Richard,

I have a patch for the i686 bootstrap problem reported in PR50763 comment 10.

pr50763-2.c looks like this before tail_merge_optimize:
...
std_canonical_va_list_type (union tree_node * typeD.1608)
{
  _BoolD.1576 pretmp.6D.2739;
  union tree_node * pretmp.5D.2738;
  _BoolD.1576 pretmp.4D.2737;
  union tree_node * pretmp.3D.2736;
  intD.6 D.2734;
  _BoolD.1576 D.2733;
  union tree_node * D.2725;
  intD.6 D.2722;
  intD.6 D.2717;

  # BLOCK 2 freq:10000
  # PRED: ENTRY [100.0%]  (fallthru,exec)
  # VUSE <.MEMD.2730_12(D)>
  D.2717_4 = typeD.1608_3(D)->baseD.1605.codeD.1597;
  if (D.2717_4 != 0)
    goto <bb 3>;
  else
    goto <bb 4>;
  # SUCC: 3 [50.0%]  (true,exec) 4 [50.0%]  (false,exec)

  # BLOCK 3 freq:5000
  # PRED: 2 [50.0%]  (true,exec)
  # VUSE <.MEMD.2730_12(D)>
  # PT = anything
  typeD.1608_5 = typeD.1608_3(D)->typedD.1606.typeD.1600;
  goto <bb 6>;
  # SUCC: 6 [100.0%]  (fallthru,exec)

  # BLOCK 4 freq:5000
  # PRED: 2 [50.0%]  (false,exec)
  # VUSE <.MEMD.2730_12(D)>
  # PT = anything
  typeD.1608_6 = typeD.1608_3(D)->typedD.1606.typeD.1600;
  # VUSE <.MEMD.2730_12(D)>
  D.2722_7 = typeD.1608_6->baseD.1605.codeD.1597;
  if (D.2722_7 != 0)
    goto <bb 5>;
  else
    goto <bb 7>;
  # SUCC: 5 [50.0%]  (true,exec) 7 [50.0%]  (false,exec)

  # BLOCK 7 freq:2500
  # PRED: 4 [50.0%]  (false,exec)
  goto <bb 6>;
  # SUCC: 6 [100.0%]  (fallthru)

  # BLOCK 5 freq:2500
  # PRED: 4 [50.0%]  (true,exec)
  # SUCC: 6 [100.0%]  (fallthru,exec)

  # BLOCK 6 freq:10000
  # PRED: 3 [100.0%]  (fallthru,exec) 7 [100.0%]  (fallthru) 5 [100.0%]
(fallthru,exec)
  # PT = anything

  # typeD.1608_1 = PHI <typeD.1608_5(3), typeD.1608_3(D)(7), typeD.1608_6(5)>
  # VUSE <.MEMD.2730_12(D)>
  # PT = anything
  D.2725_9 = typeD.1608_1->type_commonD.1607.main_variantD.1604;
  D.2733_15 = D.2725_9 != 0B;
  D.2734_14 = (intD.6) D.2733_15;
  # VUSE <.MEMD.2730_12(D)>
  return D.2734_14;
  # SUCC: EXIT [100.0%]

}
...

tail_merge_optimize discovers that block 3 and 5 are identical, and removes block 5.
In replace_block_by, we end up with phi_vuse1 == NULL_TREE and
phi_vuse2 == .MEMD.2730_12(D).

After the original fix to PR50763, this forces us to update the vuses, while
that is not necessary.

Attached patch fixes this problem by determining more precisely when the vuses
need to be updated.

bootstrapped and reg-tested on x86_64 and i686.

OK for trunk?

Thanks,
- Tom

2011-10-25  Tom de Vries  <tom@codesourcery.com>

	PR tree-optimization/50763
	* tree-ssa-tail-merge.c (replace_block_by): Update vops if phi_vuse1 or
	phi_vuse2 is NULL_TREE only if bb1 dominates or is dominated by bb2.

	* gcc.dg/pr50763-2.c: New test.