Testing libc++

Getting Started

libc++ uses LIT to configure and run its tests.

The primary way to run the libc++ tests is by using make check-cxx.

However since libc++ can be used in any number of possible configurations it is important to customize the way LIT builds and runs the tests. This guide provides information on how to use LIT directly to test libc++.

Please see the Lit Command Guide for more information about LIT.

Usage

After building libc++, you can run parts of the libc++ test suite by simply running llvm-lit on a specified test or directory. If you’re unsure whether the required libraries have been built, you can use the cxx-test-depends target. For example:

$ cd <monorepo-root>
$ make -C <build> cxx-test-depends # If you want to make sure the targets get rebuilt
$ <build>/bin/llvm-lit -sv libcxx/test/std/re # Run all of the std::regex tests
$ <build>/bin/llvm-lit -sv libcxx/test/std/depr/depr.c.headers/stdlib_h.pass.cpp # Run a single test
$ <build>/bin/llvm-lit -sv libcxx/test/std/atomics libcxx/test/std/threads # Test std::thread and std::atomic

Note

If you used the Bootstrapping build instead of the default runtimes build, the cxx-test-depends target is instead named runtimes-test-depends, and you will need to prefix <build>/runtimes/runtimes-<target>-bins/ to the paths of all tests.

In the default configuration, the tests are built against headers that form a fake installation root of libc++. This installation root has to be updated when changes are made to the headers, so you should re-run the cxx-test-depends target before running the tests manually with lit when you make any sort of change, including to the headers.

Sometimes you’ll want to change the way LIT is running the tests. Custom options can be specified using the --param <name>=<val> flag. The most common option you’ll want to change is the standard dialect (ie -std=c++XX). By default the test suite will select the newest C++ dialect supported by the compiler and use that. However, you can manually specify the option like so if you want:

$ <build>/bin/llvm-lit -sv libcxx/test/std/containers # Run the tests with the newest -std
$ <build>/bin/llvm-lit -sv libcxx/test/std/containers --param std=c++03 # Run the tests in C++03

Other parameters are supported by the test suite. Those are defined in libcxx/utils/libcxx/test/params.py. If you want to customize how to run the libc++ test suite beyond what is available in params.py, you most likely want to use a custom site configuration instead.

The libc++ test suite works by loading a site configuration that defines various “base” parameters (via Lit substitutions). These base parameters represent things like the compiler to use for running the tests, which default compiler and linker flags to use, and how to run an executable. This system is meant to be easily extended for custom needs, in particular when porting the libc++ test suite to new platforms.

Using a Custom Site Configuration

By default, the libc++ test suite will use a site configuration that matches the current CMake configuration. It does so by generating a lit.site.cfg file in the build directory from one of the configuration file templates in libcxx/test/configs/, and pointing llvm-lit (which is a wrapper around llvm/utils/lit/lit.py) to that file. So when you’re running <build>/bin/llvm-lit, the generated lit.site.cfg file is always loaded instead of libcxx/test/lit.cfg.py. If you want to use a custom site configuration, simply point the CMake build to it using -DLIBCXX_TEST_CONFIG=<path-to-site-config>, and that site configuration will be used instead. That file can use CMake variables inside it to make configuration easier.

$ cmake <options> -DLIBCXX_TEST_CONFIG=<path-to-site-config>
$ make -C <build> cxx-test-depends
$ <build>/bin/llvm-lit -sv libcxx/test # will use your custom config file

Writing Tests

When writing tests for the libc++ test suite, you should follow a few guidelines. This will ensure that your tests can run on a wide variety of hardware and under a wide variety of configurations. We have several unusual configurations such as building the tests on one host but running them on a different host, which add a few requirements to the test suite. Here’s some stuff you should know:

• All tests are run in a temporary directory that is unique to that test and cleaned up after the test is done.

• When a test needs data files as inputs, these data files can be saved in the repository (when reasonable) and referenced by the test as // FILE_DEPENDENCIES: <path-to-dependencies>. Copies of these files or directories will be made available to the test in the temporary directory where it is run.

• You should never hardcode a path from the build-host in a test, because that path will not necessarily be available on the host where the tests are run.

• You should try to reduce the runtime dependencies of each test to the minimum. For example, requiring Python to run a test is bad, since Python is not necessarily available on all devices we may want to run the tests on (even though supporting Python is probably trivial for the build-host).

Benchmarks

Libc++ contains benchmark tests separately from the test of the test suite. The benchmarks are written using the Google Benchmark library, a copy of which is stored in the libc++ repository.

For more information about using the Google Benchmark library see the official documentation.

Building Benchmarks

The benchmark tests are not built by default. The benchmarks can be built using the cxx-benchmarks target.

An example build would look like:

$ cd build
$ ninja cxx-benchmarks

This will build all of the benchmarks under <libcxx-src>/benchmarks to be built against the just-built libc++. The compiled tests are output into build/projects/libcxx/benchmarks.

The benchmarks can also be built against the platforms native standard library using the -DLIBCXX_BUILD_BENCHMARKS_NATIVE_STDLIB=ON CMake option. This is useful for comparing the performance of libc++ to other standard libraries. The compiled benchmarks are named <test>.libcxx.out if they test libc++ and <test>.native.out otherwise.

Also See:

• Building Libc++

• CMake Options

Running Benchmarks

The benchmarks must be run manually by the user. Currently there is no way to run them as part of the build.

For example:

$ cd build/projects/libcxx/benchmarks
$ ./algorithms.libcxx.out # Runs all the benchmarks
$ ./algorithms.libcxx.out --benchmark_filter=BM_Sort.* # Only runs the sort benchmarks

For more information about running benchmarks see Google Benchmark.