Code Guide and Tips

This is a document used to record tips in TVM codebase for reviewers and contributors. Most of them are summarized through lessons during the contributing and process.

C++ Code Styles

  • Use the Google C/C++ style.

  • The public facing functions are documented in doxygen format.

  • Favor concrete type declaration over auto as long as it is short.

  • Favor passing by const reference (e.g. const Expr&) over passing by value. Except when the function consumes the value by copy constructor or move, pass by value is better than pass by const reference in such cases.

  • Favor const member function when possible.

We use clang-format to enforce the code style. Because different version of clang-format might change by its version, it is recommended to use the same version of the clang-format as the main one. You can also use the following command via docker.

# Run a specific file through clang-format
docker/bash.sh ci_lint clang-format-10 [path-to-file]

# Run all linters, including clang-format
python tests/scripts/ci.py lint

clang-format is also not perfect, when necessary, you can use disble clang-format on certain code regions.

// clang-format off
void Test() {
   // clang-format will be disabled in this region.
}
// clang-format on

Because clang-format may not recognize macros, it is recommended to use macro like normal function styles.

#define MACRO_IMPL { custom impl; }
#define MACRO_FUNC(x)

// not preferred, because clang-format might recognize it as types.
virtual void Func1() MACRO_IMPL

// preferred
virtual void Func2() MACRO_IMPL;

void Func3() {
  // preferred
  MACRO_FUNC(xyz);
}

Python Code Styles

  • The functions and classes are documented in numpydoc format.

  • Check your code style using python tests/scripts/ci.py lint

  • Stick to language features in python 3.7

  • For functions with early returns, prefer if/elif/else` chains for functions with parallel and short bodies to the conditions, such as functions that apply a simple mapping to the arguments.  For more procedural functions, especially where the final ``else block would be much longer than the if and elif blocks, prefer having the final else case unindented.

    The pylint check no-else-return is disabled to allow for this distinction. See further discussion here <https://github.com/apache/tvm/pull/11327>.

    # All cases have bodies with similar flow control.  While this could
# be expressed as a sequence of if conditions, a reader would need to
# inspect the body of each condition to know that only one conditional
# body may be reached.
def sign(x):
    if x > 0:
        return "+"
    elif x < 0:
        return "-"
    else:
        return ""

# The initial special case is an early return for a special case,
# followed by a more general method.  Using an else block for the
# condition would add unnecessary indentation for the remainder of the
# function.
def num_unique_subsets(values):
    if len(values)==0:
        return 1

    # Longer, more general solution here
    ...

Writing Python Tests

We use pytest for all python testing. tests/python contains all the tests.

If you want your test to run over a variety of targets, use the tvm.testing.parametrize_targets() decorator. For example:

@tvm.testing.parametrize_targets
def test_mytest(target, dev):
  ...

will run test_mytest with target="llvm", target="cuda", and few others. This also ensures that your test is run on the correct hardware by the CI. If you only want to test against a couple targets use @tvm.testing.parametrize_targets("target_1", "target_2"). If you want to test on a single target, use the associated decorator from tvm.testing(). For example, CUDA tests use the @tvm.testing.requires_cuda decorator.

Network Resources

In CI, downloading files from the Internet is a big source of flaky test failures (e.g. remote server can go down or be slow), so try to avoid using the network at all during tests. In some cases this isn’t a reasonable proposition (e.g. the docs tutorials which need to download models).

In these cases you can re-host files in S3 for fast access in CI. A committer can upload a file, specified by a name, hash, and path in S3, using the workflow_dispatch event on the upload_ci_resource.yml GitHub Actions workflow. The sha256 must match the file or it will not be uploaded. The upload path is user-defined so it can be any path (no trailing or leading slashes allowed) but be careful not to collide with existing resources on accident. Once uploaded you should send a PR to update the URL_MAP in request_hook.py with the new URL.

Handle Integer Constant Expression

We often need to handle constant integer expressions in TVM. Before we do so, the first question we want to ask is that is it really necessary to get a constant integer. If symbolic expression also works and let the logic flow, we should use symbolic expression as much as possible. So the generated code works for shapes that are not known ahead of time.

Note that in some cases we cannot know certain information, e.g. sign of symbolic variable, it is ok to make assumptions in certain cases. While adding precise support if the variable is constant.

If we do have to get constant integer expression, we should get the constant value using type int64_t instead of int, to avoid potential integer overflow. We can always reconstruct an integer with the corresponding expression type via make_const. The following code gives an example.

Expr CalculateExpr(Expr value) {
  int64_t int_value = GetConstInt<int64_t>(value);
  int_value = CalculateExprInInt64(int_value);
  return make_const(value.type(), int_value);
}