Typical phases of a synthesis flow ---------------------------------- .. todo:: expand text - Reading and elaborating the design - Higher-level synthesis and optimization - Converting ``always``-blocks to logic and registers - Perform coarse-grain optimizations (resource sharing, const folding, ...) - Handling of memories and other coarse-grain blocks - Extracting and optimizing finite state machines - Convert remaining logic to bit-level logic functions - Perform optimizations on bit-level logic functions - Map bit-level logic gates and registers to cell library - Write results to output file Reading the design ~~~~~~~~~~~~~~~~~~ .. code-block:: yoscrypt read_verilog file1.v read_verilog -I include_dir -D enable_foo -D WIDTH=12 file2.v read_verilog -lib cell_library.v verilog_defaults -add -I include_dir read_verilog file3.v read_verilog file4.v verilog_defaults -clear verilog_defaults -push verilog_defaults -add -I include_dir read_verilog file5.v read_verilog file6.v verilog_defaults -pop Design elaboration ~~~~~~~~~~~~~~~~~~ During design elaboration Yosys figures out how the modules are hierarchically connected. It also re-runs the AST parts of the Verilog frontend to create all needed variations of parametric modules. .. code-block:: yoscrypt # simplest form. at least this version should be used after reading all input files # hierarchy # recommended form. fails if parts of the design hierarchy are missing, removes # everything that is unreachable from the top module, and marks the top module. # hierarchy -check -top top_module The :cmd:ref:`proc` command ~~~~~~~~~~~~~~~~~~~~~~~~~~~ The Verilog frontend converts ``always``-blocks to RTL netlists for the expressions and "processess" for the control- and memory elements. The :cmd:ref:`proc` command transforms this "processess" to netlists of RTL multiplexer and register cells. The :cmd:ref:`proc` command is actually a macro-command that calls the following other commands: .. code-block:: yoscrypt proc_clean # remove empty branches and processes proc_rmdead # remove unreachable branches proc_init # special handling of "initial" blocks proc_arst # identify modeling of async resets proc_mux # convert decision trees to multiplexer networks proc_dff # extract registers from processes proc_clean # if all went fine, this should remove all the processes Many commands can not operate on modules with "processess" in them. Usually a call to :cmd:ref:`proc` is the first command in the actual synthesis procedure after design elaboration. Example ^^^^^^^ .. literalinclude:: /code_examples/synth_flow/proc_01.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/proc_01.v`` .. literalinclude:: /code_examples/synth_flow/proc_01.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/proc_01.ys`` .. figure:: /_images/code_examples/synth_flow/proc_01.* :class: width-helper .. figure:: /_images/code_examples/synth_flow/proc_02.* :class: width-helper .. literalinclude:: /code_examples/synth_flow/proc_02.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/proc_02.v`` .. literalinclude:: /code_examples/synth_flow/proc_02.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/proc_02.ys`` .. figure:: /_images/code_examples/synth_flow/proc_03.* :class: width-helper .. literalinclude:: /code_examples/synth_flow/proc_03.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/proc_03.ys`` .. literalinclude:: /code_examples/synth_flow/proc_03.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/proc_03.v`` The :cmd:ref:`opt` command ~~~~~~~~~~~~~~~~~~~~~~~~~~ The :cmd:ref:`opt` command implements a series of simple optimizations. It also is a macro command that calls other commands: .. code-block:: yoscrypt opt_expr # const folding and simple expression rewriting opt_merge -nomux # merging identical cells do opt_muxtree # remove never-active branches from multiplexer tree opt_reduce # consolidate trees of boolean ops to reduce functions opt_merge # merging identical cells opt_rmdff # remove/simplify registers with constant inputs opt_clean # remove unused objects (cells, wires) from design opt_expr # const folding and simple expression rewriting while [changed design] The command :cmd:ref:`clean` can be used as alias for :cmd:ref:`opt_clean`. And ``;;`` can be used as shortcut for :cmd:ref:`clean`. For example: .. code-block:: yoscrypt hierarchy; proc; opt; memory; opt_expr;; fsm;; Example ^^^^^^^ .. figure:: /_images/code_examples/synth_flow/opt_01.* :class: width-helper .. literalinclude:: /code_examples/synth_flow/opt_01.