PR tree-optimization/37102 patch checked in to 4.3 branch

[Andrew MacLeod <amacleod at redhat dot com>]

This seems to have caused no problems in mainline, so here it is 
backported to the 4.3 branch.

Bootstrapped and no new regressions on x86_64-unknown-linux-gnu.

Andrew
2008-10-17  Andrew MacLeod  <amacleod@redhat.com>

	PR tree-optimization/37102
	* tree-outof-ssa.c (remove_gimple_phi_args): Remove all the PHI args  
	from a node. Check to see if another PHI is dead.
	(eliminate_useless_phis): Rename from eliminate_virtual_phis and remove
	real PHIs which have no uses.
	(rewrite_out_of_ssa): Call eliminate_useless_phis.

2008-10-17  Andrew MacLeod  <amacleod@redhat.com>

	PR tree-optimization/37102
	* gcc.c-torture/execute/pr37102.c: New Test.


Index: tree-outof-ssa.c
===================================================================
*** tree-outof-ssa.c	(revision 141136)
--- tree-outof-ssa.c	(working copy)
*************** replace_def_variable (var_map map, def_o
*** 606,630 ****
  }
  
  
! /* Remove any PHI node which is a virtual PHI.  */
  
  static void
! eliminate_virtual_phis (void)
  {
    basic_block bb;
!   tree phi, next;
  
    FOR_EACH_BB (bb)
      {
!       for (phi = phi_nodes (bb); phi; phi = next)
          {
  	  next = PHI_CHAIN (phi);
! 	  if (!is_gimple_reg (SSA_NAME_VAR (PHI_RESULT (phi))))
  	    {
  #ifdef ENABLE_CHECKING
  	      int i;
! 	      /* There should be no arguments of this PHI which are in
! 		 the partition list, or we get incorrect results.  */
  	      for (i = 0; i < PHI_NUM_ARGS (phi); i++)
  	        {
  		  tree arg = PHI_ARG_DEF (phi, i);
--- 606,669 ----
  }
  
  
! /* Remove each argument from PHI.  If an arg was the last use of an SSA_NAME, 
!    check to see if this allows another PHI node to be removed.  */
  
  static void
! remove_tree_phi_args (tree phi)
! {
!   use_operand_p arg_p;
!   ssa_op_iter iter;
! 
!   if (dump_file && (dump_flags & TDF_DETAILS))
!     {
!       fprintf (dump_file, "Removing Dead PHI definition: ");
!       print_generic_stmt (dump_file, phi, TDF_SLIM);
!     }
! 
!   FOR_EACH_PHI_ARG (arg_p, phi, iter, SSA_OP_USE)
!     {
!       tree arg = USE_FROM_PTR (arg_p);
!       if (TREE_CODE (arg) == SSA_NAME)
!         {
! 	  /* Remove the reference to the existing argument.  */
! 	  SET_USE (arg_p, NULL_TREE);
! 	  if (has_zero_uses (arg))
! 	    {
! 	      tree stmt = SSA_NAME_DEF_STMT (arg);
! 
! 	      /* Also remove the def if it is a PHI node.  */
! 	      if (TREE_CODE (stmt) == PHI_NODE)
! 		{
! 		  remove_tree_phi_args (stmt);
! 		  remove_phi_node (stmt, NULL_TREE, true);
! 		}
! 	    }
! 	}
!     }
! }
! 
! 
! /* Remove any PHI node which is a virtual PHI, or a PHI with no uses.  */
! 
! static void
! eliminate_useless_phis (void)
  {
    basic_block bb;
!   tree result, next, phi;
  
    FOR_EACH_BB (bb)
      {
!       for (phi = phi_nodes (bb); phi; phi = next) 
          {
  	  next = PHI_CHAIN (phi);
! 	  result = PHI_RESULT (phi);
! 	  if (!is_gimple_reg (SSA_NAME_VAR (result)))
  	    {
  #ifdef ENABLE_CHECKING
  	      int i;
! 	      /* There should be no arguments which are not virtual, or the
! 	         results will be incorrect.  */
  	      for (i = 0; i < PHI_NUM_ARGS (phi); i++)
  	        {
  		  tree arg = PHI_ARG_DEF (phi, i);
*************** eliminate_virtual_phis (void)
*** 639,645 ****
  		    }
  		}
  #endif
! 	      remove_phi_node (phi, NULL_TREE, true);
  	    }
  	}
      }
--- 678,693 ----
  		    }
  		}
  #endif
! 	      remove_phi_node (phi, NULL_TREE , true);
! 	    }
!           else
! 	    {
! 	      /* Also remove real PHIs with no uses.  */
! 	      if (has_zero_uses (result))
! 	        {
! 		  remove_tree_phi_args (phi);
! 		  remove_phi_node (phi, NULL_TREE, true);
! 		}
  	    }
  	}
      }
*************** rewrite_out_of_ssa (void)
*** 1449,1455 ****
       copies into the loop itself.  */
    insert_backedge_copies ();
  
!   eliminate_virtual_phis ();
  
    if (dump_file && (dump_flags & TDF_DETAILS))
      dump_tree_cfg (dump_file, dump_flags & ~TDF_DETAILS);
--- 1497,1505 ----
       copies into the loop itself.  */
    insert_backedge_copies ();
  
! 
!   /* Eliminate PHIs which are of no use, such as virtual or dead phis.  */
!   eliminate_useless_phis ();
  
    if (dump_file && (dump_flags & TDF_DETAILS))
      dump_tree_cfg (dump_file, dump_flags & ~TDF_DETAILS);