SSA DCE bugfix

[law at redhat dot com]

This patch fixes the bug in SSA DCE that's been recently discussed on the
main development list.

I've bootstrapped this on a private tree with SSA enabled by default on
ia64-linux.

This was a bug exposed by c-torture when using SSA opts on a different
target.  I don't recall precisely what test it was.  Regardless, we don't
need to add a new testcase to the testsuite for this bug.

	* ssa-dce.c (eliminate_dead_code): Properly handle control
	dependencies implied by PHI nodes.

Index: ssa-dce.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/ssa-dce.c,v
retrieving revision 1.2
diff -c -3 -p -r1.2 ssa-dce.c
*** ssa-dce.c	2001/06/28 17:18:36	1.2
--- ssa-dce.c	2001/06/28 17:54:25
*************** eliminate_dead_code ()
*** 558,563 ****
--- 558,598 ----
  			&propagate_necessity_through_operand,
  			(PTR) &unprocessed_instructions);
  
+ 	  /* PHI nodes are somewhat special in that each PHI alternative
+ 	     has data and control dependencies.  The data dependencies
+ 	     are handled via propagate_necessity_through_operand.  We
+ 	     handle the control dependency here.
+ 
+ 	     We consider the control dependent edges leading to the
+ 	     predecessor block associated with each PHI alternative
+ 	     as necessary.  */
+ 	  if (PHI_NODE_P (current_instruction))
+ 	    {
+ 	      rtvec phi_vec = XVEC (SET_SRC (PATTERN (current_instruction)), 0);
+ 	      int num_elem = GET_NUM_ELEM (phi_vec);
+ 	      int v;
+ 
+ 	      for (v = num_elem - 2; v >= 0; v -= 2)
+ 		{
+ 		  basic_block bb;
+ 
+ 		  bb = BASIC_BLOCK (INTVAL (RTVEC_ELT (phi_vec, v + 1)));
+ 		  EXECUTE_IF_CONTROL_DEPENDENT
+ 		    (cdbte, bb->end, edge_number,
+ 		    {
+ 		      rtx jump_insn;
+ 
+ 		      jump_insn = (INDEX_EDGE_PRED_BB (el, edge_number))->end;
+ 		      if (((GET_CODE (jump_insn) == JUMP_INSN))
+ 			  && UNNECESSARY_P (jump_insn))
+ 			{
+ 			  RESURRECT_INSN (jump_insn);
+ 			  VARRAY_PUSH_RTX (unprocessed_instructions, jump_insn);
+ 			}
+ 		    });
+ 
+ 		}
+ 	    }
  	}
      }