Docs: Merge yosys_source into extending_yosys

Move abc_flow content into synthesis/abc document.

docs/source/using_yosys/synthesis/abc.rst

@@ -1,5 +1,5 @@
 The ABC toolbox
----------------
+===============
 
 .. role:: yoscrypt(code)
    :language: yoscrypt
@@ -21,7 +21,7 @@ global view of the mapping problem.
 .. _ABC: https://github.com/berkeley-abc/abc
 
 ABC: the unit delay model, simple and efficient
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-----------------------------------------------
 
 The :cmd:ref:`abc` pass uses a highly simplified view of an FPGA:
 
@@ -66,7 +66,7 @@ But this approach has drawbacks, too:
   before clock edge) which affect the delay of a path.
 
 ABC9: the generalised delay model, realistic and flexible
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+---------------------------------------------------------
 
 ABC9 uses a more detailed and accurate model of an FPGA:
 
@@ -101,3 +101,81 @@ optimise better around those boxes, and also permute inputs to give the critical
 path the fastest inputs.
 
 .. todo:: more about logic minimization & register balancing et al with ABC
+
+Setting up a flow for ABC9
+--------------------------
+
+Much of the configuration comes from attributes and ``specify`` blocks in
+Verilog simulation models.
+
+``specify`` syntax
+~~~~~~~~~~~~~~~~~~
+
+Since ``specify`` is a relatively obscure part of the Verilog standard, a quick
+guide to the syntax:
+
+.. code-block:: verilog
+
+   specify                           // begins a specify block
+     (A => B) = 123;                 // simple combinational path from A to B with a delay of 123.
+     (A *> B) = 123;                 // simple combinational path from A to all bits of B with a delay of 123 for all.
+     if (FOO) (A => B) = 123;        // paths may apply under specific conditions.
+     (posedge CLK => (Q : D)) = 123; // combinational path triggered on the positive edge of CLK; used for clock-to-Q arrival paths.
+     $setup(A, posedge CLK, 123);    // setup constraint for an input relative to a clock.
+   endspecify                        // ends a specify block
+
+By convention, all delays in ``specify`` blocks are in integer picoseconds.
+Files containing ``specify`` blocks should be read with the ``-specify`` option
+to :cmd:ref:`read_verilog` so that they aren't skipped.
+
+LUTs
+^^^^
+
+LUTs need to be annotated with an ``(* abc9_lut=N *)`` attribute, where ``N`` is
+the relative area of that LUT model. For example, if an architecture can combine
+LUTs to produce larger LUTs, then the combined LUTs would have increasingly
+larger ``N``. Conversely, if an architecture can split larger LUTs into smaller
+LUTs, then the smaller LUTs would have smaller ``N``.
+
+LUTs are generally specified with simple combinational paths from the LUT inputs
+to the LUT output.
+
+DFFs
+^^^^
+
+DFFs should be annotated with an ``(* abc9_flop *)`` attribute, however ABC9 has
+some specific requirements for this to be valid: - the DFF must initialise to
+zero (consider using :cmd:ref:`dfflegalize` to ensure this). - the DFF cannot
+have any asynchronous resets/sets (see the simplification idiom and the Boxes
+section for what to do here).
+
+It is worth noting that in pure ``abc9`` mode, only the setup and arrival times
+are passed to ABC9 (specifically, they are modelled as buffers with the given
+delay). In ``abc9 -dff``, the flop itself is passed to ABC9, permitting
+sequential optimisations.
+
+Some vendors have universal DFF models which include async sets/resets even when
+they're unused. Therefore *the simplification idiom* exists to handle this: by
+using a ``techmap`` file to discover flops which have a constant driver to those
+asynchronous controls, they can be mapped into an intermediate, simplified flop
+which qualifies as an ``(* abc9_flop *)``, ran through :cmd:ref:`abc9`, and then
+mapped back to the original flop. This is used in :cmd:ref:`synth_intel_alm` and
+:cmd:ref:`synth_quicklogic` for the PolarPro3.
+
+DFFs are usually specified to have setup constraints against the clock on the
+input signals, and an arrival time for the ``Q`` output.
+
+Boxes
+^^^^^
+
+A "box" is a purely-combinational piece of hard logic. If the logic is exposed
+to ABC9, it's a "whitebox", otherwise it's a "blackbox". Carry chains would be
+best implemented as whiteboxes, but a DSP would be best implemented as a
+blackbox (multipliers are too complex to easily work with). LUT RAMs can be
+implemented as whiteboxes too.
+
+Boxes are arguably the biggest advantage that ABC9 has over ABC: by being aware
+of carry chains and DSPs, it avoids optimising for a path that isn't the actual
+critical path, while the generally-longer paths result in ABC9 being able to
+reduce design area by mapping other logic to larger-but-slower cells.
+