Testing the GPU C library


Running GPU tests with high parallelism is likely to cause spurious failures, out of resource errors, or indefinite hangs. limiting the number of threads used while testing using LIBC_GPU_TEST_JOBS=<N> is highly recommended.

Testing infrastructure

The LLVM C library supports different kinds of tests depending on the build configuration. The GPU target is considered a full build and therefore provides all of its own utilities to build and run the generated tests. Currently the GPU supports two kinds of tests.

  1. Hermetic tests - These are unit tests built with a test suite similar to Google’s gtest infrastructure. These use the same infrastructure as unit tests except that the entire environment is self-hosted. This allows us to run them on the GPU using our custom utilities. These are used to test the majority of functional implementations.

  2. Integration tests - These are lightweight tests that simply call a main function and checks if it returns non-zero. These are primarily used to test interfaces that are sensitive to threading.

The GPU uses the same testing infrastructure as the other supported libc targets. We do this by treating the GPU as a standard hosted environment capable of launching a main function. Effectively, this means building our own startup libraries and loader.

Testing utilities

We provide two utilities to execute arbitrary programs on the GPU. That is the loader and the start object.

Startup object

This object mimics the standard object used by existing C library implementations. Its job is to perform the necessary setup prior to calling the main function. In the GPU case, this means exporting GPU kernels that will perform the necessary operations. Here we use _begin and _end to handle calling global constructors and destructors while _start begins the standard execution. The following code block shows the implementation for AMDGPU architectures.

extern "C" [[gnu::visibility("protected"), clang::amdgpu_kernel]] void
_begin(int argc, char **argv, char **env) {
  LIBC_NAMESPACE::call_init_array_callbacks(argc, argv, env);

extern "C" [[gnu::visibility("protected"), clang::amdgpu_kernel]] void
_start(int argc, char **argv, char **envp, int *ret) {
  __atomic_fetch_or(ret, main(argc, argv, envp), __ATOMIC_RELAXED);

extern "C" [[gnu::visibility("protected"), clang::amdgpu_kernel]] void
_end(int retval) {

Loader runtime

The startup object provides a GPU executable with callable kernels for the respective runtime. We can then define a minimal runtime that will launch these kernels on the given device. Currently we provide the amdhsa-loader and nvptx-loader targeting the AMD HSA runtime and CUDA driver runtime respectively. By default these will launch with a single thread on the GPU.

$> clang++ crt1.o test.cpp --target=amdgcn-amd-amdhsa -mcpu=native -flto
$> amdhsa_loader --threads 1 --blocks 1 ./a.out
Test Passed!

The loader utility will forward any arguments passed after the executable image to the program on the GPU as well as any set environment variables. The number of threads and blocks to be set can be controlled with --threads and --blocks. These also accept additional x, y, z variants for multidimensional grids.

Running tests

Tests will only be built and run if a GPU target architecture is set and the corresponding loader utility was built. These can be overridden with the LIBC_GPU_TEST_ARCHITECTURE and LIBC_GPU_LOADER_EXECUTABLE CMake options. Once built, they can be run like any other tests. The CMake target depends on how the library was built.

  1. Cross build - If the C library was built using LLVM_ENABLE_PROJECTS or a runtimes cross build, then the standard targets will be present in the base CMake build directory.

    1. All tests - You can run all supported tests with the command:

      $> ninja check-libc
    2. Hermetic tests - You can run hermetic with tests the command:

      $> ninja libc-hermetic-tests
    3. Integration tests - You can run integration tests by the command:

      $> ninja libc-integration-tests
  2. Runtimes build - If the library was built using LLVM_ENABLE_RUNTIMES then the actual libc build will be in a separate directory.

    1. All tests - You can run all supported tests with the command:

      $> ninja check-libc-amdgcn-amd-amdhsa
      $> ninja check-libc-nvptx64-nvidia-cuda
    2. Specific tests - You can use the same targets as above by entering the runtimes build directory.

      $> ninja -C runtimes/runtimes-amdgcn-amd-amdhsa-bins check-libc
      $> ninja -C runtimes/runtimes-nvptx64-nvidia-cuda-bins check-libc
      $> cd runtimes/runtimes-amdgcn-amd-amdhsa-bins && ninja check-libc
      $> cd runtimes/runtimes-nvptx64-nvidia-cuda-bins && ninja check-libc

Tests can also be built and run manually using the respective loader utility.