Fix thread's gdbarch when SVE vector length changes

When the inferior program changes the SVE length, GDB can stop tracking
some registers as it obtains the new gdbarch that corresponds to the
updated length:

  Breakpoint 1, do_sve_ioctl_test () at sve-ioctls.c:44
  44              res = prctl(PR_SVE_SET_VL, i, 0, 0, 0, 0);
  (gdb) print i
  $2 = 32
  (gdb) info registers
          ⋮
  [ snip registers x0 to x30 ]
          ⋮
  sp             0xffffffffeff0      0xffffffffeff0
  pc             0xaaaaaaaaa8ac      0xaaaaaaaaa8ac <do_sve_ioctl_test+112>
  cpsr           0x60000000          [ EL=0 BTYPE=0 C Z ]
  fpsr           0x0                 0
  fpcr           0x0                 0
  vg             0x8                 8
  tpidr          0xfffff7fcb320      0xfffff7fcb320
  (gdb) next
  45              if (res < 0) {
  (gdb) info registers
          ⋮
  [ snip registers x0 to x30 ]
          ⋮
  sp             0xffffffffeff0      0xffffffffeff0
  pc             0xaaaaaaaaa8cc      0xaaaaaaaaa8cc <do_sve_ioctl_test+144>
  cpsr           0x200000            [ EL=0 BTYPE=0 SS ]
  fpsr           0x0                 0
  fpcr           0x0                 0
  vg             0x4                 4
  (gdb)

Notice that register tpidr disappeared when vg (which holds the vector
length) changed from 8 to 4.  The tpidr register is provided by the
org.gnu.gdb.aarch64.tls feature.

This happens because the code that searches for a new gdbarch to match the
new vector length in aarch64_linux_nat_target::thread_architecture doesn't
take into account the features present in the target description associated
with the previous gdbarch.  This patch makes it do that.

Since the id member of struct gdbarch_info is now unused, it's removed.

gdb/aarch64-linux-nat.c

@@ -900,11 +900,16 @@ aarch64_linux_nat_target::thread_architecture (ptid_t ptid)
 
   /* We reach here if the vector length for the thread is different from its
      value at process start.  Lookup gdbarch via info (potentially creating a
-     new one), stashing the vector length inside id.  Use -1 for when SVE
-     unavailable, to distinguish from an unset value of 0.  */
+     new one) by using a target description that corresponds to the new vq value
+     and the current architecture features.  */
+
+  const struct target_desc *tdesc = gdbarch_target_desc (inf->gdbarch);
+  aarch64_features features = aarch64_features_from_target_desc (tdesc);
+  features.vq = vq;
+
   struct gdbarch_info info;
   info.bfd_arch_info = bfd_lookup_arch (bfd_arch_aarch64, bfd_mach_aarch64);
-  info.id = (int *) (vq == 0 ? -1 : vq);
+  info.target_desc = aarch64_read_description (features);
   return gdbarch_find_by_info (info);
 }
 

gdb/aarch64-tdep.c

@@ -3392,6 +3392,27 @@ aarch64_get_tdesc_vq (const struct target_desc *tdesc)
   return sve_vq_from_vl (vl);
 }
 
+/* Get the AArch64 features present in the given target description. */
+
+aarch64_features
+aarch64_features_from_target_desc (const struct target_desc *tdesc)
+{
+  aarch64_features features;
+
+  if (tdesc == nullptr)
+    return features;
+
+  features.vq = aarch64_get_tdesc_vq (tdesc);
+  features.pauth
+      = (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.pauth") != nullptr);
+  features.mte
+      = (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.mte") != nullptr);
+  features.tls
+      = (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.tls") != nullptr);
+
+  return features;
+}
+
 /* Implement the "cannot_store_register" gdbarch method.  */
 
 static int
@@ -3442,17 +3463,7 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   int i, num_regs = 0, num_pseudo_regs = 0;
   int first_pauth_regnum = -1, ra_sign_state_offset = -1;
   int first_mte_regnum = -1, tls_regnum = -1;
-
-  /* Use the vector length passed via the target info.  Here -1 is used for no
-     SVE, and 0 is unset.  If unset then use the vector length from the existing
-     tdesc.  */
-  uint64_t vq = 0;
-  if (info.id == (int *) -1)
-    vq = 0;
-  else if (info.id != 0)
-    vq = (uint64_t) info.id;
-  else
-    vq = aarch64_get_tdesc_vq (info.target_desc);
+  uint64_t vq = aarch64_get_tdesc_vq (info.target_desc);
 
   if (vq > AARCH64_MAX_SVE_VQ)
     internal_error (__FILE__, __LINE__, _("VQ out of bounds: %s (max %d)"),
@@ -3472,12 +3483,8 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   /* Ensure we always have a target descriptor, and that it is for the given VQ
      value.  */
   const struct target_desc *tdesc = info.target_desc;
-  if (!tdesc_has_registers (tdesc) || vq != aarch64_get_tdesc_vq (tdesc))
-    {
-      aarch64_features features;
-      features.vq = vq;
-      tdesc = aarch64_read_description (features);
-    }
+  if (!tdesc_has_registers (tdesc))
+    tdesc = aarch64_read_description ({});
   gdb_assert (tdesc);
 
   feature_core = tdesc_find_feature (tdesc,"org.gnu.gdb.aarch64.core");

gdb/aarch64-tdep.h

@@ -121,6 +121,8 @@ struct aarch64_gdbarch_tdep : gdbarch_tdep_base
 };
 
 const target_desc *aarch64_read_description (const aarch64_features &features);
+aarch64_features
+aarch64_features_from_target_desc (const struct target_desc *tdesc);
 
 extern int aarch64_process_record (struct gdbarch *gdbarch,
 			       struct regcache *regcache, CORE_ADDR addr);

gdb/gdbarch.h

@@ -251,17 +251,8 @@ struct gdbarch_info
 
   bfd *abfd = nullptr;
 
-  union
-    {
-      /* Architecture-specific target description data.  Numerous targets
-	 need only this, so give them an easy way to hold it.  */
-      struct tdesc_arch_data *tdesc_data;
-
-      /* SPU file system ID.  This is a single integer, so using the
-	 generic form would only complicate code.  Other targets may
-	 reuse this member if suitable.  */
-      int *id;
-    };
+  /* Architecture-specific target description data.  */
+  struct tdesc_arch_data *tdesc_data;
 
   enum gdb_osabi osabi = GDB_OSABI_UNKNOWN;