Richard,

I have a fix for PR50763.

The second example from the PR looks like this:
...
int bar (int i);

void
foo (int c, int d)
{
  if (bar (c))
    bar (c);
  d = 33;
  while (c == d);
}
...

When compiled with -O2 -fno-dominator-opt, the gimple representation before
ftree-tail-merge looks like this:
...
foo (intD.6 cD.1606, intD.6 dD.1607)
{
  intD.6 D.2730;

  # BLOCK 2 freq:900
  # PRED: ENTRY [100.0%]  (fallthru,exec)
  # .MEMD.2733_6 = VDEF <.MEMD.2733_5(D)>
  # USE = nonlocal
  # CLB = nonlocal
  D.2730_2 = barD.1605 (cD.1606_1(D));
  if (D.2730_2 != 0)
    goto <bb 3>;
  else
    goto <bb 7>;
  # SUCC: 3 [29.0%]  (true,exec) 7 [71.0%]  (false,exec)

  # BLOCK 7 freq:639
  # PRED: 2 [71.0%]  (false,exec)
  goto <bb 4>;
  # SUCC: 4 [100.0%]  (fallthru)

  # BLOCK 3 freq:261
  # PRED: 2 [29.0%]  (true,exec)
  # .MEMD.2733_7 = VDEF <.MEMD.2733_6>
  # USE = nonlocal
  # CLB = nonlocal
  barD.1605 (cD.1606_1(D));
  # SUCC: 4 [100.0%]  (fallthru,exec)

  # BLOCK 4 freq:900
  # PRED: 7 [100.0%]  (fallthru) 3 [100.0%]  (fallthru,exec)
  # .MEMD.2733_4 = PHI <.MEMD.2733_6(7), .MEMD.2733_7(3)>
  if (cD.1606_1(D) == 33)
    goto <bb 8>;
  else
    goto <bb 9>;
  # SUCC: 8 [91.0%]  (true,exec) 9 [9.0%]  (false,exec)

  # BLOCK 9 freq:81
  # PRED: 4 [9.0%]  (false,exec)
  goto <bb 6>;
  # SUCC: 6 [100.0%]  (fallthru)

  # BLOCK 8 freq:819
  # PRED: 4 [91.0%]  (true,exec)
  # SUCC: 5 [100.0%]  (fallthru)

  # BLOCK 5 freq:9100
  # PRED: 8 [100.0%]  (fallthru) 10 [100.0%]  (fallthru)
  if (cD.1606_1(D) == 33)
    goto <bb 10>;
  else
    goto <bb 11>;
  # SUCC: 10 [91.0%]  (true,exec) 11 [9.0%]  (false,exec)

  # BLOCK 10 freq:8281
  # PRED: 5 [91.0%]  (true,exec)
  goto <bb 5>;
  # SUCC: 5 [100.0%]  (fallthru)

  # BLOCK 11 freq:819
  # PRED: 5 [9.0%]  (false,exec)
  # SUCC: 6 [100.0%]  (fallthru)

  # BLOCK 6 freq:900
  # PRED: 11 [100.0%]  (fallthru) 9 [100.0%]  (fallthru)
  # VUSE <.MEMD.2733_4>
  return;
  # SUCC: EXIT [100.0%]

}
...

During the first iteration, tail_merge_optimize finds that block 9 and 11, and
block 8 and 10 are equal, and removes block 11 and 10.
During the second iteration it finds that block 4 and block 5 are equal, and it
removes block 5.

Since pre had no effect, the responsibility for updating the vops lies with
tail_merge_optimize.

Block 4 starts with a virtual PHI which needs updating, but replace_block_by
decides that an update is not necessary, because vop_at_entry returns NULL_TREE
for block 5 (the vop_at_entry for block 4 is .MEMD.2733_4).
What is different from normal is that block 4 dominates block 5.

The patch makes sure that the vops are also updated if vop_at_entry is defined
for only one of bb1 and bb2.

This also forced me to rewrite the code that updates the uses, which uses
dominator info now. This forced me to keep the dominator info up-to-date. Which
forced me to move the actual deletion of the basic block and some additional
bookkeeping related to that from purge_bbs to replace_block_by.

Additionally, I fixed the case that update_vuses leaves virtual phis with only
one argument (see unlink_virtual_phi).

bootstrapped and reg-tested on x86_64. The tested patch had one addition to the
attached patch: calling verify_dominators at the end of replace_block_by.

OK for trunk?

Thanks,
- Tom

2011-10-20  Tom de Vries  <tom@codesourcery.com>

	PR tree-optimization/50763
	* tree-ssa-tail-merge.c (same_succ_flush_bb): New function, factored out
	of ...
	(same_succ_flush_bbs): Use same_succ_flush_bb.
	(purge_bbs): Remove argument.  Remove calls to same_succ_flush_bbs,
	release_last_vdef and delete_basic_block.
	(unlink_virtual_phi): New function.
	(update_vuses): Add and use vuse1_phi_args argument.  Set var to
	SSA_NAME_VAR of vuse1 or vuse2, and use var.  Handle case that def_stmt2
	is NULL.  Use phi result as phi arg in case vuse1 or vuse2 is NULL_TREE.
	Replace uses of vuse1 if vuse2 is NULL_TREE.  Fix code to limit
	replacement of uses.  Propagate phi argument for phis with a single
	argument.
	(replace_block_by): Update vops if phi_vuse1 or phi_vuse2 is NULL_TREE.
	Set vuse1_phi_args if vuse1 is a phi defined in bb1.  Add vuse1_phi_args
	as argument to call to update_vuses.  Call release_last_vdef,
	same_succ_flush_bb, delete_basic_block.  Update CDI_DOMINATORS info.
	(tail_merge_optimize): Remove argument in call to purge_bbs.  Remove
	call to free_dominance_info.  Only call calculate_dominance_info once.

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