Developer Guide

Tool Philosophy

OpenROAD is a tool to build a chip from a synthesized netlist to a physical design for manufacturing.

The unifying principle behind the design of OpenROAD is for all of the tools to reside in one tool, with one process, and one database. All tools in the flow should use Tcl commands exclusively to control them instead of external “configuration files”. File based communication between tools and forking processes is strongly discouraged. This architecture streamlines the construction of a flexible tool flow and minimizes the overhead of invoking each tool in the flow.

Tool File Organization

Every tool follows the following file structure, grouping sources, tests and headers together.

src/CMakelists.txt - add_subdirectory's src/CMakelists.txt
src/tool/src/ - sources and private headers
src/tool/include/tool/ - exported headers

OpenROAD repository

CMakeLists.txt - top level cmake file
src/ - OpenROAD class functions
src/OpenROAD.i - top level swig, %includes tool swig files
src/OpenROAD.tcl - basic read/write lef/def/db commands
include/openroad/OpenRoad.hh - OpenROAD top level class, has instances of tools
include/openroad/Error.hh - Error reporting API

Some tools such as OpenDB are submodules, which are simply subdirectories in /src that are pointers to the git submodule. They are intentionally not segregated into a separate /module.

The use of submodules for new code integrated into OpenROAD is strongly discouraged. Submodules make changes to the underlying infrastructure (OpenDB, OpenSTA etc) difficult to propagate across the dependent submodule repositories. Submodules: just say no.

Where external/third party code that a tool depends on should be placed depends on the nature of the dependency.

  • Libraries - code packaged as a linkable library. Examples are tcl, boost, zlib, eigen, lemon, spdlog.

These should be installed in the build environment and linked by OpenRoad. Document these dependencies in the top level file. The Dockerfile should be updated to illustrate where to find the library and how to install it. Adding libraries to the build enviroment requires coodination with the sys admins for the continuous integration hosts to make sure the environments include the dependency. Advanced notification should also be given to the development team so their private build environments can be updated.

Each tool cmake file builds a library that is linked by the OpenROAD application. The tools should not define a main() function. If the tool is tcl only and has no c++ code it does not need to have a cmake file. Tool cmake files should not include the following:

  • cmake_minimum_required

None of the tools have commands to read or write LEF, DEF, Verilog or database files. These functions are all provided by the OpenROAD framework for consistency.

Tools should package all state in a single class. An instance of each tool class resides in the top level OpenROAD object. This allows multiple tools to exist at the same time. If any tool keeps state in global variables (even static) only one tool can exist at a time. Many of the tools being integrated were not built with this goal in mind and will only work on one design at a time. Eventually all of the tools should be upgraded to remove this deficiency as they are re-written to work in the OpenROAD framework.

Each tool should use a unique namespace for all of its code. The same namespace should be used for Tcl functions, including those defined by a swig interface file. Internal Tcl commands stay inside the namespace, and user visible Tcl commands should be defined in the global namespace. User commands should be simple Tcl commands such as ‘global_placement’ that do not create tool instances that must be based to the commands. Defining Tcl commands for a tool class is fine for internals, but not for user visible commands. Commands have an implicit argument of the current OpenROAD class object. Functions to get individual tools from the OpenROAD object can be defined.

Initialization (c++ tools only)

The OpenRoad class has pointers to each tools with functions to get each tool. Each tool has (at a minimum) a function to make an instance of the tool class, and an initialization function that is called after all of the tools have been made, and a funtion to delete the tool. This small header does not include the class definition for the tool so that the OpenRoad framework does not have to know anything about the tool internals or include a gigantic header file.

MakeTool.hh defines the following:

Tool *makeTool();
void initTool(OpenRoad *openroad);
void deleteTool(Tool *tool);

The OpenRoad::init() function calls all of the makeTool functions and then all of the initTool() functions. The init functions are called from the bottom of the tool dependences. Each init function grabs the state it needs out of the OpenRoad instance.


Tools should provide Tcl commands to control them. Tcl object based tool interfaces are not user friendly. Define Tcl procedures that take keyword arguments that reference the OpenRoad object to get tool state. OpenSTA has Tcl utilities to parse keyword arguements (sta::parse_keyword_args). See OpenSTA/tcl/*.tcl for examples. Use swig to define internal functions to C++ functionality.p

Tcl files can be included by encoding them in cmake into a string that is evaluated at run time (See Resizer::init()).


Tools should report errors to the user using the ord::error function defined in include/openroad/Error.hh. ord::error throws ord::Exception. The variables ord::exit_on_error and ord::file_continue_on_error control how the error is handled. If ord::exit_on_error is true OpenROAD reports the error and exits. If the error is encountered while reading a file with the source or read_sdc commands and ord::file_continue_on_error is false no other commands are read from the file. The default values of both variables is false.


Each “tool” has a /test directory containing a script nameed “regression” to run “unit” tests. With no arguments it should run default unit tests.

No database files should be in tests. Read LEF/DEF/Verilog to make a database.

The regression script should not depend on the current working directory. It should be able to be run from any directory. Use filenames relative to the script name rather the the current working directory.

Regression scripts should print a consise summary of test failures. The regression script should return an exit code of zero if there are no errors and 1 if there are errors. The script should not print thousands of lines of internal tool info.

Regression scripts should pass the -no_init option to openroad so that a user’s init file is not sourced before the tests runs.

