Bazel on OpenROAD-flow-scripts(ORFS) with MegaBoom use case

This is a stand-alone project that use a wafer thin Bazel layer on top of OpenROAD-flow-scripts.

The purpose of this project is to demonstrate and develop a practical workflow for large designs in a git repository that is not a fork of OpenROAD-flow-scripts and that is using its own file layout layout: similar to a private repository for a large project.

MegaBoom(a RISC-V SoC from Chipyard) has been chosen as a test-case as it is large, familiar and interesting in itself.

There are many build flows on top of OpenROAD

There are numerous build flows on top of OpenROAD, these are some:

• ORFS. The developers of OpenROAD use this flow to test the tool. It has features specifically for reporting bugs and is simple to understand for OpenROAD developers as well as novice users. It provides a lingua franca in the community to discuss features and test cases.
• Hammer is used with Chipyard.
• https://www.zeroasic.com/ has a Python based workflow that supports both commercial tools and OpenROAD.

Why Bazel on top of ORFS?

ORFS and OpenROAD is work in progress and one should expect for large designs to get involved with the community or need a support contract with Precision Innovations (https://www.linkedin.com/in/tomspyrou/).

Using ORFS directly, instead of modifying it or creating an alternative flow, makes it easy to get the very latest features and version of OpenROAD and ORFS as well as having access to the tools, make issue and deltaDebug.py, required to articulate familiar and easily actionable github issues for the OpenROAD and ORFS maintainers.

Challenges with large designs and ORFS that Bazel helps address

• Long build times; hours, days.
• Artifacts are needed. Synthesis, for instance, can be very time consuming and it is useful to share synthesis artifacts between developers and CI servers. On a large design with multiple developers and many pull requests in flight, it can become error prone to manually track exactly what version of built stages that are still valid. Ideally one should be able to check out a pull request and automatically get the right prebuilt artifacts.
• Dependencies in ORFS are at the file level. For instance, synthesis must be redone if the clock period changes, but many other changes to .sdc do not require resynthesis. With finer grained dependencies, superfluous time consuming resynthesis, floor planning, placement, cts and routing can be avoided.
• Examining failures for global/detailed place/route, that can take many hours to build, is useful. Artifacts for failed stages are needed to examine the problem: failed .odb file as well as any reports. This workflow always existed for detailed routing: detailed routing succeeds, has exit code 0, even if there are DRC errors.
• Mocking abstracts when doing initial top-level floorplanning is needed to separate concerns. It can be useful to skip one of place, cts, route for the macros until one starts to converge on a workable top level floorplan. This is supported via mock_abstract in openroad.bzl
• Efficient local storage of build artifacts are needed as .odb files are large and they should not exist in duplicates unnecessarily. Bazel uses symbolic links. ORFS can not use symbolic links for .odb files because, long story short, make does not work properly with symbolic links. This becomes especially important when working with many branches and pull requests where there is a large degree of shared .odb files.
• Parallel builds are required for macros.
• Remote build services are required for large projects where developers machines are powerful enough to examine results, but not to run builds.
• Cross cutting builds such as completing floor planning for all macros, then place, then cts, then route is required to be able to separate concerns. When iterating on the concerns, it can be useful to complete placement under human supervision to iterate quickly, but to leave routing for CI servers to complete.
• Select level of detail of artifacts is useful throughout the development process. Initially for a macro, artifacts are useful for inspection for synthesis, floorplan, place, cts, route and abstract. Later, for stable macros, abstracts are adequate(no .odb file, only .lef, .lib and .gds).

Setup

For running bazel flow for the ORFS, megaboom uses bazel-orfs. It provides two different bazel flows for running Physical Design flow with OpenROAD-flow-scripts. For the installation and usage guide, please refer to bazel-orfs's README.

bazel-orfs must be specified in megaboom as the external dependency. It can be pinned to a specific revision of the upstream repository or the dependency can point to a local bazel-orfs workspace available on disk.

Upstream repository

In MODULE.bazel use git_override to pin bazel-orfs version at specific commit:

git_override(
    module_name = "bazel-orfs",
    remote = "https://github.com/The-OpenROAD-Project/bazel-orfs.git",
    commit = "<commit hash>",
)

Local workspace

For testing changes made to bazel-orfs it is useful to set the dependency to a local bazel-orfs workspace. In order to do that, in MODULE.bazel use local_path_override:

local_path_override(
    module_name = "bazel-orfs", path = "<path to local bazel-orfs workspace>"
)

Example of using a Bazel artifact server

If you only have a single Google account that you use for Google Cloud locally, you can use --google_default_credentials.

If you are use multiple google accounts, using the default credentials can be cumbersome when switching between projects. To avoid this, you can use the --credential_helper option instead, and pass a script that fetches credentials for the account you want to use. This account needs to have logged in using gcloud auth login and have access to the bucket specified.

To use this feature, copy the snippet below into .bazelrc and specify your username by modifying # user: myname@openroad.tools:

# user: myname@openroad.tools
build --credential_helper=%workspace%/cred_helper.py --remote_cache=https://storage.googleapis.com/megaboom-bazel-artifacts --remote_cache_compression=true

cred_helper.py will parse .bazelrc and look for the username in the comment.

NOTE: To test the credential helper, make sure to restart Bazel to avoid using a previous cached authorization:

bazel shutdown
bazel build ALUExeUnit_floorplan

To gain access to the https://storage.googleapis.com/megaboom-bazel-artifacts bucket, reach out to Tom Spyrou, Precision Innovations (https://www.linkedin.com/in/tomspyrou/).

Tutorial

This tutorial uses the docker flow to run the physical design flow with ORFS. Before starting, it is required to have available in your docker runtime a docker image with OpenROAD-flow-scripts installation. For more information, please refer to the Requirements chapter of bazel-orfs.

---

Note: orfs_env rule for downloading and loading the docker image is defined in bazel-orfs. In order to run it in the context of megaboom (the workspace that uses bazel-orfs as external dependency), it is required to use correct bazel label to this external target:

bazel run @bazel-orfs//:orfs_env

---

Hello world

A quick test-build:

# Build L1MetadataArray macro up to the placement stage and generate scripts
bazel build L1MetadataArray_test_place_gui

# View results with OpenROAD GUI
./bazel-bin/L1MetadataArray_test_place_docker gui_place

Staring at logs

In ORFS it is oftentimes useful to view the tail of the single running stage on a machine, ./out is in this project to "stare at logs":

tail -f $(./out -t)

Currently, this script depends on bazel cache residing under ~/.cache/bazel/. This directory is scanned for the newest log file which name of gets printed to the standard output.

MegaBoom RTL code

Based on: https://chipyard.readthedocs.io/en/stable/VLSI/Sky130-OpenROAD-Tutorial.html#initial-setup

**Note:** Chipyard main does not work smoothly with MegaBoom as of writing as Chipyard is mixing SFC and MFC.

Follow ucb-bar/chipyard#1623 for latest updates.

That said, the rtl/ folder was generated using latest Chipyard + some hacked files locally:

make tutorial=sky130-openroad CONFIG=MegaBoomMacroConfig verilog

Hammer hacking with ASAP7

Build everything:

./build-setup.sh --skip-ctags --skip-conda --skip-toolchain --skip-firesim --skip-marshal --skip-clean

Create Verilog code:

make CONFIG=MegaBoomConfig tech_name=asap7 VLSI_TOP=ChipTop INPUT_CONFS=example-asap7.yml TOP_MACROCOMPILER_MODE='--mode synflops' verilog