Before this commit, each simulation engine (which is only pysim at
the moment, but also cxxsim soon) was a subclass of SimulatorCore,
and every simulation engine module would essentially duplicate
the complete structure of a simulator, with code partially shared.
This was a really bad idea: it was inconvenient to use, with
downstream code having to branch between e.g. PySettle and CxxSettle;
it had no well-defined external interface; it had multiple virtually
identical entry points; and it had no separation between simulation
algorithms and glue code.
This commit completely rearranges simulation code.
1. sim._base defines internal simulation interfaces. The clarity of
these internal interfaces is important because simulation
engines mix and match components to provide a consistent API
regardless of the chosen engine.
2. sim.core defines the external simulation interface: the commands
and the simulator facade. The facade provides a single entry
point and, when possible, validates or lowers user input.
It also imports built-in simulation engines by their symbolic
name, avoiding eager imports of pyvcd or ctypes.
3. sim.xxxsim (currently, only sim.pysim) defines the simulator
implementation: time and state management, process scheduling,
and waveform dumping.
The new simulator structure has none of the downsides of the old one.
See #324.
The redesign introduces no fundamental incompatibilities, but it does
involve minor breaking changes:
* The simulator commands were moved from hdl.ast to back.pysim
(instead of only being reexported from back.pysim).
* back.pysim.DeadlineError was removed.
Summary of changes:
* The new simulator compiles HDL to Python code and is >6x faster.
(The old one compiled HDL to lots of Python lambdas.)
* The new simulator is a straightforward, rigorous implementation
of the Synchronous Reactive Programming paradigm, instead of
a pile of ad-hoc code with no particular design driving it.
* The new simulator never raises DeadlineError, and there is no
limit on the amount of delta cycles.
* The new simulator robustly handles multiclock designs.
* The new simulator can be reset, such that the compiled design
can be reused, which can save significant runtime with large
designs.
* Generators can no longer be added as processes, since that would
break reset(); only generator functions may be. If necessary,
they may be added by wrapping them into a generator function;
a deprecated fallback does just that. This workaround will raise
an exception if the simulator is reset and restarted.
* The new simulator does not depend on Python extensions.
(The old one required bitarray, which did not provide wheels.)
Fixes#28.
Fixes#34.
Fixes#160.
Fixes#161.
Fixes#215.
Fixes#242.
Fixes#262.
