At the moment the only thing it allows is redirecting `$print` cell
output in a context-dependent manner. In the future, it will allow
customizing handling of `$check` cells (where the default is to abort),
of out-of-range memory accesses, and other runtime conditions with
effects.
This context object also allows a suitably written testbench to add
Verilog-compliant `$time`/`$realtime` handling, albeit it involves
the ceremony of defining a `performer` subclass. Most people will
want something like this to customize `$time`:
int64_t time = 0;
struct : public performer {
int64_t *time_ptr;
int64_t time() const override { return *time_ptr; }
} performer = { &time };
p_top.step(&performer);
This approach to tracking simulation time was a mistake that I did not
catch in review. It has several issues:
1. There is absolutely no requirement to call `step()`, as it is
a convenience function. In particular, `steps` will not be
incremented in submodules if `-noflatten` is used.
2. The semantics of `steps` does not match that of the Verilog `$time`
construct.
3. There is no way to make the semantics of `%t` match that of Verilog.
4. The `module` interface is intentionally very barebones. It is little
more than a container for three method pointers, `reset`, `eval`,
and `commit`. Adding ancillary data to it goes against this.
If similar functionality is introduced again it should probably be
a variable that is global per toplevel design using some object that is
unique for an entire hierarchy of modules, and ideally exposed via
the C API. For now, it is being removed (in this commit) and (in next
commit) the capability is being reintroduced through a context object
that can be specified for `eval()`.
This commit achieves three roughly equally important goals:
1. To bring the rendering code in kernel/fmt.cc and in cxxrtl.h as close
together as possible, with an ideal of only having the bigint library
as the difference between the render functions.
2. To make the treatment of `$time` and `$realtime` in CXXRTL closer to
the Verilog semantics, at least in the formatting code.
3. To change the code generator so that all of the `$print`-to-`string`
conversion code is contained inside of a closure.
There are two reasons to aim for goal (3):
a. Because output redirection through definition of a global ostream
object is neither convenient nor useful for environments where
the output is consumed by other code rather than being printed on
a terminal.
b. Because it may be desirable to, in some cases, ignore the `$print`
cells that are present in the netlist based on a runtime decision.
This is doubly true for an upcoming `$check` cell implementing
assertions, since failing a `$check` would by default cause a crash.
This avoids having to devirtualize them later to get performance back,
and simplifies the code a bit.
The change is prompted by the desire to add a similar observer object
to `eval()`, and a repeated consideration of whether the dispatch on
these should be virtual in first place.
The name `on_commit` was terrible since it would not be called, as one
may conclude from the name, on each `commit()`, but only whenever that
method actually updates a value.
These are useful for formal verification with SBY where they can be used
to display solver chosen `rand const reg` signals and signals derived
from those.
The previous error message for non-constant initial $display statements
is downgraded to a log message. Constant initial $display statements
will be shown both during elaboration and become part of the RTLIL so
that the `sim` output is complete.
This allows tools like SBY to capture the $display output independent
from anything else sim might log. Additionally it provides source and
hierarchy locations for everything printed.
This makes tests/verilog/dynamic_range_lhs.v pass, after ensuring that
nowrshmsk is actually tested.
Stride is extracted from indexing of two-dimensional packed arrays and
variable slices on the form dst[i*stride +: width] = src, and is used
to optimize the generated CASE block.
Also uses less confusing variable names for indexing of lhs wires.
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.
Catherine
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