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/opt_01.ys`` .. literalinclude:: /code_examples/synth_flow/opt_01.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/opt_01.v`` .. figure:: /_images/code_examples/synth_flow/opt_02.* :class: width-helper .. literalinclude:: /code_examples/synth_flow/opt_02.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/opt_02.ys`` .. literalinclude:: /code_examples/synth_flow/opt_02.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/opt_02.v`` .. figure:: /_images/code_examples/synth_flow/opt_03.* :class: width-helper .. literalinclude:: /code_examples/synth_flow/opt_03.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/opt_03.ys`` .. literalinclude:: /code_examples/synth_flow/opt_03.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/opt_03.v`` .. figure:: /_images/code_examples/synth_flow/opt_04.* :class: width-helper .. literalinclude:: /code_examples/synth_flow/opt_04.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/opt_04.v`` .. literalinclude:: /code_examples/synth_flow/opt_04.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/opt_04.ys`` When to use :cmd:ref:`opt` or :cmd:ref:`clean` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Usually it does not hurt to call :cmd:ref:`opt` after each regular command in the synthesis script. But it increases the synthesis time, so it is favourable to only call :cmd:ref:`opt` when an improvement can be achieved. The designs in ``yosys-bigsim`` are a good playground for experimenting with the effects of calling :cmd:ref:`opt` in various places of the flow. It generally is a good idea to call :cmd:ref:`opt` before inherently expensive commands such as :cmd:ref:`sat` or :cmd:ref:`freduce`, as the possible gain is much higher in this cases as the possible loss. The :cmd:ref:`clean` command on the other hand is very fast and many commands leave a mess (dangling signal wires, etc). For example, most commands do not remove any wires or cells. They just change the connections and depend on a later call to clean to get rid of the now unused objects. So the occasional ``;;`` is a good idea in every synthesis script. The :cmd:ref:`memory` command ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the RTL netlist, memory reads and writes are individual cells. This makes consolidating the number of ports for a memory easier. The :cmd:ref:`memory` transforms memories to an implementation. Per default that is logic for address decoders and registers. It also is a macro command that calls other commands: .. code-block:: yoscrypt # this merges registers into the memory read- and write cells. memory_dff # this collects all read and write cells for a memory and transforms them # into one multi-port memory cell. memory_collect # this takes the multi-port memory cell and transforms it to address decoder # logic and registers. This step is skipped if "memory" is called with -nomap. memory_map Usually it is preferred to use architecture-specific RAM resources for memory. For example: .. code-block:: yoscrypt memory -nomap; techmap -map my_memory_map.v; memory_map Example ^^^^^^^ .. figure:: /_images/code_examples/synth_flow/memory_01.* :class: width-helper .. literalinclude:: /code_examples/synth_flow/memory_01.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/memory_01.ys`` .. literalinclude:: /code_examples/synth_flow/memory_01.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/memory_01.v`` .. figure:: /_images/code_examples/synth_flow/memory_02.* :class: width-helper .. literalinclude:: /code_examples/synth_flow/memory_02.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/memory_02.v`` .. literalinclude:: /code_examples/synth_flow/memory_02.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/memory_02.ys`` The :cmd:ref:`fsm` command ~~~~~~~~~~~~~~~~~~~~~~~~~~ The :cmd:ref:`fsm` command identifies, extracts, optimizes (re-encodes), and re-synthesizes finite state machines. It again is a macro that calls a series of other commands: .. code-block:: yoscrypt fsm_detect # unless got option -nodetect fsm_extract fsm_opt clean fsm_opt fsm_expand # if got option -expand clean # if got option -expand fsm_opt # if got option -expand fsm_recode # unless got option -norecode fsm_info fsm_export # if got option -export fsm_map # unless got option -nomap Some details on the most important commands from the ``fsm_*`` group: The :cmd:ref:`fsm_detect` command identifies FSM state registers and marks them with the ``(* fsm_encoding = "auto" *)`` attribute, if they do not have the ``fsm_encoding`` set already. Mark registers with ``(* fsm_encoding = "none" *)`` to disable FSM optimization for a register. The :cmd:ref:`fsm_extract` command replaces the entire FSM (logic and state registers) with a ``$fsm`` cell. The commands :cmd:ref:`fsm_opt` and :cmd:ref:`fsm_recode` can be used to optimize the FSM. Finally the :cmd:ref:`fsm_map` command can be used to convert the (optimized) ``$fsm`` cell back to logic and registers. The :cmd:ref:`techmap` command ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .. figure:: /_images/code_examples/synth_flow/techmap_01.