librelane

# Using Macros

The majority of chip designs are not actually synthesized, placed, routed and
streamed out all in one flow. Rather, parts of the design are usually hardened
first, which are then **integrated** into a **top-level chip**. These
pre-hardened parts of a design are called "macros."

There are a number of advantages to using Macros in {term}`PnR`, including but not
limited to:

* **Faster PnR**: If a macro has extremely complex routing, i.e. a latch-based
  SRAM for example, you can save the time of PnR on that module when
  you're iterating on your own design's PnR process.

* **Reuse**: Some macros are commonly reused between multiple designs, so the
  effort of hardening them can be skipped after the first time.

* **Black-boxing**: You can use macros by other designers as a black-box; i.e;
  you don't know and don't care how they're implemented. Macros are typically
  **verified**, i.e., have been tested in other designs or extensively simulated
  to assure functionality.

  For proprietary (closed-source) designs, this has an added benefit of being
  able to license macros to other designers as intellectual property (IP)
  without disclosing the register-transfer-level code beyond a simulation model.

All non-trivial designs taped out using OpenLane have involved the use of macros
of some kind.

## Macro Files

There are a number of views you typically need from a Macro. The two essential
views for OpenLane are the {term}`LEF` (`.lef`) and the {term}`GDSII` (`.gds`)
views- the former of which is used in PnR and the latter is used for tape-out.

* Library Exchange Format ({term}`LEF`/.`lef`): Essential
  * The "interface" of a Macro; indicating things such as its
    dimensions, locations of the pins, other structures on the metal layers
    (that may obstruct routing,) et cetera.
  * Used during PnR.
* Graphic Design System II Format ({term}`GDSII`/`.gds`): Essential
  * Contains all of the information from the LEF and also other information that
    PnR tools are not concerned with, such as the patterns on the actual silicon
    layers (vs. just the metal layer.)
  * Used during stream-out.
* The Gate-Level Netlist (`.gl.v`/`.nl.v`): Optional
  * May be used during STA (see [relevant section](#sta)).
  * Used as a fallback during hierarchy checks and linting if neither Verilog
    headers (`.vh`) nor powered gate-level netlists (`.pnl.v`) exist. It is not
    recommended for this use as the linting may fail.
  * Used as a fallback during synthesis if Verilog headers (`.vh`) do not exist.
* The Powered Gate-Level Netlist (`.pnl.v`): Optional
  * Same as the netlist, but also has the power pins connected
  * Used as a fallback during hierarchy checks and linting if Verilog headers
    (`.vh`) do not exist.
  * Used as a fallback during synthesis if neither Verilog headers nor regular
    netlists (`.gl.v`/`.nl.v`) exist. It is not recommended for this use as
    synthesis checks may fail.
* Lib file (`.lib`): Optional
  * May be used during STA (see [relevant section](#sta)).
  * Used as a last resort for synthesis if no Verilog header (`.vh`) or any
    netlists (`.nl.v`/`.gl.v`/`.pnl.v`) are available. It is not recommended for
    this use as synthesis checks may fail.
* {term}`SPEF` (`.spef`): Optional
  * may be used during STA (see [relevant section](#sta)).
* {term}`SPICE` (`.spice`): Optional
  * currently unused, but may be used in the future for hierarchical
    {term}`LVS`. As OpenLane produces a SPICE view of all macros, it is prudent
    to include it anyway.
* {term}`SDF` (`.sdf`): Optional
  * currently unused. As OpenLane produces SDF views of all macros, it is
    prudent to include it anyway.
* {term}`Yosys` JSON Header (`.json`): Optional
  * A JSON netlist produced by Yosys that provides the Powered Netlist
    in a Python-parseable format.

## In OpenLane Configurations

Macros are declared in OpenLane configurations in the global variable
{var}`::MACROS`. Macros is a dictionary, which in JSON is going to be just a
dictionary/hashmap, where the keys are the name of the macro (not instances
thereof) and the values are a Python dataclass. You can find the API reference
for the macros hashmap at {class}`openlane.common.Macro`, but a less mechanical
explanation is as follows:

