[Bug tree-optimization/58958] New: [4.9 Regression] Wrong aliasing info for variable index

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58958

            Bug ID: 58958
           Summary: [4.9 Regression] Wrong aliasing info for variable
                    index
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org

double a[10];
bool f(int n){
  a[3]=42;
  __builtin_memset(&a[n],3,sizeof(double));
  return a[3]==42;
}

is optimized to:

bool f(int) (int n)
{
  double * _4;

  <bb 2>:
  a[3] = 4.2e+1;
  _4 = &a[n_3(D)];
  __builtin_memset (_4, 3, 8);
  return 1;

}

This is the issue described here:
http://gcc.gnu.org/ml/gcc-patches/2013-10/msg02558.html

ao_ref_init_from_ptr_and_size makes an unsafe use of get_ref_base_and_extent.
Maybe relying on get_addr_base_and_unit_offset would be better?

The testcase is a regression, although the issue preexisted.