Using a non-default Python installation

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The particular installation of the Python interpreter that executes the SCons build scripts is also linked with M5 to execute runtime Python simulation scripts. (This correspondence is necessary since some Python code is shared between the build process and runtime execution.) As a result, you can get M5 to link with a non-default installation of the Python interpreter by using that instance of the interpreter to execute the SCons scripts. Typically this means either:

  1. rearranging your PATH so that scons finds the non-default 'python' first (assuming that the version you want to use is called 'python'), or
  2. explicitly invoking an alternative interpreter on the scons script, e.g.:
% python2.4 `which scons` [scons args]

If you're using a standard Python installation that's just not the default version (e.g., Python 2.4 is installed as /usr/bin/python2.4, but you have to invoke it as python2.4 because /usr/bin/python points to Python 2.3) then that's typically all there is to it.

However, if you built Python from source in some user directory, then it gets trickier. You can still use this custom Python to run scons using either of the techniques above, but you must also be sure that the same Python interpreter and Python library modules can be located at runtime too. Details of how to make this work depend on which of the (at least) three different ways of building M5 & Python you select: dynamic Python library with dynamic modules, static Python library with dynamic modules, or static Python library with static modules.

Dynamic (shared) Python library with dynamic modules

This approach works well if you'll be running M5 on the same system that you build it on, or on a set of systems that have matching directory structures (e.g., because your home directory is NFS-mounted in the same place on all of them). The steps are:

  1. When compiling Python from source, make sure you add the --enable-shared argument to configure so that the shared-library version of the interpreter is built.
  2. When running M5, you need to make sure the loader can find your Python shared library, typically by adding the directory to your LD_LIBRARY_PATH environment variable.

Note that the paths to the Python library modules are hard-coded into the interpreter when it is built, so once the shared library is found, nothing additional needs to be done to tell Python where those modules are.

Static Python library with dynamic modules

This approach results in a larger M5 binary (since the Python interpreter is statically linked), but avoids the need to set LD_LIBRARY_PATH. However, Python library modules will still be loaded dynamically, so those files must be present at the same locations on the system where you run M5 as they were on the system where you built it.

There are two ways to force the linker to use the static library:

  1. Don't add the --enable-shared argument to configure when you build Python. The static Python library gets bulit no matter what. If the linker can't find the shared library, it will be forced to use the static version.
  2. Use the compiler/linker flag (e.g., -static for gcc/g++) to force all libraries to be linked statically.

You will probably also have to:

  • tweak the path to the Python library in the m5/SConstruct file
  • add some linker options so that Python symbols not needed at link time but needed later by dynamically loaded Python library modules don't get stripped out; see [1].

Static Python library with static modules

In theory, this approach avoids the need to have any additional Python files present when running M5. It could be useful if the system installation on which you'll be running M5 is drastically different from that of the system where you're building M5.

This approach builds on the static Python library approach above, but further includes all of the Python modules needed to run your scripts in the static Python library. This is accompished by editing the Modules/Setup file in the Python source distribution before you build Python. See the comments in that file for further instructions.

In practice, we haven't tried this. Please let us know if you try it, whether you succeed or fail. I believe the static linking step will work for compiled-code modules (.so's), but not for library modules written in Python, so it's probably still not a complete solution for getting everything into a single executable file.