Improve target description check for SVE in gdbserver

The current code checks for the presence of a SVE target description by
comparing the number of registers.  This is a bit fragile since the number
of registers can change whenever we add new sets. Like PAC, for example.

If the comparison breaks, then we're left with SVE registers in the
description, but gdbserver doesn't send the registers to GDB, which in
turn displays stale information to the user.

The following patch changes the check to use the SVE feature string instead,
which hopefully should be more stable.

gdb/gdbserver/ChangeLog:

2019-11-20  Luis Machado  <luis.machado@linaro.org>

	* linux-aarch64-low.c (is_sve_tdesc): Check against target feature
	instead of register count.
	* tdesc.c (tdesc_contains_feature): New function.
	* tdesc.h (tdesc_contains_feature): New prototype.

Change-Id: I28b782cb1677560ca9a06a1be442974b25aabae4

1 files changed, 16 insertions, 0 deletions

diff --git a/gdb/gdbserver/tdesc.c b/gdb/gdbserver/tdesc.c
index 92a0a60..817e69a 100644
--- a/gdb/gdbserver/tdesc.c
+++ b/gdb/gdbserver/tdesc.c
@@ -186,3 +186,19 @@ tdesc_create_feature (struct target_desc *tdesc, const char *name)
   tdesc->features.emplace_back (new_feature);
   return new_feature;
 }
+
+/* See gdbsupport/tdesc.h.  */
+
+bool
+tdesc_contains_feature (const target_desc *tdesc, const std::string &feature)
+{
+  gdb_assert (tdesc != nullptr);
+
+  for (const tdesc_feature_up &f : tdesc->features)
+    {
+      if (f->name == feature)
+	return true;
+    }
+
+  return false;
+}