update doc (synth, overview, generics, etc.) (#1205)

* doc: extend info about setting generics through the CLI

* doc: rename 'Command Reference' to 'Additional Command Reference'

* doc: use '``' instead of ':samp:'

* doc: update info about synthesis

* doc: add references to sources in internals/Overview

* doc: add diagram to internals/Overview

1 files changed, 67 insertions, 0 deletions

diff --git a/doc/using/Synthesis.rst b/doc/using/Synthesis.rst
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+.. program:: ghdl
+.. _USING:Synthesis:
+
+Synthesis
+#########
+
+.. WARNING::
+   This is experimental and work in progress! If you find crashes or unsupported features, please :ref:`report them <reporting_bugs>`!
+
+Since ``v0.37``, GHDL features a built-in (experimental) synthesis kernel with two backends: ``synth`` and ``yosys-plugin``. Currently, synthesis is supported as a front-end of other synthesis and technology mapping tools.
+Hence, the netlists generated by GHDL are not optimised.
+
+.. NOTE::
+   Due to GHDL's modular architecture (see :ref:`INT:Overview`), the synthesis kernel shares the VHDL parsing front-end with the simulation back-ends. Hence, available options for synthesis are the same as for analysis and/or simulation elaboration (see :ref:`GHDL:options`).
+
+.. index:: synthesis command
+
+.. _Synth:command:
+
+Synthesis [``--synth``]
+=======================
+
+.. HINT::
+   This command is useful for checking that a design can be synthesized, before actually running a complete synthesis
+   tool. In fact, because this is expected to be much faster, it can be used as a frequent check.
+
+.. TIP::
+   Since GHDL's front-end supports multiple versions of the standard, but the synthesised netlists are generated using
+   a subset of VHDL 1993, GHDL's synthesis features can be used as a preprocessor with tools that do support older
+   versions of the standard, but which don't provide the most recent features.
+
+.. option:: --synth <[options] primary_unit [secondary_unit]>
+
+Elaborates for synthesis the design whose top unit is indicated by ``primary_unit [secondary_unit]``.
+
+.. ATTENTION::
+   All the units must have been analyzed; that is, the artifacts of previously executed :option:`-a` calls must exist.
+
+.. option:: --synth <[options] files... -e primary_unit [secondary_unit]>
+
+Analyses and elaborates for synthesis the files present on the command line only.
+Elaboration starts from the top unit indicated by ``primary_unit [secondary_unit]``.
+
+Currently, the output is a generic netlist using a (very simple) subset of VHDL 1993.
+See :ghdlsharp:`1174` for on-going discussion about other output formats.
+
+.. TIP::
+   Files can be provided in any order.
+
+.. _Synth:plugin:
+
+Yosys plugin
+============
+
+`ghdl-yosys-plugin <https://github.com/ghdl/ghdl-yosys-plugin>`_ is a module to use GHDL as a VHDL front-end for `Yosys
+Open Synthesis Suite <http://www.clifford.at/yosys/>`_, a framework for optimised synthesis and technology mapping.
+Artifacts generated by Yosys can be used in multiple open source and vendor tools to achieve P&R, formal verification,
+etc. A relevant feature of combining GHDL and Yosys is that mixed-language (VHDL-Verilog) synthesis with open source
+tools is possible.
+
+The command line syntax for this plugin is the same as for :option:``--synth``, except that the command name (``--synth``)
+is neither required nor supported. Instead, ``yosys``, ``yosys -m ghdl`` or ``yosys -m path/to/ghdl.so`` need to be used,
+depending of how is the plugin built. See `README <https://github.com/ghdl/ghdl-yosys-plugin>`_ for building and installation
+guidelines.
+
+.. HINT::
+   ghdl-yosys-plugin is a thin layer that converts the internal representation of :option:`--synth` to Yosys' C API. Hence, it is suggested to check the designs with :option:`--synth` before running synthesis with Yosys.