arch: Wrap InstObjParams with a class and not a function.

When parsing an ISA description, the InstObjParams class needs to have a
reference to the current parser. It does that by exposing a wrapper to
the description rather than the actual InstObjParams class. That wrapper
injects an additional argument into the InstObjParams constructor.

Originally, the wrapper which injectect the additional argument was a
function which masqueraded as a class. That made it impossible to
subclass InstObjParams.

Instead, this change replaces that function wrapper with a class
wrapper, and injects the extra argument in the __init__ method. This
preserves the fact that the InstObjParams name refers to a class, and
allows any sort of interaction that's normally allowed with a class like
subclassing.

Change-Id: I550ea2e60eadac3c7c0b9afa7d71f4607b49a5d2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39275
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/isa_parser/isa_parser.py

@@ -1436,11 +1436,12 @@ StaticInstPtr
     # END OF GRAMMAR RULES
 
     def updateExportContext(self):
-
-        # create a continuation that allows us to grab the current parser
-        def wrapInstObjParams(*args):
-            return InstObjParams(self, *args)
-        self.exportContext['InstObjParams'] = wrapInstObjParams
+        # Create a wrapper class that allows us to grab the current parser.
+        class InstObjParamsWrapper(InstObjParams):
+            def __init__(iop, *args, **kwargs):
+                super(InstObjParamsWrapper, iop).__init__(
+                        self, *args, **kwargs)
+        self.exportContext['InstObjParams'] = InstObjParamsWrapper
         self.exportContext.update(self.templateMap)
 
     def defFormat(self, id, params, code, lineno):