In commit 9faa1d37, the RTLIL backend was changed to ignore modules
without ports completely, since Yosys would recognize empty modules
as black boxes without explicit `write_verilog -noblackbox` and break
the design. That change had many flaws:
* It removed instances without ports, which are used in e.g. SoC
FPGAs to instantiate a dummy CPU.
* It removed fragments without ports, which can appear in e.g. SoC
FPGAs in case the fabric is not connected to any I/O ports.
* Finally, it was just conceptually unjustified.
This commit changes the logic to actually check for empty fragments,
and instead of removing them, it adds a dummy wire inside. It would
be possible to use the Yosys-specific (*noblackbox*) attribute.
However, it would be necessary to strip it for most targets right
away, and also the wire doubles as documentation.
Fixes#441.
Such wires are likely to trigger pathological behavior in Yosys and,
if applicable, other toolchains that consume Verilog converted from
RTLIL.
Fixes#341.
Before this commit, selecting a part that was fully out of bounds of
a value was correctly implemented as a write to a dummy wire, but
selecting a part that was only partially out of bounds resulted in
a crash.
Fixes#351.
Before this commit, there was no way to do so besides creating and
assigning an intermediate signal, which could not be extracted into
a helper function due to Module statefulness.
Fixes#292.
$verilog_initial_trigger was introduced to work around Verilog
simulation semantics issues with `always @*` statements that only
have constants on RHS and in conditions. Unfortunately, it breaks
Verilator. Since the combination of proc_prune and proc_clean passes
eliminates all such statements, it can be simply removed when both
of these passes are available, currently on Yosys master. After
Yosys 0.10 is released, we can get rid of $verilog_initial_trigger
entirely.
Also, replace `bits, sign = x.shape()` with more idiomatic
`width, signed = x.shape()`.
This unifies all properties corresponding to `len(x)` to `x.width`.
(Not all values have a `width` property.)
Fixes#210.
Because Fragment.prepare is not (currently) idempotent, it is useful
to be able to avoid calling it when converting. Even if it is made
idempotent, it can be slow on large designs, so it is advantageous
regardless of that.
Using 'x is legal RTLIL, in theory, but in practice it crashes Yosys
and when it doesn't, it causes Yosys to produce invalid Verilog.
Using a dummy wire is always safe and is not a major readability
issue as this is a rare corner case.
(It is not trivial to shorten the RHS in this case, because during
expansion of an ArrayProxy, match_shape() could be called in
a context far from the RHS handling logic.)
