combine.c (force_to_mode): Convert subtraction from a constant to NEG or NOT when conditions allow.

        * combine.c (force_to_mode) [MINUS]: Convert subtraction from
        a constant to NEG or NOT when conditions allow.

From-SVN: r34248

gcc/ChangeLog

 2000-05-29  Richard Henderson  <rth@cygnus.com>
 
+	* combine.c (force_to_mode) [MINUS]: Convert subtraction from
+	a constant to NEG or NOT when conditions allow.
+
 	* combine.c (combine_simplify_rtx): Don't create an if_then_else
 	unless both args are general_operand.  Don't canonicalize plus
 	to ior unless it helps.

gcc/combine.c

@@ -6846,7 +6846,6 @@ force_to_mode (x, mode, mask, reg, just_select)
 
       /* ... fall through ...  */
 
-    case MINUS:
     case MULT:
       /* For PLUS, MINUS and MULT, we need any bits less significant than the
 	 most significant bit in MASK since carries from those bits will
@@ -6854,6 +6853,28 @@ force_to_mode (x, mode, mask, reg, just_select)
       mask = fuller_mask;
       goto binop;
 
+    case MINUS:
+      /* If X is (minus C Y) where C's least set bit is larger than any bit
+	 in the mask, then we may replace with (neg Y).  */
+      if (GET_CODE (XEXP (x, 0)) == CONST_INT
+	  && (INTVAL (XEXP (x, 0)) & -INTVAL (XEXP (x, 0))) > mask)
+	{
+	  x = gen_unary (NEG, GET_MODE (x), GET_MODE (x), XEXP (x, 1));
+	  return force_to_mode (x, mode, mask, reg, next_select);
+	}
+
+      /* Similarly, if C contains every bit in the mask, then we may
+	 replace with (not Y).  */
+      if (GET_CODE (XEXP (x, 0)) == CONST_INT
+          && (INTVAL (XEXP (x, 0)) | mask) == INTVAL (XEXP (x, 0)))
+	{
+	  x = gen_unary (NOT, GET_MODE (x), GET_MODE (x), XEXP (x, 1));
+	  return force_to_mode (x, mode, mask, reg, next_select);
+	}
+
+      mask = fuller_mask;
+      goto binop;
+
     case IOR:
     case XOR:
       /* If X is (ior (lshiftrt FOO C1) C2), try to commute the IOR and