This commit changes the way edge detectors are represented in
generated code from a variable that is set in commit() and reset in
eval() to a function that considers .curr and .next of the clock
wire. Behavior remains the same. Besides being simpler to generate
and providing more opportunities for optimization, this commit paves
way for unbuffering module inputs.
Before this commit, any wire that was not driven by an output port of
exactly one comb cell would not be localized, even if there were no
feedback arcs through that wire. This would cause the wire to become
buffered and require (often quite a few) extraneous delta cycles
during evaluation. To alleviate this problem, -O5 was running
`splitnets -driver`.
However, this solution was mistaken. Because `splitnets -driver`
followed by `opt_clean -purge` would produce more nets with multiple
drivers, it would have to be iterated to fixpoint. Moreover, even if
this was done, it would not be sufficient because `opt_clean -purge`
does not currently remove wires with the `\init` attribute (and it
is not desirable to remove such wires, since they correspond to
registers and may be useful for debugging).
The proper solution is to consider the condition in which a wire
may be localized. Specifically, if there are no feedback arcs through
this wire, and no part of the wire is driven by an output of a sync
cell, then the wire holds no state and is localizable.
After this commit, the original condition for not localizing a wire
is replaced by a check for any sync cell driving it. This makes it
unnecessary to run `splitnets -driver` in the majority of cases
to get a design with no buffered wires, and -O5 no longer includes
that pass. As a result, Minerva SRAM SoC no longer has any buffered
wires, and runs ~27% faster.
In addition, this commit prepares the flow graph for introduction
of sync outputs of black boxes.
Co-authored-by: Jean-François Nguyen <jf@lambdaconcept.com>
Any buffered combinatorial wires (including, as a subset, feedback
wires) will prevent the design from always converging in one delta
cycle. Before this commit, only feedback wires were detected. After
this commit, any buffered combinatorial wires, including feedback
wires, are detected.
Co-authored-by: Jean-François Nguyen <jf@lambdaconcept.com>
Both parameters and attributes are necessary because the parameters
have to be the same between every instantiation of the cell, but
attributes may well vary. For example, for an UART PHY, the type
of the PHY (tty, pty, socket) would be a parameter, but configuration
of the implementation specified by the type (socket address) would
be an attribute.
This commit adds support for replacing RTLIL modules with CXXRTL
black boxes. Black box port widths may not depend on the parameters
with which it is instantiated (yet); the parameters may only be used
to change the behavior of the black box.
There is no practical benefit from using `const memory` for ROMs;
it uses an std::vector internally, which prevents contemporary
compilers from constant-propagating ROM contents. (It is not clear
whether they are permitted to do so.)
However, there is a major benefit from using non-const `memory` for
ROMs, which is the ability to dynamically fill the ROM for each
individual simulation.
This commit makes it possible to use several cxxrtl-generated files
in one application, as well as compiling cxxrtl-generated code as
a separate compilation unit.
Also, fix the semantics of SET/CLR inputs of the $dffsr cell, and
fix the scheduling of async FF cells to consider ARST/SET/CLR->Q
as a forward combinatorial arc.
This commit reduces space and time overhead for writable memories
to O(write port count) in both cases; implements handling for write
port priorities; and simplifies runtime representation of memories.
Hierarchical design simulations are generally much slower, but this
comes with a major increase in flexibility:
1. Since the `flatten` pass currently does not support flattening
of designs with processes, this is the only way to simulate such
designs with cxxrtl.
2. Support for hierarchy paves way for simulation black boxes,
which are necessary for e.g. replacing PHYs with C++ code that
integrates with the host system.
After this commit, if NDEBUG is not defined, out-of-bounds accesses
cause assertion failures for reads and writes. If NDEBUG is defined,
out-of-bounds reads return zeroes, and out-of-bounds writes are
ignored.
This commit also adds support for memories that start with a non-zero
index (`Memory::start_offset` in RTLIL).
This results in further massive gains in performance, modest decrease
in compile time, and, for designs without feedback arcs, makes it
possible to run eval() once per clock edge in certain conditions.
whitequark
David Shah