Getting Started =============== Outline of a Yosys command -------------------------- Here is a the C++ code for a "hello_world" Yosys command (hello.cc): #include "kernel/yosys.h" USING_YOSYS_NAMESPACE PRIVATE_NAMESPACE_BEGIN struct HelloWorldPass : public Pass { HelloWorldPass() : Pass("hello_world") { } void execute(vector, Design*) override { log("Hello World!\n"); } } HelloWorldPass; PRIVATE_NAMESPACE_END This can be built into a Yosys module using the following command: yosys-config --exec --cxx --cxxflags --ldflags -o hello.so -shared hello.cc --ldlibs Or short: yosys-config --build hello.so hello.cc And then executed using the following command: yosys -m hello.so -p hello_world Yosys Data Structures --------------------- Here is a short list of data structures that you should make yourself familiar with before you write C++ code for Yosys. The following data structures are all defined when "kernel/yosys.h" is included and USING_YOSYS_NAMESPACE is used. 1. Yosys Container Classes Yosys uses dict and pool as main container classes. dict is essentially a replacement for std::unordered_map and pool is a replacement for std::unordered_set. The main characteristics are: - dict and pool are about 2x faster than the std containers - references to elements in a dict or pool are invalidated by insert and remove operations (similar to std::vector on push_back()). - some iterators are invalidated by erase(). specifically, iterators that have not passed the erased element yet are invalidated. (erase() itself returns valid iterator to the next element.) - no iterators are invalidated by insert(). elements are inserted at begin(). i.e. only a new iterator that starts at begin() will see the inserted elements. - the method .count(key, iterator) is like .count(key) but only considers elements that can be reached via the iterator. - iterators can be compared. it1 < it2 means that the position of t2 can be reached via t1 but not vice versa. - the method .sort() can be used to sort the elements in the container the container stays sorted until elements are added or removed. - dict and pool will have the same order of iteration across all compilers, standard libraries and architectures. In addition to dict and pool there is also an idict that creates a bijective map from K to the integers. For example: idict si; log("%d\n", si("hello")); // will print 42 log("%d\n", si("world")); // will print 43 log("%d\n", si.at("world")); // will print 43 log("%d\n", si.at("dummy")); // will throw exception log("%s\n", si[42].c_str())); // will print hello log("%s\n", si[43].c_str())); // will print world log("%s\n", si[44].c_str())); // will throw exception It is not possible to remove elements from an idict. Finally mfp implements a merge-find set data structure (aka. disjoint-set or union-find) over the type K ("mfp" = merge-find-promote). 2. Standard STL data types In Yosys we use std::vector and std::string whenever applicable. When dict and pool are not suitable then std::map and std::set are used instead. The types std::vector and std::string are also available as vector and string in the Yosys namespace. 3. RTLIL objects The current design (essentially a collection of modules, each defined by a netlist) is stored in memory using RTLIL object (declared in kernel/rtlil.h, automatically included by kernel/yosys.h). You should glance over at least the declarations for the following types in kernel/rtlil.h: RTLIL::IdString This is a handle for an identifier (e.g. cell or wire name). It feels a lot like a std::string, but is only a single int in size. (The actual string is stored in a global lookup table.) RTLIL::SigBit A single signal bit. I.e. either a constant state (0, 1, x, z) or a single bit from a wire. RTLIL::SigSpec Essentially a vector of SigBits. RTLIL::Wire RTLIL::Cell The building blocks of the netlist in a module. RTLIL::Module RTLIL::Design The module is a container with connected cells and wires in it. The design is a container with modules in it. All this types are also available without the RTLIL:: prefix in the Yosys namespace. 4. SigMap and other Helper Classes There are a couple of additional helper classes that are in wide use in Yosys. Most importantly there is SigMap (declared in kernel/sigtools.h). When a design has many wires in it that are connected to each other, then a single signal bit can have multiple valid names. The SigMap object can be used to map SigSpecs or SigBits to unique SigSpecs and SigBits that consistently only use one wire from such a group of connected wires. For example: SigBit a = module->addWire(NEW_ID); SigBit b = module->addWire(NEW_ID); module->connect(a, b); log("%d\n", a == b); // will print 0 SigMap sigmap(module); log("%d\n", sigmap(a) == sigmap(b)); // will print 1 Using the RTLIL Netlist Format ------------------------------ In the RTLIL netlist format the cell ports contain SigSpecs that point to the Wires. There are no references in the other direction. This has two direct consequences: (1) It is very easy to go from cells to wires but hard to go in the other way. (2) There is no danger in removing cells from the netlists, but removing wires can break the netlist format when there are still references to the wire somewhere in the netlist. The solution to (1) is easy: Create custom indexes that allow you to make fast lookups for the wire-to-cell direction. You can either use existing generic index structures to do that (such as the ModIndex class) or write your own index. For many application it is simplest to construct a custom index. For example: SigMap sigmap(module); dict sigbit_to_driver_index; for (auto cell : module->cells()) for (auto &conn : cell->connections()) if (cell->output(conn.first)) for (auto bit : sigmap(conn.second)) sigbit_to_driver_index[bit] = cell; Regarding (2): There is a general theme in Yosys that you don't remove wires from the design. You can rename them, unconnect them, but you do not actually remove the Wire object from the module. Instead you let the "clean" command take care of the dangling wires. On the other hand it is safe to remove cells (as long as you make sure this does not invalidate a custom index you are using in your code). Example Code ------------ The following yosys commands are a good starting point if you are looking for examples of how to use the Yosys API: docs/source/CHAPTER_Prog/stubnets.cc manual/PRESENTATION_Prog/my_cmd.cc Script Passes ------------- The ScriptPass base class can be used to implement passes that just call other passes, like a script. Examples for such passes are: techlibs/common/prep.cc techlibs/common/synth.cc In some cases it is easier to implement such a pass as regular pass, for example when ScriptPass doesn't provide the type of flow control desired. (But many of the script passes in Yosys that don't use ScriptPass simply predate the ScriptPass base class.) Examples for such passes are: passes/opt/opt.cc passes/proc/proc.cc Whether they use the ScriptPass base-class or not, a pass should always either call other passes without doing any non-trivial work itself, or should implement a non-trivial algorithm but not call any other passes. The reason for this is that this helps containing complexity in individual passes and simplifies debugging the entire system. Exceptions to this rule should be rare and limited to cases where calling other passes is optional and only happens when requested by the user (such as for example `techmap -autoproc`), or where it is about commands that are "top-level commands" in their own right, not components to be used in regular synthesis flows (such as the `bugpoint` command). A pass that would "naturally" call other passes and also do some work itself should be re-written in one of two ways: 1) It could be re-written as script pass with the parts that are not calls to other passes factored out into individual new passes. Usually in those cases the new sub passes share the same prefix as the top-level script pass. 2) It could be re-written so that it already expects the design in a certain state, expecting the calling script to set up this state before calling the pass in questions. Many back-ends are examples for the 2nd approach. For example, `write_aiger` does not convert the design into AIG representation, but expects the design to be already in this form, and prints an `Unsupported cell type` error message otherwise. Notes on the existing codebase ------------------------------ For historical reasons not all parts of Yosys adhere to the current coding style. When adding code to existing parts of the system, adhere to this guide for the new code instead of trying to mimic the style of the surrounding code.