stats: Add support for hierarchical stats

This change makes the stat system aware of the hierarchical nature of
stats. The aim is to achieve the following goals:

  * Make the SimObject hierarchy explicit in the stat system (i.e.,
    get rid of name() + ".foo"). This makes stat naming less fragile
    and makes it possible to implement hierarchical formats like
    XML/HDF5/JSON in a clean way.

  * Make it more convenient to split stats into a separate
    struct/class that can be bound to a SimObject. This makes the
    namespace cleaner and makes stat accesses a bit more obvious.

  * Make it possible to build groups of stats in C++ that can be used
    in subcomponents in a SimObject (similar to what we do for
    checkpoint sections). This makes it easier to structure large
    components.

  * Enable partial stat dumps. Some of our internal users have been
    asking for this since a full stat dump can be large.

  * Enable better stat access from Python.

This changeset implements solves the first three points by introducing
a class (Stats::Group) that owns statistics belonging to the same
object. SimObjects inherit from Stats::Group since they typically have
statistics.

New-style statistics need to be associated with a parent group at
instantiation time. Instantiation typically sets the name and the
description, other parameters need to be set by overriding
Group::regStats() just like with legacy stats. Simple objects with
scalar stats can typically avoid implementing regStats() altogether
since the stat name and description are both specified in the
constructor.

For convenience reasons, statistics groups can be merged into other
groups. This means that a SimObject can create a stat struct that
inherits from Stats::Group and merge it into the parent group
(SimObject). This can make the code cleaner since statistics tracking
gets grouped into a single object.

Stat visitors have a new API to expose the group structure. The
Output::beginGroup(name) method is called at the beginning of a group
and the Output::endGroup() method is called when all stats, and
sub-groups, have been visited. Flat formats (e.g., the text format)
typically need to maintain a stack to track the full path to a stat.

Legacy, flat, statistics are still supported after applying this
change. These stats don't belong to any group and stat visitors will
not see a Output::beginGroup(name) call before their corresponding
Output::visit() methods are called.

Change-Id: I9025d61dfadeabcc8ecf30813ab2060def455648
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19368
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

2 files changed, 6 insertions, 25 deletions

diff --git a/src/sim/sim_object.cc b/src/sim/sim_object.cc
index 7b794a0b7..eb6e15ae1 100644
--- a/src/sim/sim_object.cc
+++ b/src/sim/sim_object.cc
@@ -56,7 +56,9 @@ SimObject::SimObjectList SimObject::simObjectList;
 // SimObject constructor: used to maintain static simObjectList
 //
 SimObject::SimObject(const Params *p)
-    : EventManager(getEventQueue(p->eventq_index)), _params(p)
+    : EventManager(getEventQueue(p->eventq_index)),
+      Stats::Group(nullptr),
+      _params(p)
 {
 #ifdef DEBUG
     doDebugBreak = false;
@@ -98,19 +100,6 @@ SimObject::startup()
 {
 }
 
-//
-// no default statistics, so nothing to do in base implementation
-//
-void
-SimObject::regStats()
-{
-}
-
-void
-SimObject::resetStats()
-{
-}
-
 /**
  * No probe points by default, so do nothing in base.
  */
diff --git a/src/sim/sim_object.hh b/src/sim/sim_object.hh
index 5c9bf0019..c938ba5e5 100644
--- a/src/sim/sim_object.hh
+++ b/src/sim/sim_object.hh
@@ -52,6 +52,7 @@
 #include <string>
 #include <vector>
 
+#include "base/stats/group.hh"
 #include "params/SimObject.hh"
 #include "sim/drain.hh"
 #include "sim/eventq.hh"
@@ -92,7 +93,8 @@ class ProbeManager;
  * SimObject.py). This has the effect of calling the method on the
  * parent node <i>before</i> its children.
  */
-class SimObject : public EventManager, public Serializable, public Drainable
+class SimObject : public EventManager, public Serializable, public Drainable,
+                  public Stats::Group
 {
   private:
     typedef std::vector<SimObject *> SimObjectList;
@@ -146,16 +148,6 @@ class SimObject : public EventManager, public Serializable, public Drainable
     virtual void initState();
 
     /**
-     * Register statistics for this object.
-     */
-    virtual void regStats();
-
-    /**
-     * Reset statistics associated with this object.
-     */
-    virtual void resetStats();
-
-    /**
      * Register probe points for this object.
      */
     virtual void regProbePoints();