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Tutorial: Run Taichi programs in C++ application

Taichi makes it easy to write high-performance programs with efficient parallelism, but in many applications we cannot simply deploy the Python scripts. Taichi offers a runtime library (TiRT) with a C interface so that your Taichi kernels can be launched in any native application. In this tutorial, we'll walkthrough the steps to deploy a Taichi program in a C++ application.

Overview

AOT E2E

In Python, when you call a function decorated with @ti.kernel, Taichi immediately compiles the kernel and send it to the device for execution. This is called just-in-time (JIT) compilation. But generally speaking, we don't want to compile the kernels on a mobile phone, or to leak the source code to the users. So Taichi introduced ahead-of-time (AOT) compilation so that you can compile kernels on a development machine, and launch them on user devices via TiRT.

  1. Compile Taichi kernels from Python and save the artifacts.
  2. Load AOT modules with TiRT and launch them in your applications.

Although this tutorial only demonstrates integrating Taichi in a C++ application, thanks to the versatility of the C interface, TiRT can communicate with many programming languages including C/C++, Swift, Rust, C# (via P/Invoke) and Java (via JNI).

1. Write kernels for AOT compilation

A Taichi kernel describes two aspects of a computer program: The computation itself, and the data it operates on. Because we don't know what kind of data will be fed into the kernel before execution, we have to clearly annotate the argument types for the AOT compiler.

Taichi supports the following argument types:

  • ti.i32
  • ti.f32
  • ti.Ndarray

Despite integers and floating-point numbers, we have a commonly-used data container called Ndarray. It's similar to an ndarray in NumPy, or a Tensor in PyTorch. It has multiple dimensions to index and the data is laid out continuously in memory. If you have more experience with C++ than Python, You can treat it as a nested array type like float[6][14].

To give an example, the following init kernel accepts an ND-array argument called x. We want to inform the compiler that the ND-array stores floating-point data and it only has a single dimension to index, so dtype is ti.f32, and field_dim is set to 1. When executed, every element in x will be assigned with a constant 0.

@ti.kernel
def init(x: ti.types.ndarray(ti.f32, element_shape=tuple(), field_dim=1)):
    for i in x:
        x[i] = 0

After initialization, in kernel add_base, we want to add a floating-point number base to those in x in each frame.

@ti.kernel
def add_base(x: ti.types.ndarray(field_dim=1), base: ti.f32):
    for i in range(x.shape[0]):
        x[i] += base

You can also create an ND-array and launch the kernels in the same script to ensure they do everything you expect.

x = ti.ndarray(ti.f32, shape=(8192))
init(x)

N_ITER = 50
for _ in range(N_ITER):
    add_base(x, 0.1)

2. Compile and save the artifacts

Now let's compile the kernels into an AOT module.

A compiled taichi kernel consists of all compiled artifacts when compiling a ti.kernel with the types of its parameters. Take kernel add_base as an example, argument base's type is ti.f32. Its type information is used to compile the kernel and thus encoded in the compiled artifact, while it can be called with any floating point number at runtime.

mod = ti.aot.Module(ti.vulkan)
mod.add_kernel(init, template_args={'x': x})
mod.add_kernel(add_base, template_args={'x': x})
mod.save(target_dir)

ti.types.ndarray is a bit more complicated since it requires both dtype and ndim as its type information. To compile Taichi kernels with ti.types.ndarray arguments, you'll have to supply that information either directly in the type annotation, or provide an example input via template_args.

Now that we're done with Kernel compilation, let's take a look at the generated artifacts and its layout:

// FUTURE WORK: This is just a zip. Replace tcb with readable JSON
// Structure of compiled artifacts
.
├── demo
│   ├── add_base_c78_0_k0001_vk_0_t00.spv
│   ├── init_c76_0_k0000_vk_0_t00.spv
│   ├── metadata.json
│   └── metadata.tcb
└── demo.py

3. Get Taichi Runtime Library (TiRT)

TiRT

Alright, you are now done with Python and well prepared to build your application! The compiled artifacts we just saved and TiRT are all you need.

