[PATCH v2] gdb: testsuite: Test whether PC register is expedited in gdb.server/server-run.exp

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

One thing GDB always does when the inferior stops is finding out where
it's stopped at, by way of querying the value of the program counter
register.

To save a packet round trip, the remote target can send the PC
value (often alongside other frequently consulted registers such as the
stack pointer) in the stop reply packet as an "expedited register".

Test that this is actually done for the targets where gdbserver is
supposed to.

Extend the "maintenance print remote-registers" command output with an
"Expedited" column which says "yes" if the register was seen by GDB in
the last stop reply packet it received, and is left blank otherwise.

Tested for regressions on aarch64-linux-gnu native-extended-remote.

The testcase was tested on aarch64-linux-gnu, i686-linux-gnu and
x86_64-linux-gnu native-remote and native-extended-remote targets.
---
 gdb/NEWS                                |  7 ++++++
 gdb/doc/gdb.texinfo                     |  6 ++++-
 gdb/regcache-dump.c                     | 19 +++++++++-----
 gdb/remote.c                            | 27 +++++++++++++++++++-
 gdb/remote.h                            |  5 ++++
 gdb/testsuite/gdb.server/server-run.exp | 33 +++++++++++++++++++++++++
 6 files changed, 89 insertions(+), 8 deletions(-)

Changes in v2:
- Added NEWS entry and updated documentation for maint print
  remote-registers command (suggested by Andrew).
- Updated help string for maint print remote-registers command.
- Adopted Andrew's gdb_test_multiple invocation to avoid leaving unmatched
  ouptut in Expect's buffer.

v1 is here:
https://inbox.sourceware.org/gdb-patches/20240831005607.2478217-2-thiago.bauermann@linaro.org/

diff --git a/gdb/NEWS b/gdb/NEWS
index 2f5d5ebbcef1..8820fadb6436 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -40,6 +40,13 @@
   history has been reached.  It also specifies that the forward execution can
   continue, and the recording will also continue.
 
+* Changed commands
+
+maintenance print remote-registers
+  Add an "Expedited" column to the output of the command.  It indicates
+  which registers were included in the last stop reply packet received by
+  GDB.
+
 * New commands
 
 maintenance info inline-frames [ADDRESS]
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 8cde2f22637b..26c7b609f86f 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -42048,7 +42048,11 @@ including registers which aren't available on the target nor visible
 to user; the command @code{maint print register-groups} includes the
 groups that each register is a member of; and the command @code{maint
 print remote-registers} includes the remote target's register numbers
-and offsets in the `G' packets.
+and offsets in the `G' packets, as well as an indication of which
+registers were included in the last stop reply packet received by
+@value{GDBN} (@pxref{Stop Reply Packets}).  Please note that the list
+of registers included in a stop reply can change from one stop to the
+next.
 
 These commands take an optional parameter, a file name to which to
 write the information.
diff --git a/gdb/regcache-dump.c b/gdb/regcache-dump.c
index bc665dc08a67..6b711bf6c2a0 100644
--- a/gdb/regcache-dump.c
+++ b/gdb/regcache-dump.c
@@ -162,7 +162,7 @@ class register_dump_remote : public register_dump
   {
     if (regnum < 0)
       {
-	gdb_printf (file, "Rmt Nr  g/G Offset");
+	gdb_printf (file, "Rmt Nr  g/G Offset  Expedited");
       }
     else if (regnum < gdbarch_num_regs (m_gdbarch))
       {
@@ -170,7 +170,12 @@ class register_dump_remote : public register_dump
 
 	if (remote_register_number_and_offset (m_gdbarch, regnum,
 					       &pnum, &poffset))
-	  gdb_printf (file, "%7d %11d", pnum, poffset);
+	  {
+	    if (remote_register_is_expedited (regnum))
+	      gdb_printf (file, "%7d %11d  yes", pnum, poffset);
+	    else
+	      gdb_printf (file, "%7d %11d", pnum, poffset);
+	  }
       }
   }
 };
@@ -324,9 +329,11 @@ _initialize_regcache_dump ()
 	     "Takes an optional file parameter."),
 	   &maintenanceprintlist);
   add_cmd ("remote-registers", class_maintenance,
-	   maintenance_print_remote_registers, _("\
-Print the internal register configuration including remote register number "
-"and g/G packets offset.\n\
-Takes an optional file parameter."),
+	   maintenance_print_remote_registers,
+	   _("Print the internal register configuration including remote "
+	     "register number and g/G packets offset.\n"
+	     "Also prints which registers were sent in the last stop reply "
+	     "packet (i.e. expedited).\n"
+	     "Takes an optional file parameter."),
 	   &maintenanceprintlist);
 }
diff --git a/gdb/remote.c b/gdb/remote.c
index 2c3988cb5075..c17572d51c8d 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -693,6 +693,10 @@ class remote_state
      qSupported.  */
   gdb_thread_options supported_thread_options = 0;
 