* The keys contain the name of the Macro itself (not instances thereof.)
* The values are:
  * A dictionary of instance names to instance objects
    * The instance objects in turn consist off:
      * `location`: A tuple of two numbers, in microns, indicating the location
        of the macro (optional)
      * `orientation`: The orientation of the placed macro-- see page 250 of the
        {term}`LEFDEFREF` for a definition and visual.
  * `gds`: List of GDS files comprising the macro (usually only one)
  * `lef`: List of LEF files comprising the macro (usually only one)
  * `vh`: List of Verilog Headers (if any)
  * `nl`: List of Gate-level Netlists (if any)
  * `pnl`: List of Powered Gate-level Netlists (if any)
  * `spice`: List of SPICE files (if any)
  * `lib`: Dictionary of LIB files.
    * The keys of the dictionary are wildcards matching {term}`timing corners <timing corner>`,
      and the values are paths for lib files that are characterized for those
      timing corners. For example:
    * `"*_tt_025C_1v80": ["dir::lib0.lib"]` will match `lib0.lib` to all of
      `min_tt_025C_1v80`, `max_tt_025C_1v80`, `and nom_tt_025C_1v80`.
  * `spef`: Dictionary of SPEF files, similar to above.
  * `sdf`: Dictionary of SDF files, similar to above.
  * `json_h`: (Optional) path to a singular Yosys-style JSON header for the
    design

```{warning}
The Macro class will throw an error if either `gds` or `lef` do not have at
least one file.
```

Let's presume the following Macro as an example, a Macro named `aes_example`
with one instance named `mprj` placed in the north orientation at $(10µm, 20µm)$
would be declared as:

```json
  "aes_example": {
      "instances": {
          "mprj": {
              "location": [
                  10,
                  20
              ],
              "orientation": "N"
          }
      },
      "gds": [
          "dir::./gds/aes_example.gds.gz"
      ],
      "lef": [
          "dir::./lef/aes_example.lef"
      ],
      "nl": [
          "dir::./gl/aes_example.v"
      ],
      "spef": {
          "min_*": [
              "dir::./spef/multicorner/aes_example.min.spef.gz"
          ],
          "nom_*": [
              "dir::./spef/multicorner/aes_example.nom.spef.gz"
          ],
          "max_*": [
              "dir::./spef/multicorner/aes_example.max.spef.gz"
          ]
      },
      "lib": {},
      "spice": [],
      "sdf": {}
  }
```

````{admonition} On Instance Names
:class: tip

Instance names should match the name as instantiated in Verilog, i.e.,
without escaping any characters for layout formats.

For example, if you instantiate an array of macros `spm spm_inst[1:0]`,
the first instance's name should be `spm_inst[0]`, not
`spm_inst\[0\]` or similar.

---

If your macros are within other instances, the names get a bit more complicated
depending on whether you flatten the hierarchy during synthesis or not. It is
the designer's responsibility to use either notation in the Instances dictionary
depending on which hierarchy mode is utilized. 

**Hierarchy flattened during Synthesis (default, recommended)**

When flattening the hierarchy, Yosys will rename instances to use the dot
notation, i.e, the name of an instance inside another instance will be
`instance_a.instance_b`. Note that they are the names of instances and not
the names of modules.

**Hierarchy maintainted during Synthesis**

Using macros with hierarchy is maintained during Synthesis is
**currently unsupported** in OpenLane, as the netlist is flattened upon moving
into OpenROAD as its hierarchical support is rather experimental. This will be
remedied in a future update to OpenLane.

In the meantime, if you do choose to keep the hierarchy after Synthesis
regardless by using the attribute `(* keep_hierarchy = "TRUE" *)`, you're on
your own, but here are a couple tips.
* Naming will fall upon OpenROAD, which use the `/` notation regardless of your
  technology LEF's divider characters property, i.e., `instance_a/instance_b`.
  Note once again that are the names of instances and not the names of modules.
* The power and ground pins of the macro must match that of the standard cell
  library as `Odb.SetPowerConnections` won't work for these macros.
  
**Please do not do this:**

Please do not use any of these characters in instance names by using the `\`
prefix, as the example below shows:

* `/` 
* `[]`
* `:`

Using these is considered **undefined behavior** and the flow will likely crash
because of assumptions about the hierarchy involving these characters.

```verilog
CellType \instance/name (…);
```
````

## STA

Without certain files, STA is performed in what is known as **black-boxed**
mode. This has a tendency to hide what are known as "errors at the boundary,"
where if STA passes cleanly for the macro individually and for the top-level
integration as a black-box; STA still fails for the overall design.

