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