[Bug tree-optimization/25243] Jump threading opportunity missed in tree-ssa but caught in jump1

[steven at gcc dot gnu dot org]

------- Comment #2 from steven at gcc dot gnu dot org  2005-12-03 14:37 -------
Actually VRP doesn't catch it.

Do:
-    if (e[i] == 16)
+    if (e[i] == 16)
so that store-CCP doesn't load e[0] anymore to find that it is 16.  With that,
the .vrp dump at -O2 looks like this:

baz (r)
{
  long unsigned int i;
  int D.1638;
  long unsigned int i.0;

<bb 0>:
  goto <bb 3> (<L2>);

<L0>:;
  i_4 = i_1;
  D.1638_6 = e[i_1];
  if (D.1638_6 == 17) goto <L3>; else goto <L1>;

<L1>:;
  i_7 = i_1 + 1;

  # i_1 = PHI <0(0), i_7(2)>;
<L2>:;
  if (i_1 <= 3) goto <L0>; else goto <L3>;

<L3>:;
  if (i_1 == 4) goto <L4>; else goto <L5>;

<L4>:;
  foo (r_3);

<L5>:;
  return;

}

The tree loop optimizers turn "(i_1 <= 3)" into "(i_1 != 4)", but there are no
passes after VRP that use the ASSERT_EXPRs to propagate "i_1 == 4" down along
the false-edge going to the block starting with label L3.

So even at -O2 we don't catch this jump threading opportunity at the tree
level.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25243