docs: More netlist documentation

Signed-off-by: David Shah <dave@ds0.me>

1 files changed, 31 insertions, 0 deletions

diff --git a/docs/netlist.md b/docs/netlist.md
index 09bf53d0..0f9a8969 100644
--- a/docs/netlist.md
+++ b/docs/netlist.md
@@ -40,3 +40,34 @@ Other structures used by these basic structures include:
     - Each entry in `wires` maps from *sink* wire in the routing tree to its driving pip, and the binding strength of that pip (e.g. how freely the router may rip up the pip)
     - Manipulation of this structure is done automatically by `Arch::bindWire`, `Arch::unbindWire`, `Arch::bindPip` and `Arch::unbindPip`; which should almost always be used in lieu of manual manipulation
  - `attrs` stores metadata about the wire (which may come from the JSON or be added by passes)
+ - `clkconstr` contains the period constraint if the wire is a constrained clock; or is empty otherwise
+ - `region` is a reference to a `Region` if the net is constrained to a device region or `nullptr` otherwise (_N.B. not supported by the current router_).
+
+## BaseCtx/Context
+
+Relevant fields from a netlist point of view are:
+ - `cells` is a map from cell name to a `unique_ptr<CellInfo>` containing cell data
+ - `nets` is a map from net name to a `unique_ptr<NetInfo>` containing net data
+ - `net_aliases` maps every alias for a net to its canonical name (i.e. index into `nets`) - net aliases often occur when a net has a name both inside a submodule and higher level module
+ - `ports` is a list of top level ports, primarily used during JSON export (e.g. to produce a useful post-PnR simulation model)
+
+Context also has a method `check()` that ensures all of the contracts met above are satisfied. It is strongly suggested to run this after any pass that may modify the netlist.
+
+## Performance Improvements
+
+Two features are provided to enable performance improvements in some algorithms, generally by reducing the number of `unordered_map` accesses.
+
+The first is `udata`. This is a field of both nets and cells that can be used to give an index into algorithm-specific structures, such as a flat `vector` of cells. Placers and routers may use this for any purpose, but it should not be used to exchange data between passes.
+
+The second is `ArchCellInfo` and `ArchNetInfo`. These are provided by architectures and used as base classes for `CellInfo` and `NetInfo` respectively. They allow architectures to tag information that is needed frequently - for example the clock polarity and clock net for a flipflop are needed for placement validity checking. They should only be used inside arch-specific code, and are lost when netlists are saved/loaded thus must not be used as primary storage - usually these should mirror attributes/parameters. `assignArchInfo` should set these up accordingly.
+
+## Helper Functions - Context
+
+`Context` and its subclass `BaseCtx` provides several helper functions that are often needed inside CAD algorithms.
+
+ - `nameOfBel`, `nameOfWire`, and `nameOfPip` gets the name of an identified object as a C string, often used in conjunction with the logging functions
+ - `nameOf` is similar to above but for netlist objects that have a `name` field (e.g. cells, nets, etc)
+ - `getNetinfoSourceWire` gets the physical wire `WireId` associated with the source of a net
+ - `getNetinfoSinkWire` gets the physical wire `WireId` associated with a given sink (specified by `PortRef`)
+ - `getNetinfoRouteDelay` gets the routing delay - actual if the net is fully routed, estimated otherwise - between the source and a given sink of a net
+ - `getNetByAlias` returns the pointer to a net given any of its aliases - this should be used in preference to a direct lookup in `nets` whenever a net name is provided by the user
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