[PATCH] Fix for PR52009 - Another missed tail merging opportunity

[Tom de Vries]

Richard,

this patch fixes PR52009.

Consider this test-case:
...
int z;

void
foo (int y)
{
  if (y == 6)
    z = 5;
  else
    z = 5;
}
...

Currently, compiling with -O2 gives us this representation at 
pr51879-7.c.094t.pre:
...
  # BLOCK 3 freq:1991
  # PRED: 2 [19.9%]  (true,exec)
  # .MEMD.1710_4 = VDEF <.MEMD.1710_3(D)>
  zD.1702 = 5;
  goto <bb 5>;
  # SUCC: 5 [100.0%]  (fallthru,exec)

  # BLOCK 4 freq:8009
  # PRED: 2 [80.1%]  (false,exec)
  # .MEMD.1710_5 = VDEF <.MEMD.1710_3(D)>
  zD.1702 = 5;
  # SUCC: 5 [100.0%]  (fallthru,exec)

  # BLOCK 5 freq:10000
  # PRED: 3 [100.0%]  (fallthru,exec) 4 [100.0%]  (fallthru,exec)
  # .MEMD.1710_2 = PHI <.MEMD.1710_4(3), .MEMD.1710_5(4)>
  # VUSE <.MEMD.1710_2>
  return;
...

Blocks 3 and 4 are not tail-merged.

The patch allows the example to be tail-merged by:
- value numbering .MEMD.1710_4 and .MEMD.1710_5 equal
- comparing gimple_vdef value numbers for assignments during tail-merge

Bootstrapped and reg-tested on x86_64.

OK for stage1?

Thanks,
- Tom

2012-01-31  Tom de Vries  <t...@codesourcery.com>

        PR tree-optimization/52009
        * tree-ssa-tail-merge.c (gimple_equal_p): For GIMPLE_ASSIGN, compare
        value numbers of gimple_vdef.
        * tree-ssa-sccvn.h (struct vn_reference_s): Add vdef field.
        (vn_reference_insert): Add vdef parameter to prototype.
        * tree-ssa-sccvn.c (copy_reference_ops_from_ref): Handle MODIFY_EXPR.
        (vn_reference_insert): Add and handle vdef parameter.
        (visit_reference_op_load): Add argument to vn_reference_insert call.
        (visit_reference_op_store): Find value number of vdef of store.  Insert
        value number of vdef of store.

        * gcc.dg/pr51879-7.c: New test.