[Bug java/15525] suggestion to enable cast elimination

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

------- Additional Comments From tromey at gcc dot gnu dot org  2004-09-02 01:39 -------
Here's my test case.  This is idiomatic Java code,
the cast is checked but the check is completely redundant
and can be eliminated in theory; that's the goal.

I compiled with:

gcj -c -O3 -fdump-tree-all q.java

{
  int f;
  public boolean equals(Object x)
  {
    if (! (x instanceof q))
      return false;
    q v = (q) x;
    return f == v.f;
  }
}


Before the patch, I seethis in q.java.t60.vars:


;; Function q.equals(java.lang.Object) (_ZN1q6equalsEPN4java4lang6ObjectE)

q.equals(java.lang.Object) (this, x)
{
  int T.2;

<bb 0>:
  if (_Jv_IsInstanceOf (x, &q.class) == 0) goto <L4>; else goto <L1>;

<L4>:;
  T.2 = 0;
  goto <bb 2> (<L2>);

<L1>:;
  T.2 = this->f == _Jv_CheckCast (&q.class, x)->f;

<L2>:;
  return T.2;

}



;; Function () (_ZN1qC1Ev)

() (this)
{
<bb 0>:
  <init> (this) [tail call];
  return;

}


After the patch, q.java.t63.vars:

;; Function q.equals(java.lang.Object) (_ZN1q6equalsEPN4java4lang6ObjectE)

q.equals(java.lang.Object) (this, x)
{
  struct q * iftmp.10;
  int T.6;
  boolean T.3;

<bb 0>:
  T.3 = _Jv_IsInstanceOf (x, &q.class);
  if (x == 0B || !T.3) goto <L11>; else goto <L1>;

<L11>:;
  T.6 = 0;
  goto <bb 7> (<L8>);

<L1>:;
  if (x != 0B) goto <L2>; else goto <L12>;

<L12>:;
  iftmp.10 = 0B;
  goto <bb 6> (<L7>);

<L2>:;
  if (T.3) goto <L3>; else goto <L4>;

<L3>:;
  iftmp.10 = (struct q *) x;
  goto <bb 5> (<L5>);

<L4>:;
  _Jv_ThrowCast (&q.class, x);

<L5>:;

<L7>:;
  T.6 = this->f == iftmp.10->f;

<L8>:;
  return T.6;

}



;; Function () (_ZN1qC1Ev)

() (this)
{
<bb 0>:
  <init> (this) [tail call];
  return;

}

This is an improvement, since we've eliminated an InstanceOf
(the patch works by breaking CheckCast into explicit InstanceOf +
ThrowCast; you won't see the redundant InstanceOf in the first
dump).

With VRP we could also remove the dead call to ThrowCast.
This call is only reached from <L2>, and note earlier that
T.3 is never 0.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15525