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108 lines
5.1 KiB
ReStructuredText
108 lines
5.1 KiB
ReStructuredText
Technology mapping
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==================
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.. todo:: less academic, check text is coherent
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Previous chapters outlined how HDL code is transformed into an RTL netlist. The
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RTL netlist is still based on abstract coarse-grain cell types like arbitrary
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width adders and even multipliers. This chapter covers how an RTL netlist is
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transformed into a functionally equivalent netlist utilizing the cell types
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available in the target architecture.
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Technology mapping is often performed in two phases. In the first phase RTL
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cells are mapped to an internal library of single-bit cells (see
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:ref:`sec:celllib_gates`). In the second phase this netlist of internal gate
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types is transformed to a netlist of gates from the target technology library.
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When the target architecture provides coarse-grain cells (such as block ram or
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ALUs), these must be mapped to directly form the RTL netlist, as information on
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the coarse-grain structure of the design is lost when it is mapped to bit-width
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gate types.
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Cell substitution
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-----------------
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The simplest form of technology mapping is cell substitution, as performed by
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the techmap pass. This pass, when provided with a Verilog file that implements
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the RTL cell types using simpler cells, simply replaces the RTL cells with the
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provided implementation.
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When no map file is provided, techmap uses a built-in map file that maps the
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Yosys RTL cell types to the internal gate library used by Yosys. The curious
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reader may find this map file as :file:`techlibs/common/techmap.v` in the Yosys
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source tree.
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Additional features have been added to techmap to allow for conditional mapping
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of cells (see :doc:`/cmd/techmap`). This can for example be useful if the target
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architecture supports hardware multipliers for certain bit-widths but not for
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others.
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A usual synthesis flow would first use the techmap pass to directly map some RTL
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cells to coarse-grain cells provided by the target architecture (if any) and
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then use techmap with the built-in default file to map the remaining RTL cells
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to gate logic.
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Subcircuit substitution
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-----------------------
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Sometimes the target architecture provides cells that are more powerful than the
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RTL cells used by Yosys. For example a cell in the target architecture that can
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calculate the absolute-difference of two numbers does not match any single RTL
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cell type but only combinations of cells.
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For these cases Yosys provides the extract pass that can match a given set of
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modules against a design and identify the portions of the design that are
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identical (i.e. isomorphic subcircuits) to any of the given modules. These
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matched subcircuits are then replaced by instances of the given modules.
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The extract pass also finds basic variations of the given modules, such as
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swapped inputs on commutative cell types.
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In addition to this the extract pass also has limited support for frequent
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subcircuit mining, i.e. the process of finding recurring subcircuits in the
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design. This has a few applications, including the design of new coarse-grain
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architectures :cite:p:`intersynthFdlBookChapter`.
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The hard algorithmic work done by the extract pass (solving the isomorphic
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subcircuit problem and frequent subcircuit mining) is performed using the
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SubCircuit library that can also be used stand-alone without Yosys (see
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:ref:`sec:SubCircuit`).
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.. _sec:techmap_extern:
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Gate-level technology mapping
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-----------------------------
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.. todo:: newer techmap libraries appear to be largely ``.v`` instead of ``.lib``
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On the gate-level the target architecture is usually described by a "Liberty
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file". The Liberty file format is an industry standard format that can be used
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to describe the behaviour and other properties of standard library cells .
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Mapping a design utilizing the Yosys internal gate library (e.g. as a result of
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mapping it to this representation using the techmap pass) is performed in two
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phases.
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First the register cells must be mapped to the registers that are available on
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the target architectures. The target architecture might not provide all
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variations of d-type flip-flops with positive and negative clock edge,
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high-active and low-active asynchronous set and/or reset, etc. Therefore the
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process of mapping the registers might add additional inverters to the design
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and thus it is important to map the register cells first.
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Mapping of the register cells may be performed by using the dfflibmap pass. This
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pass expects a Liberty file as argument (using the -liberty option) and only
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uses the register cells from the Liberty file.
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Secondly the combinational logic must be mapped to the target architecture. This
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is done using the external program ABC via the abc pass by using the -liberty
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option to the pass. Note that in this case only the combinatorial cells are used
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from the cell library.
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Occasionally Liberty files contain trade secrets (such as sensitive timing
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information) that cannot be shared freely. This complicates processes such as
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reporting bugs in the tools involved. When the information in the Liberty file
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used by Yosys and ABC are not part of the sensitive information, the additional
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tool yosys-filterlib (see :ref:`sec:filterlib`) can be used to strip the
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sensitive information from the Liberty file.
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