[RFA 5/4 take 2] Improved linker-debugger interface

[Gary Benson <gbenson@redhat.com>]

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Hi all,

I did some profiling and realised that the probes patch I just mailed
had reintroduced calls to update_section_map, a slow function I did
some work to avoid calling last year:

  http://www.cygwin.com/ml/gdb-patches/2011-07/msg00460.html
  http://www.cygwin.com/ml/gdb-patches/2011-10/msg00361.html

Attached is a patch to avoid calling update_section_map from the
probes interface.  Updated timings are as follows:

  no of solibs    100     250     500     1000    2000    5000
  ------------------------------------------------------------
  old interface     1       3       9       35     141     942
  new interface     0       0       1        4      14      89
  (times in seconds)

So, with this patch GDB is not three but ten times faster.

Thanks,
Gary

-- 
http://gbenson.net/

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2012-07-19  Gary Benson  <gbenson@redhat.com>

	* objfiles.h (inhibit_section_map_updates): New function
	declaration.
	(resume_section_map_updates): Likewise.
	* objfiles.c (objfile_pspace_info): New field "inhibit_updates".
	(find_pc_section): Do not update the section map if
	inhibit_updates is set.
	(inhibit_section_map_updates): New function.
	(resume_section_map_updates): Likewise.
	* solib-svr4.c (svr4_handle_solib_event): Inhibit section map
	updates for calls to evaluate_probe_argument.

diff --git a/gdb/objfiles.h b/gdb/objfiles.h
index 0df5798..c7db1f2 100644
--- a/gdb/objfiles.h
+++ b/gdb/objfiles.h
@@ -522,6 +522,19 @@ extern void set_objfile_data (struct objfile *objfile,
 extern void *objfile_data (struct objfile *objfile,
 			   const struct objfile_data *data);
 
+/* In normal use, the section map will be rebuilt by FIND_PC_SECTION
+   if objfiles have been added, removed or relocated since it was last
+   called.  Calling INHIBIT_SECTION_MAP_UPDATES will inhibit this
+   behavior until RESUME_SECTION_MAP_UPDATES is called.  If you call
+   INHIBIT_SECTION_MAP_UPDATES you must ensure that every call to
+   FIND_PC_SECTION in the inhibited region relates to a section that
+   is already in the section map and has not since been removed or
+   relocated.  */
+extern void inhibit_section_map_updates (void);
+
+/* Resume automatically rebuilding the section map as required.  */
+extern void resume_section_map_updates (void);
+
 extern void default_iterate_over_objfiles_in_search_order
   (struct gdbarch *gdbarch,
    iterate_over_objfiles_in_search_order_cb_ftype *cb,
diff --git a/gdb/objfiles.c b/gdb/objfiles.c
index 5ff0eb2..327b5d6 100644
--- a/gdb/objfiles.c
+++ b/gdb/objfiles.c
@@ -70,6 +70,9 @@ struct objfile_pspace_info
   int objfiles_changed_p;
   struct obj_section **sections;
   int num_sections;
+
+  /* Nonzero if section map updates should be inhibited.  */
+  int inhibit_updates;
 };
 
 /* Per-program-space data key.  */
@@ -1289,7 +1292,7 @@ find_pc_section (CORE_ADDR pc)
     return s;
 
   pspace_info = get_objfile_pspace_data (current_program_space);
-  if (pspace_info->objfiles_changed_p != 0)
+  if (pspace_info->objfiles_changed_p && !pspace_info->inhibit_updates)
     {
       update_section_map (current_program_space,
 			  &pspace_info->sections,
@@ -1457,6 +1460,23 @@ objfiles_changed (void)
   get_objfile_pspace_data (current_program_space)->objfiles_changed_p = 1;
 }
 
+/* See comments in objfiles.h.  */
+
+void
+inhibit_section_map_updates (void)
+{
+  get_objfile_pspace_data (current_program_space)->inhibit_updates = 1;
+
+}
+
+/* See comments in objfiles.h.  */
+
+void
+resume_section_map_updates (void)
+{
+  get_objfile_pspace_data (current_program_space)->inhibit_updates = 0;
+}
+
 /* The default implementation for the "iterate_over_objfiles_in_search_order"
    gdbarch method.  It is equivalent to use the ALL_OBJFILES macro,
    searching the objfiles in the order they are stored internally,
diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index 17395f7..d3c388b 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -1820,6 +1820,18 @@ svr4_handle_solib_event (bpstat bs)
   if (action == NAMESPACE_NO_ACTION)
     return;
 
+  /* EVALUATE_PROBE_ARGUMENT looks up symbols in the dynamic linker
+     using FIND_PC_SECTION.  FIND_PC_SECTION is accelerated by a cache
+     called the section map.  The section map is invalidated every
+     time a shared library is loaded or unloaded, and if the inferior
+     is generating a lot of shared library events then the section map
+     will be updated every time SVR4_HANDLE_SOLIB_EVENT is called.
+     We called FIND_PC_SECTION in SVR4_CREATE_SOLIB_EVENT_BREAKPOINTS,
+     so we can guarantee that the dynamic linker's sections are in the
+     section map.  We can therefore inhibit section map updates across
+     these calls to EVALUATE_PROBE_ARGUMENT and save a lot of time.  */
+  inhibit_section_map_updates ();
+
   val = evaluate_probe_argument (pi->probe, 0);
   if (val == NULL)
     goto error;
@@ -1851,6 +1863,8 @@ svr4_handle_solib_event (bpstat bs)
 	action = NAMESPACE_RELOAD;
     }
 
+  resume_section_map_updates ();
+
   if (action == NAMESPACE_UPDATE_OR_RELOAD)
     {
       if (namespace_update_incremental (info, lmid, lm, is_initial_ns))
@@ -1873,6 +1887,7 @@ svr4_handle_solib_event (bpstat bs)
   warning (_("Probes-based dynamic linker interface failed.\n"
 	     "Reverting to original interface.\n"));
 
+  resume_section_map_updates ();
   free_namespace_table (info);
   free_probes (info);
   info->using_probes = 0;