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

gdb/gdbserver/ChangeLog

@@ -1,3 +1,10 @@
+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.
+
 2019-11-15  Christian Biesinger  <cbiesinger@google.com>
 
 	* Makefile.in: Add safe-strerror.c.

gdb/gdbserver/linux-aarch64-low.c

@@ -83,7 +83,7 @@ is_sve_tdesc (void)
 {
   struct regcache *regcache = get_thread_regcache (current_thread, 0);
 
-  return regcache->tdesc->reg_defs.size () == AARCH64_SVE_NUM_REGS;
+  return tdesc_contains_feature (regcache->tdesc, "org.gnu.gdb.aarch64.sve");
 }
 
 static void

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;
+}

gdb/gdbserver/tdesc.h

@@ -93,4 +93,9 @@ void init_target_desc (struct target_desc *tdesc,
 
 const struct target_desc *current_target_desc (void);
 
+/* Return true if TDESC contains the feature described by string FEATURE.
+   Return false otherwise.  */
+bool tdesc_contains_feature (const target_desc *tdesc,
+			     const std::string &feature);
+
 #endif /* GDBSERVER_TDESC_H */