For now TiRT is shipped along with taichi-nightly Python wheels. Be aware that there's no strong version compatibility enforced yet, so it's highly recommended to use the python taichi and TiRT built from exactly the same commit.

TODO: We'll figure out a proper way to release it once the versioning issue is improved.

# Install python taichi:
pip install -i https://pypi.taichi.graphics/simple/ taichi-nightly
# Get the runtime library:
pip download --no-deps -i https://pypi.taichi.graphics/simple/ taichi-nightly
# For example
unzip taichi_nightly-1.3.0.post20221102-cp38-cp38-manylinux_2_27_x86_64.whl
export TAICHI_C_API_INSTALL_DIR=$PWD/taichi_nightly-1.3.0.post20221102.data/data/c_api/

Currently, only TiRT for Linux systems is included in the nightly distributions. If you need one for Android / Windows, please see the FAQ below to build it from source.

Integrate TiRT to your CMakeLists.txt:

# Find built taichi C-API library in `TAICHI_C_API_INSTALL_DIR`.
find_library(taichi_c_api taichi_c_api HINTS ${TAICHI_C_API_INSTALL_DIR}/lib NO_CMAKE_FIND_ROOT_PATH)
if (NOT EXISTS ${taichi_c_api})
    message(FATAL_ERROR "Couldn't find C-API library in ${TAICHI_C_API_INSTALL_DIR}")
endif()

# Make sure your target is properly linked!
set(TAICHI_TUTORIAL_DEMO_NAME "0_tutorial_kernel")
message("-- Building ${TAICHI_TUTORIAL_DEMO_NAME}")
add_executable(${TAICHI_TUTORIAL_DEMO_NAME} ${CMAKE_CURRENT_SOURCE_DIR}/app.cpp)
target_include_directories(${TAICHI_TUTORIAL_DEMO_NAME} PUBLIC ${TAICHI_C_API_INSTALL_DIR}/include)
target_link_libraries(${TAICHI_TUTORIAL_DEMO_NAME} ${taichi_c_api})

4. Run taichi kernels in your application

TiRT provides a fundamental C interface to help achieve optimal portability, however we also kindly provide a header-only C++ wrapper to save you from writing verbose C code. For simplicity purpose, we'll stick with the C++ wrapper in this tutorial.

Calling Taichi in C++ as easy as what you'd imagine:

  • Create a Taichi runtime with target arch
  • Load the compiled artifacts from disk through TiRT's load_aot_module interface.
  • Load kernels from the module to k_init_ and k_add_base_
  • Prepare the inputs: ndarray x_ and float base
  • Launch the kernels!

A complete C++ application with embedded Taichi is shown below:

#include <taichi/cpp/taichi.hpp>

struct App0_tutorial {
  static const uint32_t NPARTICLE = 8192 * 2;
  static const uint32_t N_ITER = 50;

  ti::Runtime runtime_;
  ti::AotModule module_;
  ti::Kernel k_init_;
  ti::Kernel k_add_base_;
  ti::NdArray<float> x_;

  App0_tutorial() {
    runtime_ = ti::Runtime(TI_ARCH_VULKAN);
    module_ = runtime_.load_aot_module("0_tutorial_kernel/assets/tutorial");
    k_init_ = module_.get_kernel("init");
    k_add_base_ = module_.get_kernel("add_base");
    x_ = runtime_.allocate_ndarray<float>({NPARTICLE}, {}, true);
    std::cout << "Initialized!" << std::endl;
  }

  bool run() {
    float base = 0.2;

    k_init_.push_arg(x_);
    k_init_.launch();
    k_add_base_.push_arg(x_);
    k_add_base_.push_arg(base);
    for (int i = 0; i < N_ITER; i++) {
      k_add_base_.launch();
    }
    runtime_.wait();
    return true;
  }
};

int main(int argc, const char** argv) {
  App0_tutorial app;
  app.run();
  return 0;
}

FAQ

Map your Taichi data types from Python to C++

PythonC++
scalarC++ scalar type
ti.vector / ti.matrixstd::vector
ti.ndarrayti::Ndarray
ti.Textureti::Texture
ti.fieldWIP

Does Taichi support device import/export?

