[Bug tree-optimization/56654] New: uninit warning behaves erratically

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56654

             Bug #: 56654
           Summary: uninit warning behaves erratically
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rguenth@gcc.gnu.org
                CC: xinliangli@gmail.com


For c-c++-common/uninit-17.c we currently expect the uninit use at:

static void bar(int a, int *ptr)
{
  do
  {
    int b;   /* { dg-message "declared" } */
    if (b < 40) {
      ptr[0] = b;
    }
    b += 1; /* { dg-warning "may be used uninitialized" } */
    ptr++;
  }
  while (--a != 0);
}

where I would expect it at the if stmt.  Now, when I exchange the late
DOM and the late VRP passes I instead get the warning two lines earlier
(inside the guarded BB).  But there is no change in the IL:

> diff -u a/uninit-17.c.131t.uninit1 b/uninit-17.c.131t.uninit1 
--- a/uninit-17.c.131t.uninit1  2013-03-18 14:27:16.677116283 +0100
+++ b/uninit-17.c.131t.uninit1  2013-03-18 14:26:58.956922876 +0100
@@ -3,7 +3,13 @@

 [WORKLIST]: add to initial list: b_5 = PHI <b_13(D)(2), b_7(6)>
 [CHECK]: examining phi: b_5 = PHI <b_13(D)(2), b_7(6)>
-[CHECK]: Found unguarded use: b_7 = b_5 + 1;
+
+Use in stmt *ptr_6 = b_5;
+is guarded by :
+if (b_5 <= 39)
+
+[CHECK] Found def edge 1 in b_5 = PHI <b_13(D)(2), b_7(6)>
+[CHECK]: Found unguarded use: *ptr_6 = b_5;
 void foobar(int, int*) (int a, int * ptr)
 {
   int b;

the only difference is in the order of b_5 immediate uses.

That is, find_uninit_use should impose an ordering when walking over
immediate uses - for example visit the most dominating use first.