Regression scripts should add output files or directories to .gitignore so that running does note leave the source repository “dirty”.

The Nangate45 open source library data used by many tests is in test/Nangate45. Use the following command to add a link in the tool command

cd tool/test
ln -s ../../../test/Nangate45

After the link is installed, the test script can read the liberty file with the command shown below.

read_liberty Nangate45/Nangate45_typ.lib


Checking out the OpenROAD repo with –recursive installs all of the OpenRoad tools and their submodules.

git clone --recusive
cd OpenROAD
mkdir build
cd build
cmake ..

All tools build using cmake and must have a CMakeLists.txt file in their tool directory.

This builds the openroad executable in /build.

Note that removing submodules from a repo when moving it into OpenROAD is less than obvious. Here are the steps:

git submodule deinit <path_to_submodule>
git rm <path_to_submodule>
git commit-m "Removed submodule "
rm -rf .git/modules/<path_to_submodule>

Tools should compile with no compile warnings in gcc or clang with -Wall.

Tool Work Flow

To work on one of the tools inside OpenROAD when it is a submodule requires updating the OpenROAD repo to integrate your changes. Submodules point to a specific version (hash) of the submodule repo and do not automatically track changes to the submodule repo.

Work on OpenROAD should be done in the openroad branch. Stable commits on the openroad branch are periodically pushed to the master branch for public consumption.

To make changes to a submodule, first check out a branch of the submodule (git clone –recursive does not check out a branch, just a specific commit).

cd src/<tool>
git checkout <branch>

<branch> is the branch used for development of the tool when it is inside OpenROAD. The convention is for to be named ‘openroad’.

After making changes inside the tool source tree, stage and commit them to the tool repo and push them to the remote repo.

git add ...
git commit -m "massive improvement"
git push

If instead you have done development in a different branch or source tree, merge those changes into the branch used for OpenROAD.

Once the changes are in the OpenROAD submodule source tree it will show them as a diff in the hash for the directory.

cd openroad
git stage <tool_submodule_dir>
git commit -m "merge tool massive improvement"
git push

Example of Adding a Tool to OpenRoad

The patch file “add_tool.patch” illustrates how to add a tool to OpenRoad. Use

patch -p < doc/add_tool.patch`
cd src/tool/test
ln -s ../../../test/regression.tcl regression.tcl

to add the sample tool. This adds a directory OpenRoad/src/tool that illustrates a tool named “Tool” that uses the file structure described and defines a command to run the tool with keyword and flag arguments as illustrated below:

% toolize foo
Helping 23/6
Gotta pos_arg1 foo
Gotta param1 0.000000
Gotta flag1 false

% toolize -flag1 -key1 2.0 bar
Helping 23/6
Gotta pos_arg1 bar
Gotta param1 2.000000
Gotta flag1 true

% help toolize
toolize [-key1 key1] [-flag1] pos_arg1


Tool commands should be documented in the top level OpenROAD file. Detailed documentation should be the tool/ file.

Tool Flow

  • Verilog to DB (dbSTA)
  • Init Floorplan (OpenROAD)
  • I/O placement (ioPlacer)
  • PDN generation (pdngen)
  • Tapcell and Welltie insertion (tapcell)
  • I/O placement (ioPlacer)
  • Macro placement (TritonMacroPlace)
  • Global placement (RePlAce)
  • Gate Resizing and buffering (Resizer)
  • Detailed placement (OpenDP)
  • Clock Tree Synthesis (TritonCTS)
  • Repair Hold Violations (Resizer)
  • Global route (FastRoute)
  • Detailed route (TritonRoute)
  • Final timing/power report (OpenSTA)

Tool Checklist

Tools should make every attempt to minimize external dependencies. Linking libraries other than those currently in use complicates the builds and sacrifices the portability of OpenROAD. OpenROAD should be portable to many different compiler/operating system versions and dependencies make this vastly more complicated.

OpenROAD submodules reference tool openroad branch head. No git develop, openroad_app, or openroad_build branches.

Submodules used by more than one tool belong in /src, not duplicated in each tool repo.

CMakeLists.txt does not use add_compile_options include_directories link_directories link_libraries Use target_ versions instead. See

CMakeLists.txt does not use glob. Use explicit lists of source files and headers instead.

CMakeLists.txt does not define CFLAGS CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE Let the top level and defaults control these.

No main.cpp or main procedure.

No compiler warnings for gcc or clang with optimization enabled.

Does not call flute::readLUT (called once by OpenRoad).

Tcl command(s) documented in top level in flow order.

Command line tool documentation in tool README.

Conforms to Tcl command naming standards (no camel case).

Does not read configuration files. Use command arguments or support commands.

.clang-format at tool root directory to aid foreign programmers.

No jenkins/, Jenkinsfile, Dockerfile in tool directory.

regression script named “test/regression” with no arguments that runs tests. Not tests/, not test/ etc.

regression script should run independent of current directory. For example, ../test/regression should work.

regression should only print test results or summary, not belch 1000s of lines of output.

Test scripts use OpenROAD tcl commands (not itcl, not internal accessors).

regression script should only write files in a directory that is in the tool’s .gitignore so the hierarchy does not have modified files in it as a result or running the regressions.

Regressions report no memory errors with valgrind (stretch goal).

Regressions report no memory leaks with valgrind (difficult).

James Cherry, Dec 2019