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.
In the past I was calling the ILANG_BACKEND::dump_const() to dump
values to an output stream. When these values were strings, the
function used to add quotes around them. The firrtl compiler, in turn,
escaped these quotes and the result was double-quoted strings which
were hard to read.
However I'm now calling design_entity->get_src_attribute() directly
and there is no additional quote being put around it, so we can
safely remove the unnecessary call to str.erase() here.
Yosys puts quotes around the string that represents the fileinfo whereas
firrtl does not. So when firrtl sees quotes, it escapes them with an extra
backslash which makes it hard to read afterwards.
