[tree-ssa] moving phi nodes?

[Richard Henderson <rth at redhat dot com>]

One jump threading case that we're currently missing, sorely, 
is of the form:

int foo(int p, int q)
{
  int i = 0;
  if (p)
    i = q;
  if (i != 0)
    bar ();
  return i;
}

>From the t28.dce2 dump:

  # BLOCK 0
  # PRED: ENTRY (fallthru,exec)
  if (p_3 != 0) goto <Uc910>; else goto <Uc984>;
  # SUCC: 2 (false,exec) 1 (true,exec)

  # BLOCK 1
  # PRED: 0 (true,exec)
<Uc910>:;
  # SUCC: 2 (fallthru,exec)

  # BLOCK 2
  # PRED: 0 (false,exec) 1 (fallthru,exec)
  # i_1 = PHI <0(0), q_4(1)>;
<Uc984>:;
  if (i_1 != 0) goto <Uc9f8>; else goto <Uca6c>;
  # SUCC: 4 (false,exec) 3 (true,exec)

  # BLOCK 3
  # PRED: 2 (true,exec)
<Uc9f8>:;
  bar ();
  # SUCC: 4 (fallthru,exec)

  # BLOCK 4
  # PRED: 2 (false,exec) 3 (fallthru,exec)
<Uca6c>:;
  return i_1;
  # SUCC: EXIT

We really should be threading the edge BB0->BB4, since it is known
that I == 0 when incoming to block 2.  Except if we do this, we need
to move the PHI node for I:

  # BLOCK 0
  # PRED: ENTRY (fallthru,exec)
  if (p_3 != 0) goto <Uc910>; else goto <Uca6c>;
  # SUCC: 4 (false,exec) 1 (true,exec)

  # BLOCK 1
  # PRED: 0 (true,exec)
<Uc910>:;
  # SUCC: 2 (fallthru,exec)

  # BLOCK 2
  # PRED: 1 (fallthru,exec)
<Uc984>:;
  if (q_4 != 0) goto <Uc9f8>; else goto <Uca6c>;
  # SUCC: 4 (false,exec) 3 (true,exec)

  # BLOCK 3
  # PRED: 2 (true,exec)
<Uc9f8>:;
  bar ();
  # SUCC: 4 (fallthru,exec)

  # BLOCK 4
  # PRED: 0 (false,exec) 2 (false,exec) 3 (fallthru,exec)
  # i_1 = PHI <0(0), 0(2), q_4(3)>;
<Uca6c>:;
  return i_1;
  # SUCC: EXIT

Moving the PHI node is non-trivial.  Anyone know of a way to do it that
doesn't require just dropping all the SSA names and starting over?  Do
we have any other gimple pass planned or desired that would take care 
of this as a side effect?

Incidentally, this optimization is performed by the rtl .14.bypass 
gcse pass.


r~