Call target_can_download_tracepoint if there are tracepoints to download

Nowadays, GDB calls target_can_download_tracepoint at the entry of
download_tracepoint_locations, which is called by.
update_global_location_list.  Sometimes, it is not needed to call
target_can_download_tracepoint at all because there is no tracepoint
created.  In remote target, target_can_download_tracepoint send
qTStatus to the remote in order to know whether tracepoint can be
downloaded or not.  This means some redundant qTStatus packets are
sent.

This patch is to teach GDB to call target_can_download_tracepoint
lazily, only on the moment there are tracepoint to download.
gdb.perf/single-step.exp (with a local patch to measure RSP packets)
shows the number of RSP packets is reduced because there is no
tracepoint at all, so GDB doesn't send qTStatus any more.

                       # of RSP packets
                       original  patched
single-step rsp 1000   7000      6000
single-step rsp 2000   14000     12000
single-step rsp 3000   21000     18000
single-step rsp 4000   28000     24000

gdb:

2015-09-10  Yao Qi  <yao.qi@linaro.org>

	* breakpoint.c (download_tracepoint_locations): New local
	can_download_tracepoint.  Check the result of
	target_can_download_tracepoint and save it in
	can_download_tracepoint if there are tracepoints to download.
	* linux-nat.h (enum tribool): Move it to ...
	* common/common-types.h: ... here.

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2015-09-10  Yao Qi  <yao.qi@linaro.org>
+
+	* breakpoint.c (download_tracepoint_locations): New local
+	can_download_tracepoint.  Check the result of
+	target_can_download_tracepoint and save it in
+	can_download_tracepoint if there are tracepoints to download.
+	* linux-nat.h (enum tribool): Move it to ...
+	* common/common-types.h: ... here.
+
 2015-09-09  Pedro Alves  <palves@redhat.com>
 
 	* inf-loop.c (inferior_event_handler): Delete INF_TIMER case.

gdb/breakpoint.c

@@ -12146,9 +12146,7 @@ download_tracepoint_locations (void)
 {
   struct breakpoint *b;
   struct cleanup *old_chain;
-
-  if (!target_can_download_tracepoint ())
-    return;
+  enum tribool can_download_tracepoint = TRIBOOL_UNKNOWN;
 
   old_chain = save_current_space_and_thread ();
 
@@ -12163,6 +12161,17 @@ download_tracepoint_locations (void)
 	   : !may_insert_tracepoints))
 	continue;
 
+      if (can_download_tracepoint == TRIBOOL_UNKNOWN)
+	{
+	  if (target_can_download_tracepoint ())
+	    can_download_tracepoint = TRIBOOL_TRUE;
+	  else
+	    can_download_tracepoint = TRIBOOL_FALSE;
+	}
+
+      if (can_download_tracepoint == TRIBOOL_FALSE)
+	break;
+
       for (bl = b->loc; bl; bl = bl->next)
 	{
 	  /* In tracepoint, locations are _never_ duplicated, so

gdb/common/common-types.h

@@ -58,4 +58,6 @@ typedef unsigned long long ULONGEST;
 /* * The largest CORE_ADDR value.  */
 #define CORE_ADDR_MAX (~ (CORE_ADDR) 0)
 
+enum tribool { TRIBOOL_UNKNOWN = -1, TRIBOOL_FALSE = 0, TRIBOOL_TRUE = 1 };
+
 #endif /* COMMON_TYPES_H */

gdb/linux-nat.h

@@ -116,7 +116,6 @@ struct lwp_info
 extern struct lwp_info *lwp_list;
 
 /* Does the current host support PTRACE_GETREGSET?  */
-enum tribool { TRIBOOL_UNKNOWN = -1, TRIBOOL_FALSE = 0, TRIBOOL_TRUE = 1 };
 extern enum tribool have_ptrace_getregset;
 
 /* Iterate over each active thread (light-weight process).  */