perf probe: Show better error message when failed to find variable

Indicate to check variable location range in error message when we got
failed to find the variable.

Before this patch:

  $ perf probe --add 'generic_perform_write+118 bytes'
  Failed to find the location of bytes at this address.
   Perhaps, it has been optimized out.
    Error: Failed to add events.

After this patch:

  $ perf probe --add 'generic_perform_write+118 bytes'
  Failed to find the location of the 'bytes' variable at this address.
   Perhaps it has been optimized out.
   Use -V with the --range option to show 'bytes' location range.
    Error: Failed to add events.
Signed-off-by: He Kuang <hekuang@huawei.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1431336304-16863-3-git-send-email-hekuang@huawei.com
[ Improve the error message based on lkml thread ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-finder.c

@@ -527,10 +527,12 @@ static int convert_variable(Dwarf_Die *vr_die, struct probe_finder *pf)
 
 	ret = convert_variable_location(vr_die, pf->addr, pf->fb_ops,
 					&pf->sp_die, pf->tvar);
-	if (ret == -ENOENT || ret == -EINVAL)
-		pr_err("Failed to find the location of %s at this address.\n"
-		       " Perhaps, it has been optimized out.\n", pf->pvar->var);
-	else if (ret == -ENOTSUP)
+	if (ret == -ENOENT || ret == -EINVAL) {
+		pr_err("Failed to find the location of the '%s' variable at this address.\n"
+		       " Perhaps it has been optimized out.\n"
+		       " Use -V with the --range option to show '%s' location range.\n",
+		       pf->pvar->var, pf->pvar->var);
+	} else if (ret == -ENOTSUP)
 		pr_err("Sorry, we don't support this variable location yet.\n");
 	else if (ret == 0 && pf->pvar->field) {
 		ret = convert_variable_fields(vr_die, pf->pvar->var,