if_then_else simplification

[Richard Henderson <rth at cygnus dot com>]

Consider

	int f()
	{
	  int x = 0, p;
	  p = foo();
	  if (p)
	    x = 1;
	  return x;
	}

Clearly this last assignment is equivalent to

	  x = (p != 0);

but we currently won't transform things that far.  One could argue
for stronger ifcvt in this case, but this is actually a simplification
of something in cpplib, in which foo() is actually a 5 node subgraph. 
At which point things stop seeming quite so reasonable.

CSE is quite happy, however, to substitute constants such that we get

	(set (reg:SI 100)
	     (if_then_else:SI (eq (reg:SI 101) (const_int 0))
		(const_int 0)
		(const_int 1)))

With this patch we take the extra step to reduce this to ne:SI.

Tested on alpha and i686, though i686 requires other changes to 
get this substitution to happen.  (Those secondary changes might
be a bad idea though, since they could potentially allow combine
to perform transformations that we do not want.  More investigation
is required here.)



r~

        * simplify-rtx.c (simplify_ternary_operation): Try to simplify
        IF_THEN_ELSE to a setcc form.

Index: simplify-rtx.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/simplify-rtx.c,v
retrieving revision 1.16
diff -c -p -d -r1.16 simplify-rtx.c
*** simplify-rtx.c	2000/05/04 21:15:03	1.16
--- simplify-rtx.c	2000/05/22 08:18:51
*************** simplify_ternary_operation (code, mode, 
*** 1963,1968 ****
--- 1963,1987 ----
  	    return op2;
  	  else if (temp == const1_rtx)
  	    return op1;
+ 	  else if (temp)
+ 	    op0 = temp;
+ 
+ 	  /* Look for happy constants in op1 and op2.  */
+ 	  if (GET_CODE (op1) == CONST_INT && GET_CODE (op2) == CONST_INT)
+ 	    {
+ 	      HOST_WIDE_INT t = INTVAL (op1);
+ 	      HOST_WIDE_INT f = INTVAL (op2);
+ 	      
+ 	      if (t == STORE_FLAG_VALUE && f == 0)
+ 	        code = GET_CODE (op0);
+ 	      else if (t == 0 && f == STORE_FLAG_VALUE
+ 		       && can_reverse_comparison_p (op0, NULL_RTX))
+ 		code = reverse_condition (GET_CODE (op0));
+ 	      else
+ 		break;
+ 
+ 	      return gen_rtx_fmt_ee (code, mode, XEXP (op0, 0), XEXP (op0, 1));
+ 	    }
  	}
        break;