This makes clk2fflogic add an attr to $ff cells that carry the state of
the emulated async FF. The $ff output doesn't have any async updates
that happened in the current cycle, but the $ff input does, so the $ff
input corresponds to the async FF's output in the original design.
Hence this patch also makes the following changes to passes besides
clk2fflogic (but only for FFs with the clk2fflogic attr set):
* opt_clean treats the input as a register name (instead of the
output)
* rename -witness ensures that the input has a public name
* the formal backends (smt2, btor, aiger) will use the input's
name for the initial state of the FF in witness files
* when sim reads a yw witness that assigns an initial value to the
input signal, the state update is redirected to the output
This ensures that yosys witness files for clk2fflogic designs have
useful and stable public signal names. It also makes it possible to
simulate a clk2fflogic witness on the original design (with some
limitations when the original design is already using $ff cells).
It might seem like setting the output of a clk2fflogic FF to update the
input's initial value might not work in general, but it works fine for
these reasons:
* Witnesses for FFs are only present in the initial cycle, so we do
not care about any later cycles.
* The logic that clk2fflogic generates loops the output of the
genreated FF back to the input, with muxes in between to apply any
edge or level sensitive updates. So when there are no active updates
in the current gclk cycle, there is a combinational path from the
output back to the input.
* The logic clk2fflogic generates makes sure that an edge sensitive
update cannot be active in the first cycle (i.e. the past initial
value is assumed to be whatever it needs to be to avoid an edge).
* When a level sensitive update is active in the first gclk cycle, it
is actively driving the output for the whole gclk cycle, so ignoring
any witness initialization is the correct behavior.
When assigning values to input ports of nested modules in cosimulation,
sim needs to find the actual driver of the signal to perform the
assignment. The existing code didn't handle unconnected inputs in that
scenario.
Writeback of simulation state into initial state was only working for `run()` and `run_cosim_fst()`.
This change moves the writeback into the `write_output_files()` function so that all simulation modes work with the writeback option.
This matches the behavior of smtbmc.
This also updates the sim internal memory API to allow masked writes
where State::Sa bits (internal don't care - not a valid value for a
signal) leave the memory content unchanged.
The previous approach only initialized past_d and past_ad while for FST
cosim we also need to initialize the other past values like past_clk,
etc. Also to properly initialize them, we need to run a combinational
update step in case any of the wires feeding into the FF are private or
otherwise not part of the FST.
Uses the regex below to search (using vscode):
^\t\tlog\("(.{10,}(?<!\\n)|.{81,}\\n)"\);
Finds any log messages double indented (which help messages are)
and checks if *either* there are is no newline character at the end,
*or* the number of characters before the newline is more than 80.
These can be used to protect undefined flip-flop initialization values
from optimizations that are not sound for formal verification and can
help mapping all solver-provided values in witness traces for flows that
use different backends simultaneously.
Martin Povišer
Jannis Harder
Krystine Sherwin
Claire Xenia Wolf
KrystalDelusion
Miodrag Milanovic