This effectively reverts and reimplements half of commit 82903e49.
I was confused and did not realize that RTLIL does, in fact, have
attributes on switches.
After this commit, processes no longer have any source locations.
This is semantically fine, as the processes we emit are purely
artificial (because of LHS grouping), but I have not checked how
downstream tooling handles this.
According to RTLIL semantics (that was undocumented before today),
the only purpose of `sync always` is to enable inference of latches,
because there is no other way to express them in terms of RTLIL
processes without ending up with a combinatorial loop. But, nMigen
specifically avoids latches, so this is not necessary.
This change results in major improvements in Verilog readability.
See also #98.
This means that instead of:
with m.Case(0b00):
<body>
with m.Case(0b01):
<body>
it is legal to write:
with m.Case(0b00, 0b01):
<body>
with no change in semantics, and slightly nicer RTLIL or Verilog
output.
Fixes#103.
The main purpose of this rework is cleanup, to avoid specifying
the direction of input ports in an implicit, ad-hoc way using
the named ports and ports dictionaries.
While working on this I realized that output ports can be connected
to anything that is valid on LHS, so this is now supported too.
Otherwise the following code fails to compile:
index = Signal(1)
array = Array(range(2))
with m.If(0 == array[index]):
m.d.sync += index.eq(0)
Fixes#51.
These used to serve a useful purpose being public, back when the RTLIL
backend was immature. Not anymore; now they merely clutter up views
in gtkwave and so on.
This makes simulation work correctly (by introducing delta cycles,
and therefore, making the overall Verilog simulation deterministic)
at the price of pessimizing mux trees generated by Yosys and Synplify
frontends, sometimes severely.
This avoids reading 'x from the memory in simulation. In general,
FPGA memories can only be initialized in block granularity, and
zero-initializing is cheap, so this is not a significant issue with
resource consumption.
