Reduce Hg packet (select remote general thread) bouncing

A patch I wrote made GDB pull the thread list sooner when debugging
with target remote, and I noticed an intended consequence.  GDB
started bouncing around the currently selected remote/general thread
more frequently.  E.g.:

  Sending packet: $qTMinFTPILen#3b...Packet received: 5
 +Sending packet: $Hgp726d.726d#53...Packet received: OK
  Sending packet: $m400680,40#2f...Packet received: 85c0741455bff00d60004889e5ffd05de97bffffff0f1f00e973ffffff0f1f00554889e5c745fc00000000c745fc01000000e900000000c745fc02000000b800
 +Sending packet: $Hgp726d.7278#28...Packet received: OK
  Sending packet: $m4006b2,1#28...Packet received: e9
  Fast tracepoint 2 at 0x4006b2: file gdb/testsuite/gdb.trace/range-stepping.c, line 53.
  Sending packet: $qTStatus#49...Packet received: T0;tnotrun:0;tframes:0;tcreated:0;tfree:500000;tsize:500000;circular:0;disconn:0;starttime:0;stoptime:0;username:;notes::

This ended up breaking "tstart" when one has fast tracepoints set,
because gdbserver isn't expecting an Hg packet in response to
qRelocInsn:

 (gdb) ftrace *set_point
 Fast tracepoint 3 at 0x4006b2: file gdb/testsuite/gdb.trace/range-stepping.c, line 53.
 (gdb) PASS: gdb.trace/range-stepping.exp: ftrace: ftrace *set_point
 tstart
 gdbserver: Malformed response to qRelocInsn, ignoring: Hgp2783.2783

 Target does not support this command.
 (gdb) FAIL: gdb.trace/range-stepping.exp: ftrace: tstart

remote_trace_start should probably start by making sure the remote
current thread matches inferior_ptid (calling set_general_thread), but
still, reducing unnecessary bouncing is a good idea.  It happens
because the memory/symbol/breakpoint routines use
switch_to_program_space_and_thread to do something in the right
context and then revert back to the previously current thread.

The fix is to simply make any_thread_of_process,
find_inferior_for_program_space, etc. give preference to the current
thread/inferior it if matches.

gdb/
2014-10-02  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (any_thread_of_process, any_live_thread_of_process):
	Adjust comments.
	* inferior.c (find_inferior_for_program_space): Give preference to
	the current inferior.
	* inferior.h (find_inferior_for_program_space): Update comment.
	* progspace.c (switch_to_program_space_and_thread): Prefer the
	current inferior if it's bound to the program space requested.  If
	the inferior found doesn't have a PID yet, don't bother looking up
	a thread.
	* progspace.h (switch_to_program_space_and_thread): Adjust
	comment.
	* thread.c (any_thread_of_process, any_live_thread_of_process):
	Give preference to the current thread.

1 files changed, 34 insertions, 6 deletions

diff --git a/gdb/thread.c b/gdb/thread.c
index 65890e1..bceaf49 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -466,7 +466,13 @@ any_thread_of_process (int pid)
 {
   struct thread_info *tp;
 
-  for (tp = thread_list; tp; tp = tp->next)
+  gdb_assert (pid != 0);
+
+  /* Prefer the current thread.  */
+  if (ptid_get_pid (inferior_ptid) == pid)
+    return inferior_thread ();
+
+  ALL_NON_EXITED_THREADS (tp)
     if (ptid_get_pid (tp->ptid) == pid)
       return tp;
 
@@ -476,18 +482,40 @@ any_thread_of_process (int pid)
 struct thread_info *
 any_live_thread_of_process (int pid)
 {
+  struct thread_info *curr_tp = NULL;
   struct thread_info *tp;
   struct thread_info *tp_executing = NULL;
 
-  for (tp = thread_list; tp; tp = tp->next)
-    if (tp->state != THREAD_EXITED && ptid_get_pid (tp->ptid) == pid)
+  gdb_assert (pid != 0);
+
+  /* Prefer the current thread if it's not executing.  */
+  if (ptid_get_pid (inferior_ptid) == pid)
+    {
+      /* If the current thread is dead, forget it.  If it's not
+	 executing, use it.  Otherwise, still choose it (below), but
+	 only if no other non-executing thread is found.  */
+      curr_tp = inferior_thread ();
+      if (curr_tp->state == THREAD_EXITED)
+	curr_tp = NULL;
+      else if (!curr_tp->executing)
+	return curr_tp;
+    }
+
+  ALL_NON_EXITED_THREADS (tp)
+    if (ptid_get_pid (tp->ptid) == pid)
       {
-	if (tp->executing)
-	  tp_executing = tp;
-	else
+	if (!tp->executing)
 	  return tp;
+
+	tp_executing = tp;
       }
 
+  /* If both the current thread and all live threads are executing,
+     prefer the current thread.  */
+  if (curr_tp != NULL)
+    return curr_tp;
+
+  /* Otherwise, just return an executing thread, if any.  */
   return tp_executing;
 }