Yes! We understand that in real applications it's pretty common to hook Taichi in your existing Vulkan pipeline. As a result, you can choose to import an external device for Taichi to use, or export a device that Taichi creates to share with the external application

Which backends & hardware are supported?

Currently ti.vulkan, ti.opengl, ti.x86 and ti.cuda are supported. ti.metal is not yet supported.

How can I debug a C++ application with embedded Taichi?

  1. Check ti_get_last_error() whenever you call a Taichi C API.
  2. Enable backward-cpp in your application to locate the source of crashes. E.g. https://github.com/taichi-dev/taichi-aot-demo/pull/69
  3. Get values of ndarrays back on host using ndarray.read(), e.g. https://github.com/taichi-dev/taichi-aot-demo/pull/57/files#diff-d94bf1ff63835d9cf87e700ca3c37d1e9a3c09e5994944db2adcddf132a71d0cR32
  4. Enable printing in shaders, e.g. https://github.com/taichi-dev/taichi-aot-demo/pull/55

Does Taichi support generating shaders for different deployment targets?

Yea, it's a common feature request that people want to generate shaders for their Android application on a Linux desktop. If you know exactly what your target device is capable of, you can specify in the ti.aot.Module(arch=, caps=[]). Some example caps are spirv_has_atomic_float, spirv_has_physical_storage_buffer etc.

Supporting compiling to an arch different from the one in ti.init() is a planned feature but not yet supported.

Are Taichi compiled artifacts versioned?

(Pre-release version, until JSON is resolved)

We highly recommend using python taichi and C++ taichi runtime built from exactly the same commit.

Can I hook Taichi into a render pipeline?

Yes! If you already have a rendering pipeline, you can interop with Taichi via https://docs.taichi-lang.org/docs/taichi_vulkan.

If you don't have one already, please check out our demos at https://github.com/taichi-dev/taichi-aot-demo

I just want to use raw shaders generated by Taichi. Where can I find them?

Yea you can find it in the target folder of aot save. But please be aware that launching Taichi shaders requires some special setup which relates to implementation details in Taichi and may subject to change without notice.

But if you have super strict size limitation for the whole application where the provided runtime is too large to fit, you may consider writing a minimal taichi runtime in C which consumes those raw shaders.

Can I build the libtaichi_c_api.so from source?

Usually, for simplicity and stability, we recommend using the official nightly taichi wheels and the c_api shipped inside the wheel. But if you want a runtime library with special build configuration:

TAICHI_CMAKE_ARGS="-DTI_WITH_VULKAN:BOOL=ON -DTI_WITH_C_API:BOOL=ON" python setup.py develop

# Other commonly used CMake options
- TI_WITH_OPENGL
- TI_WITH_CPU
- TI_WITH_CUDA

You can find the built libtaichi_c_api.so and its headers in the _skbuild/ folder.

Taichi/C API Reference Manual

https://docs.taichi-lang.org/docs/taichi_core#api-reference

When do I need to recompile my artifacts?

You should recompile the artifacts when the following happens:

  • Kernels and their launching logic in Python are updated
  • You need a newer version for either Python Taichi or runtime library.
  • Your target device has a different set of capabilites
  • If you update some Python constants (instead of kernel arguments), they're likely encoded as a constant in Taichi compiled artifacts so you'll need a recompilation.

Note given the nature of how Ndarrays are handled in the Taichi compilation, shaders generated can be used for ndarrays with different shapes as long as their ranks match. Which really comes to handy if you need one set of shaders for everything, e.g. different screen sizes on Android phones.

How can I set values for ndarrays in C++?

In the C++ wrapper we provide these convenient read/write() methods on NdArray class. https://github.com/taichi-dev/taichi/blob/master/c_api/include/taichi/cpp/taichi.hpp#L192-L215

In C API you can allocate your memory as host accessible and then use map/unmap. https://docs.taichi-lang.org/docs/taichi_core

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