This commit adds a reader/writer implementation for a file format
optimized for fast, single-pass storage and retrieval of design state
changes, as well as a recorder/replayer that integrate with the eval
and commit simulation steps to create replay logs and reproduce them
later.
This feature makes it possible to run a simulation once, recording
the stimulus as well as changes to the registers, and navigate to
a past time point in the simulation later without rerunning it.
Both the changes in inputs (stimulus) and changes in state are saved
so that navigation does not require calling `eval()` or `commit()`;
only a series of memory copy operations.
On a representative example of a SoC netlist, saving the replay log
while simulating it takes 150% of the time it would take to simulate
the same design without logging, which is a much lower overhead than
writing an equivalent full view (including memories) VCD waveform dump.
The replay log is also several times smaller than the VCD dump, and
more space savings are available as low hanging fruit.
Replaying the log has not been optimized and currently takes about
the same time as running the simulation in first place. However, it
is still useful since it provides fast navigation to an arbitrary time
point, something that rerunning the simulation does not allow for.
The current file format should be considered preliminary. It is not
very space-efficient, and my testing shows that a lot of time is spent
in the write() syscall in the kernel. Most likely, compression and/or
writing in another thread could improve performance by 10-20%. This
may be done at a later time.
The commit observer is a structure containing a callback that is invoked
whenever the `commit()` method changes a wire or a memory. This allows
code external to the compiled netlist to react to changes in the design
state in a very efficient way. One example of how this feature can be
used is an efficient implementation of record/replay.
Note that the VCD writer does not benefit from this feature because it
must be able to react to changes in any debug items and not just those
that contain design state.
While the VCD format separates the timescale and the timestep (likely
to allow representing the timestep with a small integer type), time in
CXXRTL is represented using a uniform 96-bit number, which allows for
a ±100 year range at femtosecond resolution.
The implementation uses `value<96>`, which provides fast arithmetic and
comparison operations, as well as conversion to/from a more common
representation of integer seconds plus femtoseconds.
In preparation for substantial expansion of CXXRTL's runtime, this commit
reorganizes the files used by the implementation. Only minimal changes are
required in a consumer.
First, change:
-I$(yosys-config --datdir)/include
to:
-I$(yosys-config --datdir)/include/backends/cxxrtl/runtime
Second, change:
#include <backends/cxxrtl/cxxrtl.h>
to:
#include <cxxrtl/cxxrtl.h>
(and do the same for cxxrtl_vcd.h, etc.)
Catherine
Martin Povišer
Merry