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.
Prior to this fix, the `CxxrtlBackend` used the entire path for the include
directive when a separated interface file is generated (via the `-header`
option). This commit updates the code to use the base name of the interface
file.
Since the C++11 standard is used by default, we cannot take advantage of
the `std::filesystem` to get the basename.
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.)
We add a new flow graph node type, PRINT_SYNC, as they don't get handled
with regular CELL_EVALs. We could probably move this grouping out of
the dump method.
Removing some signed checks and logic where we've already guaranteed the
values to be positive. Indeed, in these cases, if a negative value got
through (per my realisation in the signed fuzz harness), it would cause
an infinite loop due to flooring division.
Before this commit, values, wires, and memories with an initializer
were value-initialized in emitted C++ code. After this commit, all
values, wires, and memories are default-initialized, and the default
constructor of generated modules calls the reset() method, which
assigns the members that have an initializer.
The following VCD file crashes GTKWave's VCD loader:
$var wire 1 ! x:1 $end
$enddefinitions $end
In practice, a colon can be a part of a variable name that is
translated from a Verilog function, something like:
update$func$.../hdl/hazard3_csr.v:350$2534.$result
Catherine
Martin Povišer
Merry
Henri Nurmi
Asherah Connor
Charlotte
Michael Nolan
Marcelina Kościelnicka