For example, let's presume a situation where a top-level chip A integrates a
macro B. Macro B passes STA successfully, and Top-level A also passes STA
successfully, albeit with B as a black-box. However:

* Macro B undergoes static timing analysis with an input delay of 5ns for all
  pins
* The actual delay of one of B's inputs connected to a register in A has an
  actual input delay of 2ns.

This means that the value of the signal connected to B may in fact be changing
too quickly, causing a hold violation at the boundary that would not be
detectable without hierarchical STA.

---

Depending on the value of {var}`OpenROAD.STAPrePNR::STA_MACRO_PRIORITIZE_NL`;
these files could be used:

* If `true`, a combination of `.nl.v` and `.spef` are prioritized (if
  available,) and if either is not available, `.lib` is used as a fallback, and
  if that doesn't exist, STA is done in black-boxed mode.
* If `false`, `.lib` is used, and if it is not available, `.nl.v` and `.spef`
  are used as a fallback. If neither are available, STA is done in black-boxed
  mode.

In commercial flows, typically, just `.lib` is used. However, the lack of an
accurate open source `.lib` file generator and OpenSTA producing very accurate
timing results with `.nl.v` and `.spef` leads us to prioritize these by default.

However, if you're using a macro that has been hardened using proprietary tools,
you may want to use the `.lib` file.

Regrettably, `STA_MACRO_PRIORITIZE_NL` is universal, i.e., either you prioritize
`.lib` for all macros or prioritize `.nl.v`+`.spef` for all macros. You may
however say, exclude `.spef` files from the Macro definition so it will
automatically fall-back to `.lib` for a certain design, and so on.

## Working with Macros in your RTL

In your RTL, Macros must be instantiated as follows, where the `USE_POWER_PINS`
preprocessor guard corresponds to the value of the variable
{var}`Yosys.Synthesis::VERILOG_POWER_DEFINE`.

```verilog
macro_name instance_name(
`ifdef USE_POWER_PINS
  .power_pin1(power_net1),
  .ground_pin1(ground_net1),
`endif
  .signal_pin1(signal_net1),
  .signal_pin2(signal_net2)
);
```

This is used by flows to create a hierarchy of power nets for the design,
which is later used in constructing the power distribution network and
connecting macros together.

### Deciding on Macros *ex post facto*

Sometimes, as a PnR engineer, you're tasked with hardening a giant RTL design
where some components that should logically be macros aren't.

In that scenario, you typically exclude the RTL from synthesis and replace it
with a Macro's netlist that was hardened separately. The problem with that is,
some of these submodules that you want to turn to Macro may have Verilog
parameters.

For example, let's assume you want to harden this submodule as a Macro:

```verilog
// BarrelShifter.v
module BarrelShifter #(
  parameter width = 32
) (
  VPWR,
  VGND,
  A,
  O,
  shmt
);
// ...
endmodule

// Top.v
module Top(
  // ...
);
BarrelShifter #(
  width=16
) barrelShifter(
`ifdef USE_POWER_PINS
  .VPWR(VPWR),
  .VGND(VGND),
`endif
  .A(in),
  .O(out)
  .shamt(shamt)
);
endmodule
```

As part of the hardening process, the width parameter becomes "baked-in"; i.e.
it can no longer be changed, and `BarrelShifter.nl.v` will have a baked in width
value of `16`. However, this means that during linting and synthesis, as they
use the Verilog Header or Netlist, the Synthesizer will say that "width" is an
unknown parameter.

You may think the solution is simply to then remove the parameter. The problem
is during RTL simulation where you use `BarrelShifter.v`, that will end up using
THAT default value, which is `32`, breaking the simulation process.

One solution is as follows:

```verilog
module Top(
  // ...
);

`ifdef __pnr__
BarrelShifter barrelShifter (
`ifdef USE_POWER_PINS
  .VPWR(VPWR),
  .VGND(VGND),
`endif
  .A(in),
  .O(out)
  .shamt(shamt)
);
`else
BarrelShifter #(
  width=16
) barrelShifter(
  .A(in),
  .O(out)
  .shamt(shamt)
);
`endif
endmodule
```

This will preserve the original RTL for simulation, but when Synthesizing/
Linting using OpenLane, `__pnr__` will be defined, thereby using the header for
the hardened version of the Macro. Additionally, as the power pins have no relevance
for the RTL, they can simply be left out when PnR is not defined.