CONTRIB.md

Core Contributor Guidelines

Code formatting

For code contributions, all C/C++ source code in the repository, including the test code, must adhere to the source-code formatting and style guidelines described here.

For doc contributions, we use restructured text, not markdown.

Adding ops to nGraph Core

Our design philosophy is that the graph is not a script for running kernels; rather, the graph is a snapshot of the computation's building blocks which we call ops. Compilation should match ops to appropriate kernels for the backend(s) in use. Thus, we expect that adding of new Core ops should be infrequent and that most functionality instead gets added with new functions that build sub-graphs from existing core ops.

The coding style described here should apply to both Core ops, and to any functions that build out (upon) sub-graphs from the core.

Coding style

We have a coding standard to help us to get development done. If part of the standard is impeding progress, we either adjust that part or remove it. To this end, we employ coding standards that facilitate understanding of what nGraph components are doing. Programs are easiest to understand when they can be understood locally; if most local changes have local impact, you do not need to dig through multiple files to understand what something does.

Names

Names should briefly describe the thing being named and follow these casing standards:

• Define C++ class or type names with CamelCase.
• Assign template parameters with UPPER_SNAKE_CASE.
• Case variable and function names with snake_case.

Method names for basic accessors are prefixed by get_ or set_ and should have simple $\mathcal{O}(1)$ implementations:

• A get_ method should be externally idempotent. It may perform some simple initialization and cache the result for later use.
• An is_ may be used instead of get_ for boolean accessors. Trivial get_ methods can be defined in a header file.
• A set_ method should change the value returned by the corresponding get_ method.
  • Use set_is_ if using is_ to get a value.
  • Trivial set_ methods may be defined in a header file.
• Names of variables should indicate the use of the variable.
  • Member variables should be prefixed with m_.
  • Static member variables should be rare and be prefixed with s_.
• Do not use using to define a type alias at top-level in header file. If the abstraction is useful, give it a class.
  • C++ does not enforce the abstraction. For example if X and Y are aliases for the same type, you can pass an X to something expecting a Y.
  • If one of the aliases were later changed, or turned into a real type, many callers could require changes.

Namespaces

• ngraph is for the public API, although this is not currently enforced.

  • Use a nested namespace for implementation classes.

  • Use an unnamed namespace or static for file-local names. This helps prevent unintended name collisions during linking and when using shared and dynamically-loaded libraries.

  • Never use using at top-level in a header file.

    • Doing so leaks the alias into users of the header, including headers that follow.

    • It is okay to use using with local scope, such as inside a class definiton.

  • Be careful of C++'s implicit namespace inclusions. For example, if a parameter's type is from another namespace, that namespace can be visible in the body.

  • Only use using std and/or using ngraph in .cpp files. using a nested namespace has can result in unexpected behavior.

File Names

• Do not use the same file name in multiple directories. At least one IDE/debugger ignores the directory name when setting breakpoints.
• Use .hpp for headers and .cpp for implementation.
• Reflect the namespace nesting in the directory hierarchy.
• Unit test files are in the tests directory.

  • Tranformer-dependent tests are tests running on the default transformer or specifying a transformer. For these, use the form

    TEST(file_name, test_name)
    

  • Transformer-independent tests:

    • File name is file_name.in.cpp

    • Add #include "test_control.hpp" to the file's includes

    • Add the line static std::string s_manifest = "${MANIFEST}"; to the top of the file.

    • Use

      NGRAPH_TEST(${BACKEND_NAME}, test_name)
      

      for each test. Files are generated for each transformer and the ${BACKEND_NAME} is replaced with the transformer name.

      Individual unit tests may be disabled by adding the name of the test to the unit_test.manifest file found in the transformer's source file directory.

Formatting

Things that look different should look different because they are different. We use clang format to enforce certain formatting. Although not always ideal, it is automatically enforced and reduces merge conflicts.

• The .clang-format file located in the root of the project specifies our format.

  • The script maint/apply-code-format.sh enforces that formatting at the C/C++ syntactic level.
  • The script at maint/check-code-format.sh verifies that the formatting rules are met by all C/C++ code (again, at the syntax level). The script has an exit code of 0 when code meets the standard and non-zero otherwise. This script does not modify the source code.

• Formatting with #include files:

  • Put headers in groups separated by a blank line. Logically order the groups downward from system-level to 3rd-party to ngraph.

  • Formatting will keep the files in each group in alphabetic order.

  • Use this syntax for files that do not change during development; they will not be checked for changes during builds. Normally this will be everything but the ngraph files:

    #include <file>

  • Use this syntax for files that are changing during development; they will be checked for changes during builds. Normally this will be ngraph headers:

    #include "file"

  • Use this syntax for system C headers with C++ wrappers:

    #include <c...>

• To guard against multiple inclusion, avoid using the #define X_H style. Use this syntax instead:

  #pragma once

• The initialization

  Foo x{4, 5};

  is preferred over

  Foo x(4, 5);

• Indentation should be accompanied by braces; this includes single-line bodies for conditionals and loops.

• Exception checking:

  • Throw an exception to report a problem.
  • Nothing that calls abort, exit or terminate should be used. Remember that ngraph is a guest of the framework.
  • Do not use exclamation points in messages!
  • Be as specific as practical. Keep in mind that the person who sees the error is likely to be on the other side of the framework and the message might be the only information they see about the problem.

• If you use auto, know what you are doing. auto uses the same type-stripping rules as template parameters. If something returns a reference, auto will strip the reference unless you use auto&:

  • Don't do things like

    auto s = Shape{2,3};
    

    Instead, use

    Shape s{2, 3};
    

  • Indicate the type in the variable name.

• One variable declaration/definition per line

  • Don't use the C-style

    int x, y, *z;
    

    Instead, use:

    int x;
    int y;
    int* z;