sim: Clean up and simplify main().

Use pybind11 to simplify the python parts, update some inaccurate
comments, rename m5Main to gem5Main, remove code which supported python
versions less than 3.

Change-Id: I139af7d3d1052cfbce779a87e34d7ce997876a60
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49414
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/importer.cc

@@ -29,10 +29,13 @@
 #include "pybind11/pybind11.h"
 #include "python/m5ImporterCode.hh"
 
+#include "sim/init.hh"
+
 namespace py = pybind11;
 
 PYBIND11_EMBEDDED_MODULE(importer, m)
 {
+    m.def("_init_all_embedded", gem5::EmbeddedPython::initAll);
     py::str importer_code(
             reinterpret_cast<const char *>(gem5::Blobs::m5ImporterCode),
             gem5::Blobs::m5ImporterCode_len);

src/python/importer.py

@@ -74,7 +74,12 @@ class CodeImporter(object):
 # Create an importer and add it to the meta_path so future imports can
 # use it.  There's currently nothing in the importer, but calls to
 # add_module can be used to add code.
-import sys
-importer = CodeImporter()
-add_module = importer.add_module
-sys.meta_path.insert(0, importer)
+def install():
+    importer = CodeImporter()
+    global add_module
+    add_module = importer.add_module
+    import sys
+    sys.meta_path.insert(0, importer)
+
+    # Injected into this module's namespace by the c++ code that loads it.
+    _init_all_embedded()

src/sim/init.cc

@@ -202,7 +202,7 @@ PYBIND11_EMBEDDED_MODULE(_m5, _m5)
  * main function.
  */
 int
-m5Main(int argc, char **argv)
+gem5Main(int argc, char **argv)
 {
 #if HAVE_PROTOBUF
     // Verify that the version of the protobuf library that we linked

src/sim/init.hh

@@ -98,7 +98,7 @@ class EmbeddedPyBind
     static std::map<std::string, EmbeddedPyBind *> &getMap();
 };
 
-int m5Main(int argc, char **argv);
+int gem5Main(int argc, char **argv);
 
 } // namespace gem5
 

src/sim/main.cc

@@ -28,45 +28,40 @@
 
 #include <Python.h>
 
+#include "pybind11/embed.h"
+#include "pybind11/pybind11.h"
+
 #include "sim/init.hh"
 #include "sim/init_signals.hh"
 
 using namespace gem5;
 
+namespace py = pybind11;
+
 // main() is now pretty stripped down and just sets up python and then
-// calls initM5Python which loads the various embedded python modules
-// into the python environment and then starts things running by
-// calling m5Main.
+// calls EmbeddedPython::initAll which loads the various embedded python
+// modules into the python environment and then starts things running by
+// calling gem5Main.
 int
 main(int argc, char **argv)
 {
-    int ret;
-
-    // Initialize m5 special signal handling.
+    // Initialize gem5 special signal handling.
     initSignals();
 
-#if PY_MAJOR_VERSION >= 3
+    // Convert argv[0] to a wchar_t string, using python's locale and cleanup
+    // functions.
     std::unique_ptr<wchar_t[], decltype(&PyMem_RawFree)> program(
-        Py_DecodeLocale(argv[0], NULL),
+        Py_DecodeLocale(argv[0], nullptr),
         &PyMem_RawFree);
+
+    // This can help python find libraries at run time relative to this binary.
+    // It's probably not necessary, but is mostly harmless and might be useful.
     Py_SetProgramName(program.get());
-#else
-    Py_SetProgramName(argv[0]);
-#endif
 
-    // initialize embedded Python interpreter
-    Py_Initialize();
+    py::scoped_interpreter guard;
 
-    // Initialize the embedded m5 python library
-    ret = EmbeddedPython::initAll();
+    auto importer = py::module_::import("importer");
+    importer.attr("install")();
 
-    if (ret == 0) {
-        // start m5
-        ret = m5Main(argc, argv);
-    }
-
-    // clean up Python intepreter.
-    Py_Finalize();
-
-    return ret;
+    return gem5Main(argc, argv);
 }