+  /* Contains the regnums of the expedited registers in the last stop
+     reply packet.  */
+  std::set<int> last_seen_expedited_registers;
+
 private:
   /* Asynchronous signal handle registered as event loop source for
      when we have pending events ready to be passed to the core.  */
@@ -1490,6 +1494,20 @@ is_remote_target (process_stratum_target *target)
   return as_remote_target (target) != nullptr;
 }
 
+/* See remote.h.  */
+
+bool
+remote_register_is_expedited (int regnum)
+{
+  remote_target *rt = as_remote_target (current_inferior ()->process_target ());
+
+  if (rt == nullptr)
+    return false;
+
+  remote_state *rs = rt->get_remote_state ();
+  return rs->last_seen_expedited_registers.count (regnum) > 0;
+}
+
 /* Per-program-space data key.  */
 static const registry<program_space>::key<char, gdb::xfree_deleter<char>>
   remote_pspace_data;
@@ -8518,6 +8536,10 @@ remote_target::process_stop_reply (stop_reply_up stop_reply,
 {
   *status = stop_reply->ws;
   ptid_t ptid = stop_reply->ptid;
+  struct remote_state *rs = get_remote_state ();
+
+  /* Forget about last reply's expedited registers.  */
+  rs->last_seen_expedited_registers.clear ();
 
   /* If no thread/process was reported by the stub then select a suitable
      thread/process.  */
@@ -8544,7 +8566,10 @@ remote_target::process_stop_reply (stop_reply_up stop_reply,
 					stop_reply->arch);
 
 	  for (cached_reg_t &reg : stop_reply->regcache)
-	    regcache->raw_supply (reg.num, reg.data.get ());
+	    {
+	      regcache->raw_supply (reg.num, reg.data.get ());
+	      rs->last_seen_expedited_registers.insert (reg.num);
+	    }
 	}
 
       remote_thread_info *remote_thr = get_remote_thread_info (this, ptid);
diff --git a/gdb/remote.h b/gdb/remote.h
index cb0a66da66e5..bfe3c65b4637 100644
--- a/gdb/remote.h
+++ b/gdb/remote.h
@@ -121,4 +121,9 @@ extern void send_remote_packet (gdb::array_view<const char> &buf,
 
 extern bool is_remote_target (process_stratum_target *target);
 
+/* Return true if REGNUM was returned as an expedited register in the last
+   stop reply we received.  */
+
+extern bool remote_register_is_expedited (int regnum);
+
 #endif
diff --git a/gdb/testsuite/gdb.server/server-run.exp b/gdb/testsuite/gdb.server/server-run.exp
index 92eb38bd9db0..af5a5f53ca03 100644
--- a/gdb/testsuite/gdb.server/server-run.exp
+++ b/gdb/testsuite/gdb.server/server-run.exp
@@ -52,3 +52,36 @@ if { [istarget *-*-linux*] } {
 
 gdb_breakpoint main
 gdb_test "continue" "Breakpoint.* main .*" "continue to main"
+
+if { [istarget "aarch64*-*-*"]
+     || [istarget "arm*-*-*"]
+     || [istarget "csky*-*-*"]
+     || [istarget "loongarch*-*-*"]
+     || [istarget "riscv*-*-*"] } {
+    set pc_regname "pc"
+} elseif { [is_amd64_regs_target] } {
+    set pc_regname "rip"
+} elseif { [is_x86_like_target] } {
+    set pc_regname "eip"
+} elseif { [istarget "tic6x-*-*"] } {
+    set pc_regname "PC"
+}
+
+# Sending the PC register in advance is good practice.  Test that this is
+# actually done for the targets where gdbserver is supposed to.
+set expedited_pc_test_name "send PC as expedited register in stop reply"
+if { [info exists pc_regname] } {
+    set seen_line false
+    gdb_test_multiple "maintenance print remote-registers" \
+	     $expedited_pc_test_name -lbl {
+	-re " ${pc_regname}\[\[:space:\]\]+${decimal}.*${decimal}  yes" {
+	    set seen_line true
+	    exp_continue
+	}
+	-re "\r\n$gdb_prompt $" {
+	    gdb_assert { $seen_line } $gdb_test_name
+	}
+    }
+} else {
+    untested $expedited_pc_test_name
+}

base-commit: 43af2e08dc0af7796b557d14f13317c0c24f948a