(build_binary_op): For TRUNC_MOD_EXPR and FLOOR_MOD_EXPR,

don't shorten when divisor might be -1.

From-SVN: r5893

gcc/c-typeck.c

@@ -1982,7 +1982,14 @@ build_binary_op (code, orig_op0, orig_op1, convert_p)
     case TRUNC_MOD_EXPR:
     case FLOOR_MOD_EXPR:
       if (code0 == INTEGER_TYPE && code1 == INTEGER_TYPE)
-	shorten = 1;
+	/* Although it would be tempting to shorten always here, doing so loses
+	   on some targets, since the modulo instruction is undefined if the
+	   quotient can't be represented in the computation mode.  We shorten
+	   only if unsigned or if dividing by something we know != -1.  */
+	shorten = (TREE_UNSIGNED (orig_op0)
+		   || (TREE_CODE (op1) == INTEGER_CST
+		       && (TREE_INT_CST_LOW (op1) != -1
+			   || TREE_INT_CST_HIGH (op1) != -1)));
       break;
 
     case TRUTH_ANDIF_EXPR: