[Bug tree-optimization/43402] [4.5 Regression] dom1 miscompiles binary search

[rguenth at gcc dot gnu dot org]

------- Comment #8 from rguenth at gcc dot gnu dot org  2010-03-17 16:42 -------
(In reply to comment #7)
> Hmm, I wonder how that could cause the bug.  Probably because we can't rely
> on SSA form being uptodate during cfgcleanup, and hence looking up PHI
> arguments is wrong, at least for those SSA names that are registered for
> updating.

But you can't have SSA names around when you rename its symbol.  And only
with SSA names the lookup is performed.

After DOM (but before cfgcleanup completes) we have

<bb 4>:
  # start_1 = PHI <start_16(3)>
  # limit_2 = PHI <limit_19(3)>
  # lastMid_3 = PHI <mid_10(3)>
  D.1978_9 = start_1 + limit_2;
  mid_10 = D.1978_9 >> 1;
  if (lastMid_3 == mid_10)
    goto <bb 8>;
  else
    goto <bb 5>;

which is then simplified.

Hm.  It seems we have a SSA name replacement table, thus probably the
cfgcleanup lookup code has to honor this.

The following seems to fix it:

Index: tree-cfgcleanup.c
===================================================================
*** tree-cfgcleanup.c   (revision 157512)
--- tree-cfgcleanup.c   (working copy)
*************** cleanup_control_expr_graph (basic_block
*** 100,123 ****
          {
            tree lhs = gimple_cond_lhs (stmt);
            tree rhs = gimple_cond_rhs (stmt);
!           /* For conditions try harder and lookup single-argument
!              PHI nodes.  Only do so from the same basic-block though
!              as other basic-blocks may be dead already.  */
!           if (TREE_CODE (lhs) == SSA_NAME)
              {
!               gimple def_stmt = SSA_NAME_DEF_STMT (lhs);
!               if (gimple_code (def_stmt) == GIMPLE_PHI
!                   && gimple_phi_num_args (def_stmt) == 1
!                   && gimple_bb (def_stmt) == gimple_bb (stmt))
!                 lhs = PHI_ARG_DEF (def_stmt, 0);
!             }
!           if (TREE_CODE (rhs) == SSA_NAME)
!             {
!               gimple def_stmt = SSA_NAME_DEF_STMT (rhs);
!               if (gimple_code (def_stmt) == GIMPLE_PHI
!                   && gimple_phi_num_args (def_stmt) == 1
!                   && gimple_bb (def_stmt) == gimple_bb (stmt))
!                 rhs = PHI_ARG_DEF (def_stmt, 0);
              }
            val = fold_binary_loc (loc, gimple_cond_code (stmt),
                                   boolean_type_node, lhs, rhs);
--- 100,126 ----
          {
            tree lhs = gimple_cond_lhs (stmt);
            tree rhs = gimple_cond_rhs (stmt);
!           if (!name_mappings_registered_p ())
              {
!               /* For conditions try harder and lookup single-argument
!                  PHI nodes.  Only do so from the same basic-block though
!                  as other basic-blocks may be dead already.  */
!               if (TREE_CODE (lhs) == SSA_NAME)
!                 {
!                   gimple def_stmt = SSA_NAME_DEF_STMT (lhs);
!                   if (gimple_code (def_stmt) == GIMPLE_PHI
!                       && gimple_phi_num_args (def_stmt) == 1
!                       && gimple_bb (def_stmt) == gimple_bb (stmt))
!                     lhs = PHI_ARG_DEF (def_stmt, 0);
!                 }
!               if (TREE_CODE (rhs) == SSA_NAME)
!                 {
!                   gimple def_stmt = SSA_NAME_DEF_STMT (rhs);
!                   if (gimple_code (def_stmt) == GIMPLE_PHI
!                       && gimple_phi_num_args (def_stmt) == 1
!                       && gimple_bb (def_stmt) == gimple_bb (stmt))
!                     rhs = PHI_ARG_DEF (def_stmt, 0);
!                 }
              }
            val = fold_binary_loc (loc, gimple_cond_code (stmt),
                                   boolean_type_node, lhs, rhs);

though the essence of r157093 was to capture single-valued PHI nodes
with constant operands, so we could restrict the lookup to consider
constants only.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rguenth at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2010-03-17 15:21:02         |2010-03-17 16:42:29
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43402