[review v3] [AArch64, SVE] Improve target description check for SVE in gdbserver

[Luis Machado (Code Review)]

Change URL: https://gnutoolchain-gerrit.osci.io/r/c/binutils-gdb/+/690
......................................................................

[AArch64, SVE] 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:

2019-11-18  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.

Signed-off-by: Luis Machado <luis.machado@linaro.org>
Change-Id: I28b782cb1677560ca9a06a1be442974b25aabae4
---
M gdb/gdbserver/linux-aarch64-low.c
M gdb/gdbserver/tdesc.c
M gdb/gdbserver/tdesc.h
3 files changed, 24 insertions(+), 1 deletion(-)



diff --git a/gdb/gdbserver/linux-aarch64-low.c b/gdb/gdbserver/linux-aarch64-low.c
index 87a21a0..9e234e0 100644
--- a/gdb/gdbserver/linux-aarch64-low.c
+++ b/gdb/gdbserver/linux-aarch64-low.c
@@ -83,7 +83,7 @@
 {
   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
diff --git a/gdb/gdbserver/tdesc.c b/gdb/gdbserver/tdesc.c
index 92a0a60..20e6263 100644
--- a/gdb/gdbserver/tdesc.c
+++ b/gdb/gdbserver/tdesc.c
@@ -186,3 +186,20 @@
   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)
+{
+  if (tdesc != nullptr && !tdesc->features.empty ())
+    {
+      for (const tdesc_feature_up &f : tdesc->features)
+	{
+	  if (f->name == feature)
+	    return true;
+	}
+    }
+
+  return false;
+}
diff --git a/gdb/gdbserver/tdesc.h b/gdb/gdbserver/tdesc.h
index b93f53c..d74c6b0 100644
--- a/gdb/gdbserver/tdesc.h
+++ b/gdb/gdbserver/tdesc.h
@@ -93,4 +93,10 @@
 
 const struct target_desc *current_target_desc (void);
 
+/* Return true if TDESC contains the feature described by string FEATURE.
+   If TDESC has no features or if the feature string FEATURE was not found in
+   TDESC, return false.  */
+bool tdesc_contains_feature (const target_desc *tdesc,
+			     const std::string &feature);
+
 #endif /* GDBSERVER_TDESC_H */

-- 
Gerrit-Project: binutils-gdb
Gerrit-Branch: master
Gerrit-Change-Id: I28b782cb1677560ca9a06a1be442974b25aabae4
Gerrit-Change-Number: 690
Gerrit-PatchSet: 3
Gerrit-Owner: Luis Machado <luis.machado@linaro.org>
Gerrit-Reviewer: Luis Machado <luis.machado@linaro.org>
Gerrit-CC: Simon Marchi <simon.marchi@polymtl.ca>
Gerrit-MessageType: newpatchset