigned int n)
{
  unsigned int i;
  unsigned int j;
  if (n<8)
  for (j=3D0;j<n;j++)
   {
     i =3D j;
     do
        a[i+1]=3Da[i];
     while (i--);
   }
}

here we unroll the inner do loop 7 times based on the array bounds derived =
from
[i] access.=20=20
Afterwards VRP proves that j is always at most 7 and thus the loop walks ou=
t of
bounds. Here we may be able to determine that the loop accesses both i and =
i+1
and we could actually unroll only 6 times, but we handle each of array acce=
ss
independently.