* :class: width-helper The :cmd:ref:`techmap` command replaces cells with implementations given as verilog source. For example implementing a 32 bit adder using 16 bit adders: .. literalinclude:: /code_examples/synth_flow/techmap_01_map.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/techmap_01_map.v`` .. literalinclude:: /code_examples/synth_flow/techmap_01.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/techmap_01.v`` .. literalinclude:: /code_examples/synth_flow/techmap_01.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/techmap_01.ys`` stdcell mapping ^^^^^^^^^^^^^^^ When :cmd:ref:`techmap` is used without a map file, it uses a built-in map file to map all RTL cell types to a generic library of built-in logic gates and registers. The built-in logic gate types are: ``$_NOT_``, ``$_AND_``, ``$_OR_``, ``$_XOR_``, and ``$_MUX_``. The register types are: ``$_SR_NN_``, ``$_SR_NP_``, ``$_SR_PN_``, ``$_SR_PP_``, ``$_DFF_N_``, ``$_DFF_P_ $_DFF_NN0_``, ``$_DFF_NN1_``, ``$_DFF_NP0_``, ``$_DFF_NP1_``, ``$_DFF_PN0_``, ``$_DFF_PN1_``, ``$_DFF_PP0_ $_DFF_PP1_``, ``$_DFFSR_NNN_``, ``$_DFFSR_NNP_``, ``$_DFFSR_NPN_``, ``$_DFFSR_NPP_``, ``$_DFFSR_PNN_ $_DFFSR_PNP_``, ``$_DFFSR_PPN_``, ``$_DFFSR_PPP_``, ``$_DLATCH_N_``, and ``$_DLATCH_P_``. See :doc:`/yosys_internals/formats/cell_library` for more about the internal cells used. The :cmd:ref:`abc` command ~~~~~~~~~~~~~~~~~~~~~~~~~~ The :cmd:ref:`abc` command provides an interface to ABC_, an open source tool for low-level logic synthesis. .. _ABC: http://www.eecs.berkeley.edu/~alanmi/abc/ The :cmd:ref:`abc` command processes a netlist of internal gate types and can perform: - logic minimization (optimization) - mapping of logic to standard cell library (liberty format) - mapping of logic to k-LUTs (for FPGA synthesis) Optionally :cmd:ref:`abc` can process registers from one clock domain and perform sequential optimization (such as register balancing). ABC is also controlled using scripts. An ABC script can be specified to use more advanced ABC features. It is also possible to write the design with :cmd:ref:`write_blif` and load the output file into ABC outside of Yosys. Example ^^^^^^^ .. literalinclude:: /code_examples/synth_flow/abc_01.v :language: verilog :caption: ``docs/source/code_examples/synth_flow/abc_01.v`` .. literalinclude:: /code_examples/synth_flow/abc_01.ys :language: yoscrypt :caption: ``docs/source/code_examples/synth_flow/abc_01.ys`` .. figure:: /_images/code_examples/synth_flow/abc_01.* :class: width-helper Other special-purpose mapping commands ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :cmd:ref:`dfflibmap` This command maps the internal register cell types to the register types described in a liberty file. :cmd:ref:`hilomap` Some architectures require special driver cells for driving a constant hi or lo value. This command replaces simple constants with instances of such driver cells. :cmd:ref:`iopadmap` Top-level input/outputs must usually be implemented using special I/O-pad cells. This command inserts this cells to the design. Example Synthesis Script ~~~~~~~~~~~~~~~~~~~~~~~~ .. code-block:: yoscrypt # read and elaborate design read_verilog cpu_top.v cpu_ctrl.v cpu_regs.v read_verilog -D WITH_MULT cpu_alu.v hierarchy -check -top cpu_top # high-level synthesis proc; opt; fsm;; memory -nomap; opt # substitute block rams techmap -map map_rams.v # map remaining memories memory_map # low-level synthesis techmap; opt; flatten;; abc -lut6 techmap -map map_xl_cells.v # add clock buffers select -set xl_clocks t:FDRE %x:+FDRE[C] t:FDRE %d iopadmap -inpad BUFGP O:I @xl_clocks # add io buffers select -set xl_nonclocks w:* t:BUFGP %x:+BUFGP[I] %d iopadmap -outpad OBUF I:O -inpad IBUF O:I @xl_nonclocks # write synthesis results write_edif synth.edif The weird :cmd:ref:`select` expressions at the end of this script are discussed later in :doc:`using_yosys/more_scripting/selections`.