sim: tag-based checkpoint versioning

This commit addresses gem5 checkpoints' linear versioning bottleneck.
Since development is distributed across many private trees, there exists
a sort of 'race' for checkpoint version numbers: internally a checkpoint
version may be used but then resynchronizing with the external tree causes
a conflict on that version.  This change replaces the linear version number
with a set of unique strings called tags.  Now the only conflicts that can
arise are of tag names, where collisions are much easier to avoid.

The checkpoint upgrader (util/cpt_upgrader.py) upgrades the version
representation, as one would expect. Each tag version implements its
upgrader code in a python file in the util/cpt_upgraders directory
rather than adding a function to the upgrader script itself.

The version tags are stored in the 'Globals' section rather than 'root'
(as the version was previously) because 'Globals' gets unserialized
first and can provide a warning before any other unserialization errors
can occur.

1 files changed, 0 insertions, 9 deletions

diff --git a/src/sim/serialize.hh b/src/sim/serialize.hh
index 4a2cc04e9..9a1170a9e 100644
--- a/src/sim/serialize.hh
+++ b/src/sim/serialize.hh
@@ -71,15 +71,6 @@ class EventQueue;
 typedef std::ostream CheckpointOut;
 
 
-/** The current version of the checkpoint format.
- * This should be incremented by 1 and only 1 for every new version, where a new
- * version is defined as a checkpoint created before this version won't work on
- * the current version until the checkpoint format is updated. Adding a new
- * SimObject shouldn't cause the version number to increase, only changes to
- * existing objects such as serializing/unserializing more state, changing sizes
- * of serialized arrays, etc. */
-static const uint64_t gem5CheckpointVersion = 0x000000000000000f;
-
 template <class T>
 void paramOut(CheckpointOut &cp, const std::string &name, const T &param);