cksum: document the --debug option

This should have been part of commit v8.32-113-gb73b9fcb1

* doc/coreutils.texi (cksum invocation): Add the --debug description.
* src/cksum.c (usage): Likewise.
(main): Also give explicit indication when using generic hardware.

doc/coreutils.texi

@@ -3949,8 +3949,14 @@ The CRC algorithm is specified by the POSIX standard.  It is not
 compatible with the BSD or System V @command{sum} algorithms (see the
 previous section); it is more robust.
 
-The only options are @option{--help} and @option{--version}.  @xref{Common
-options}.
+The program accepts the following options.  Also see @ref{Common options}.
+
+@table @samp
+
+@item --debug
+@opindex --debug
+Output extra information to stderr, like the checksum implementation being used.
+@end table
 
 @exitstatus
 

src/cksum.c

@@ -344,6 +344,9 @@ Usage: %s [FILE]...\n\
       fputs (_("\
 Print CRC checksum and byte counts of each FILE.\n\
 \n\
+"), stdout);
+      fputs (_("\
+      --debug    indicate which implementation used\n\
 "), stdout);
       fputs (HELP_OPTION_DESCRIPTION, stdout);
       fputs (VERSION_OPTION_DESCRIPTION, stdout);
@@ -392,7 +395,10 @@ main (int argc, char **argv)
 # if USE_PCLMUL_CRC32
   if (pclmul_supported ())
      cksum_fp = cksum_pclmul;
+  else
 # endif /* USE_PCLMUL_CRC32 */
+  if (debug)
+    error (0, 0, "%s", _("using generic hardware support"));
 
   if (optind == argc)
     ok = cksum ("-", false);