[Bug tree-optimization/29708] New: Alias can go funny with pointer addition

[pinskia at gcc dot gnu dot org]

With the following testcase:
int a[100], b[100], d[100];

void link_error (void);
int g(int i, int j)
{
  int *c;
  int t;
  c = i?a:b;
  c++;
  d[j] = 1;
  c[j] = 2;
  if (d[j] != 1)
    link_error ();
  return 0;
}
----
May-alias should figure out that c can only pointer to a or b but it gets
confused as we get:
  j.2_11 = (unsigned int) j_7;
  D.1537_12 = j.2_11 * 4;
  D.1538_13 = (int *) D.1537_12;
  D.1539_14 = D.1538_13 + c_6;
  *D.1539_14 = 2;

as may-alias only looks at the left hand side of a PLUS expression, we get the
following result:
D.1538_13 = &ANYTHING
D.1539_14 = D.1538_13

Note 4.1 optimzed this testcase but I think that was by accident and is not
actually getting it correct.

If we add PTR_PLUS_EXPR we get the correct answer all the time unless someone
does some real weird tricks by doing something like:

size_t t = (size_t)a;
int *c = (int*)(size_t)j;
*(c+t) = 2;


-- 
           Summary: Alias can go funny with pointer addition
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29708