AArch64 SVE: Check for vector length change when getting gdbarch

Override the thread_architecture method, similar to SPU.  If the vector
length has changed, then find the arch using info, making sure the vector
length is passed down to the init routine.

In the init routine, ensure the arch has the correct vector length.

Example output. Program is stopped in thread 2, just before it calls prctl
to change the vector length

(gdb) info threads
  Id   Target Id                                     Frame
  1    Thread 0xffffbf6f4000 (LWP 3188) "sve_change" 0x0000ffffbf6ae130 in pthread_join ()
* 2    Thread 0xffffbf55e200 (LWP 3189) "sve_change" thread1 (arg=0xfeedface) at sve_change_size.c:28
(gdb) print $vg
$1 = 8
(gdb) print $z0.s.u
$2 = {623191333, 623191333, 623191333, 623191333, 0 <repeats 12 times>}
(gdb) n
29	  int ret = prctl(PR_SVE_SET_VL, vl/2);
(gdb) n
30	  printf ("Changed: ret\n", ret);
(gdb) print $vg
$4 = 4
(gdb) print $z0.s.u
$5 = {623191333, 623191333, 623191333, 623191333, 0, 0, 0, 0}
(gdb) thr 1
[Switching to thread 1 (Thread 0xffffbf6f4000 (LWP 3181))]
(gdb) print $vg
$6 = 8
(gdb) print $z0.s.u
$7 = {623191333, 623191333, 623191333, 623191333, 0 <repeats 12 times>}

gdb/ChangeLog:

	* aarch64-linux-nat.c
	(aarch64_linux_nat_target::thread_architecture): Add override.
	* aarch64-tdep.c (aarch64_gdbarch_init): Ensure different tdesc for
	each VQ.

1 files changed, 21 insertions, 5 deletions

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index ea3f5ec..cb185ee 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -3198,20 +3198,36 @@ 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, pauth_ra_state_offset = -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);
+
+  if (vq > AARCH64_MAX_SVE_VQ)
+    internal_error (__FILE__, __LINE__, _("VQ out of bounds: %ld (max %d)"),
+		    vq, AARCH64_MAX_SVE_VQ);
+
   /* If there is already a candidate, use it.  */
   for (gdbarch_list *best_arch = gdbarch_list_lookup_by_info (arches, &info);
        best_arch != nullptr;
        best_arch = gdbarch_list_lookup_by_info (best_arch->next, &info))
     {
       struct gdbarch_tdep *tdep = gdbarch_tdep (best_arch->gdbarch);
-      if (tdep)
+      if (tdep && tdep->vq == vq)
 	return best_arch->gdbarch;
     }
 
-  /* Ensure we always have a target description.  */
+  /* 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))
-    tdesc = aarch64_read_description (0, false);
+  if (!tdesc_has_registers (tdesc) || vq != aarch64_get_tdesc_vq (tdesc))
+    tdesc = aarch64_read_description (vq, false);
   gdb_assert (tdesc);
 
   feature_core = tdesc_find_feature (tdesc,"org.gnu.gdb.aarch64.core");
@@ -3304,7 +3320,7 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   tdep->lowest_pc = 0x20;
   tdep->jb_pc = -1;		/* Longjump support not enabled by default.  */
   tdep->jb_elt_size = 8;
-  tdep->vq = aarch64_get_tdesc_vq (tdesc);
+  tdep->vq = vq;
   tdep->pauth_reg_base = first_pauth_regnum;
   tdep->pauth_ra_state_regnum = (feature_pauth == NULL) ? -1
 				: pauth_ra_state_offset + num_regs;