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.

1 files changed, 24 insertions, 17 deletions

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index f747ebda1ab..ba9b2d896b7 100644
--- a/gdb/aarch64-tdep.c
+++ b/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");