loop.c:basic_induction_var does not accept ZERO_EXTEND

[David Edelsohn]

	While investigating loop optimizations on 64-bit PowerPC on a
32-bit host, I noticed that no biv was being found for a simple loop.  I
tracked this down to loop.c:basic_induction_var() not having a case for
ZERO_EXTEND.  The comment at the beginning of the function mentions
detecting BIVs in a promoted variable which is sign-extended or
zero-extended, but the switch statement only has a case for SIGN_EXTEND,
not ZERO_EXTEND.

	Naively, I would expect ZERO_EXTEND should be treated identically
to SIGN_EXTEND.  Was there a technical reason to exclude zero-extended
values or an oversight?  Is there some subtle corner-case problem with
zero-extended values as induction variables?  If there is a reason, the
comment at the top of the function should be clarified.

Thanks, David


Index: loop.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/loop.c,v
retrieving revision 1.248
diff -c -p -r1.248 loop.c
*** loop.c	2000/05/19 03:44:58	1.248
--- loop.c	2000/05/22 18:26:35
*************** basic_induction_var (loop, x, mode, dest
*** 6071,6076 ****
--- 6071,6077 ----
   	return 0;
  
      case SIGN_EXTEND:
+     case ZERO_EXTEND:
        return basic_induction_var (loop, XEXP (x, 0), GET_MODE (XEXP (x, 0)),
  				  dest_reg, p, inc_val, mult_val, location,
  				  multi_insn_incr);