[Bug tree-optimization/29156] New: Misscompilation due to alias analysis

["rakdver at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

The testcase below gets misscompiled at -O2.

The alias info looks this way:

Dereferenced pointers

xa, UID 1527, struct test1 *, symbol memory tag: SMT.4, default def: xa_4
xb, UID 1528, struct test2 *, symbol memory tag: SMT.5, default def: xb_3

Symbol memory tags

SMT.4, UID 1547, struct test1, is addressable, is global, call clobbered, may
aliases: { global }
SMT.5, UID 1548, struct test2, is addressable, is global, call clobbered, may
aliases: { global }

The alias sets for SMT.4 and SMT.5 intersect, so everything looks OK here. 
However, access_can_touch_variable (correctly) determines that the dereferences
of xa and xb cannot touch "global"; hence we create the following virual
operands:

  #   SMT.5_8 = V_MAY_DEF <SMT.5_7>;
  xb_3->sub.a = 1;
  #   SMT.4_10 = V_MAY_DEF <SMT.4_9>;
  xa_4->a = 8;
  #   VUSE <SMT.5_8>;
  D.1531_5 = xb_3->sub.a;

The accesses to xa and xb appear to be independent, which leads to a
misscompilation.

struct test1
{
  int a;
  int b;
};
struct test2
{
  float d;
  struct test1 sub;
};

int global;

int bla(struct test1 *xa, struct test2 *xb)
{
  global = 1;
  xb->sub.a = 1;
  xa->a = 8;
  return xb->sub.a;
}

int main(void)
{
  struct test2 pom;

  if (bla (&pom.sub, &pom) != 8)
    abort ();

  return 0;
}


-- 
           Summary: Misscompilation due to alias analysis
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rakdver at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux
  GCC host triplet: i686-pc-linux
GCC target triplet: i686-pc-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29156