[Bug tree-optimization/51964] New: Missed tail merging opportunity

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51964

             Bug #: 51964
           Summary: Missed tail merging opportunity
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vries@gcc.gnu.org


pr51879-5.c:
...
int bar (int);
void baz (int);
void foo2 (void);
void
foo (int y, int z)
{
  int a;
  if (y == 6)
    {
      if (z)
    foo2 ();
      a = bar (7);
    }
  else
    a = bar (7);
  baz (a);
}
...

compile:
...
gcc -O2 pr51879-5.c -S -fdump-tree-all-all
...

pr51879-5.c.094t.pre:
...
  # BLOCK 5 freq:4877
  # PRED: 8 [100.0%]  (fallthru) 4 [100.0%]  (fallthru,exec)
  # .MEMD.1719_6 = PHI <.MEMD.1719_8(D)(8), .MEMD.1719_9(4)>
  # .MEMD.1719_10 = VDEF <.MEMD.1719_6>
  # USE = nonlocal 
  # CLB = nonlocal 
  aD.1712_4 = barD.1703 (7);
  goto <bb 7>;
  # SUCC: 7 [100.0%]  (fallthru,exec)

  # BLOCK 6 freq:5123
  # PRED: 2 [51.2%]  (false,exec)
  # .MEMD.1719_11 = VDEF <.MEMD.1719_8(D)>
  # USE = nonlocal 
  # CLB = nonlocal 
  aD.1712_5 = barD.1703 (7);
  # SUCC: 7 [100.0%]  (fallthru,exec)

  # BLOCK 7 freq:10000
  # PRED: 5 [100.0%]  (fallthru,exec) 6 [100.0%]  (fallthru,exec)
  # aD.1712_1 = PHI <aD.1712_4(5), aD.1712_5(6)>
  # .MEMD.1719_7 = PHI <.MEMD.1719_10(5), .MEMD.1719_11(6)>
  # .MEMD.1719_12 = VDEF <.MEMD.1719_7>
  # USE = nonlocal 
  # CLB = nonlocal 
  bazD.1705 (aD.1712_1);
  # VUSE <.MEMD.1719_12>
  return;
  # SUCC: EXIT [100.0%] 
...

Blocks 5 and 6 are not merged by tail_merge_optimize (they are merged by rtl
cross-jumping though).

The reason the blocks are not merged by tail_merge_optimize is that
tail_merge_optimize uses value numbering to determine equivalence of blocks.
And since the calls have a different vuse (.MEMD.1719_6 and .MEMD.1719_8(D))
the results of the calls won't have the same value number (even after fixing
PR51879).

However, the reason we can merge the calls is not because the calls have the
same result. It's because the results are used in the same way. To detect this
we should use a different comparison mechanism than the current.