Shrink using sed code.
   text	   data	    bss	    dec	    hex	filename
    301	      0	      0	    301	    12d	coreutils.org/tac.o
     94	      0	      0	     94	     5e	coreutils/tac.o

--- busybox-1.19.0/coreutils/tac.c
+++ busybox-1.19.0/coreutils/tac.c
@@ -33,6 +33,19 @@
 int tac_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int tac_main(int argc UNUSED_PARAM, char **argv)
 {
+#ifdef CONFIG_SED
+	int status = 0, i = 1;
+	char *s, *sed_argv[4];
+	
+	sed_argv[1] = "1!G;h;$!d";
+	sed_argv[3] = NULL;
+	do {
+		s = sed_argv[2] = argv[i];
+		if (!s || *s != '-')
+			status = sed_main(0, sed_argv);
+	} while (status == 0 && s && argv[++i]);
+	return status;
+#else
 	char **name;
 	FILE *f;
 	struct lstring *line = NULL;
@@ -108,4 +121,5 @@
 	}
 
 	return retval;
+#endif
 }