Run-time selecion of simulator engine

Allows alternating between cxxsim and pysim at run-time, with minimal
code changes to its users.

diff --git a/src/nmutil/sim_tmp_alternative.py b/src/nmutil/sim_tmp_alternative.py
new file mode 100644 (file)
index 0000000..4af3c07
--- /dev/null
+++ b/src/nmutil/sim_tmp_alternative.py
@@ -0,0 +1,63 @@
+"""Run-time selection of simulator engine
+
+Usage::
+
+    from nmutil.sim_tmp_alternative import Simulator
+
+Then, use :py:class:`Simulator` as usual.
+
+This should be backwards compatible to old developer versions of nMigen.
+
+To use cxxsim, export ``NMIGEN_SIM_MODE=cxxsim`` from the shell.
+Be sure to check out the ``cxxsim`` branch of nMigen, and update yosys
+to the latest commit as well.
+
+To use pysim, just keep ``NMIGEN_SIM_MODE`` unset.
+Alternatively, export ``NMIGEN_SIM_MODE=pysim``.
+
+Example::
+
+    $ export NMIGEN_SIM_MODE=...  # pysim or cxxsim, default is pysim
+    $ python ...
+
+or, even::
+
+    $ NMIGEN_SIM_MODE=...  python ...
+"""
+
+import os
+
+try:
+    from nmigen.sim import Simulator as RealSimulator
+    detected_new_api = True
+except ImportError:
+    detected_new_api = False
+    try:
+        from nmigen.sim.pysim import Simulator as RealSimulator
+    except ImportError:
+        from nmigen.back.pysim import Simulator as RealSimulator
+
+nmigen_sim_environ_variable = os.environ.get("NMIGEN_SIM_MODE") \
+                              or "pysim"
+"""Detected run-time engine from environment"""
+
+
+def Simulator(*args, **kwargs):
+    """Wrapper that allows run-time selection of simulator engine"""
+    if detected_new_api:
+        kwargs['engine'] = nmigen_sim_environ_variable
+    return RealSimulator(*args, **kwargs)
+
+
+def is_engine_cxxsim():
+    """Returns ``True`` if the selected engine is cxxsim"""
+    return nmigen_sim_environ_variable == "cxxsim"
+
+
+def is_engine_pysim():
+    """Returns ``True`` if the selected engine is pysim"""
+    return nmigen_sim_environ_variable == "pysim"
+
+
+nmigen_sim_top_module = "top." if is_engine_pysim() else ""
+"""Work-around for cxxsim not defining the top-level module"""