canonicalize BImode true

[Richard Henderson <rth at cygnus dot com>]

Without this, we sign-extend from bit 0 to get -1 as the canonical
value for true.  If that were all, that would be no big deal.  

However, if STORE_FLAG_VALUE is 1 (the most useful value for C,
which is otherwise arbitrary on ia64), combine decides that it
needs to insert various negations to get from -1 to 1, which
prevents various combinations from ocurring.

Since all that's required of trunc_int_for_mode et al is that they
be consistent, I choose to canonicalize BImode on STORE_FLAG_VALUE.



r~


        * combine.c (if_then_else_cond): Canonicalize BImode true to
        STORE_FLAG_VALUE.
        * explow.c (trunc_int_for_mode): Likewise.

Index: combine.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/combine.c,v
retrieving revision 1.153
diff -c -p -d -r1.153 combine.c
*** combine.c	2000/09/18 18:24:31	1.153
--- combine.c	2000/09/18 18:28:00
*************** if_then_else_cond (x, ptrue, pfalse)
*** 7418,7423 ****
--- 7433,7446 ----
  	   || ((cond0 = get_last_value (x)) != 0 && CONSTANT_P (cond0)))
      ;
  
+   /* If we're in BImode, canonicalize on 0 and STORE_FLAG_VALUE, as that
+      will be least confusing to the rest of the compiler.  */
+   else if (mode == BImode)
+     {
+       *ptrue = GEN_INT (STORE_FLAG_VALUE), *pfalse = const0_rtx;
+       return x;
+     }
+ 
    /* If X is known to be either 0 or -1, those are the true and
       false values when testing X.  */
    else if (x == constm1_rtx || x == const0_rtx
Index: explow.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/explow.c,v
retrieving revision 1.52
diff -c -p -d -r1.52 explow.c
*** explow.c	2000/07/23 17:12:04	1.52
--- explow.c	2000/09/18 18:28:00
*************** trunc_int_for_mode (c, mode)
*** 52,57 ****
--- 52,61 ----
  {
    int width = GET_MODE_BITSIZE (mode);
  
+   /* Canonicalize BImode to 0 and STORE_FLAG_VALUE.  */
+   if (mode == BImode)
+     return c & 1 ? STORE_FLAG_VALUE : 0;
+ 
    /* We clear out all bits that don't belong in MODE, unless they and our
       sign bit are all one.  So we get either a reasonable negative
       value or a reasonable unsigned value.  */