namespace tf {

/** @page CompileTaskflowWithCUDA Compile Taskflow with CUDA

@tableofcontents

@section InstallCUDACompiler Install CUDA Compiler

To compile %Taskflow with CUDA code, you need a @c nvcc compiler.
Please visit the official page of 
<a href="https://developer.nvidia.com/cuda-downloads">Downloading CUDA Toolkit</a>.

@section CompileTaskflowWithCUDADirectly Compile Source Code Directly

%Taskflow's GPU programming interface for CUDA is tf::cudaFlow.
Consider the following `simple.cu` program that launches a single kernel
function to output a message:

@code{.cpp}
#include <taskflow/taskflow.hpp>
#include <taskflow/cudaflow.hpp>  

int main(int argc, const char** argv) {

  // create a CUDA graph with a single-threaded task
  tf::cudaGraph cg;
  cf.single_task([] __device__ () { printf("hello CUDA Graph!\n"); });
  
  // instantiate an executable CUDA graph and run it through a stream
  tf::cudaStream stream;
  tf::cudaGraphExec exec(cg);

  stream.run(cg).synchronize();

  return 0;
}
@endcode

The easiest way to compile %Taskflow with CUDA code (e.g., %cudaFlow, kernels)
is to use @nvcc:

@code{.bash}
~$ nvcc -std=c++17 -I path/to/taskflow/ --extended-lambda simple.cu -o simple
~$ ./simple
hello cudaFlow!
@endcode


@section CompileTaskflowWithCUDASeparately Compile Source Code Separately

Large GPU applications often compile a program into separate objects
and link them together to form an executable or a library.
You can compile your CPU code and GPU code separately with %Taskflow
using @c nvcc and other compilers (such as @c g++ and @c clang++).
Consider the following example that defines two tasks
on two different pieces (@c main.cpp and @c cudaflow.cpp) of source code:

@code{.cpp}
// main.cpp
#include <taskflow/taskflow.hpp>

tf::Task make_cudaflow(tf::Taskflow& taskflow);  // create a cudaFlow task

int main() {

  tf::Executor executor;
  tf::Taskflow taskflow;

  tf::Task task1 = taskflow.emplace([](){ std::cout << "main.cpp!\n"; })
                           .name("cpu task");
  tf::Task task2 = make_cudaflow(taskflow);

  task1.precede(task2);

  executor.run(taskflow).wait();

  return 0;
}
@endcode

@code{.cpp}
// cudaflow.cpp
#include <taskflow/taskflow.hpp>
#include <taskflow/cudaflow.hpp>

tf::Task make_cudaflow(tf::Taskflow& taskflow) {
  return taskflow.emplace([](){
    // create a CUDA graph with a single-threaded task
    tf::cudaGraph cg;
    cf.single_task([] __device__ () { printf("hello CUDA Graph!\n"); });
    
    // instantiate an executable CUDA graph and run it through a stream
    tf::cudaStream stream;
    tf::cudaGraphExec exec(cg);

    stream.run(cg).synchronize();
  }).name("gpu task");
}
@endcode

Compile each source to an object (@c g++ as an example):

@code{.bash}
~$ g++ -std=c++17 -I path/to/taskflow -c main.cpp -o main.o
~$ nvcc -std=c++17 --extended-lambda -x cu -I path/to/taskflow \
        -dc cudaflow.cpp -o cudaflow.o
~$ ls
# now we have the two compiled .o objects, main.o and cudaflow.o
main.o cudaflow.o 
@endcode

The @c --extended-lambda option tells @c nvcc to generate GPU code
for the lambda defined with <tt>__device__</tt>.
The <tt>-x cu</tt> tells @c nvcc to treat the input files as @c .cu files 
containing both CPU and GPU code. 
By default, @c nvcc treats @c .cpp files as CPU-only code. 
This option is required to have @c nvcc generate device code here, 
but it is also a handy way to avoid renaming source files in larger projects.
The @c –dc option tells @c nvcc to generate device code for later linking.

You may also need to specify the target architecture to tell @c nvcc to target
on a compatible SM architecture using the option @-arch.
For instance, the following command requires 
device code linking to have compute capability 7.5 or later:

@code{.bash}
~$ nvcc -std=c++17 --extended-lambda -x cu -arch=sm_75 -I path/to/taskflow \
        -dc cudaflow.cpp -o cudaflow.o
@endcode

@subsection CompileTaskflowWithCUDANaiveLinking Link Objects Using nvcc

Using @c nvcc to link compiled object code is nothing special but
replacing the normal compiler with @c nvcc and it takes care of all the 
necessary steps:

@code{.bash}
~$ nvcc main.o cudaflow.o -o main

# run the main program 
~$ ./main
main.cpp!
cudaflow.cpp!
@endcode

@subsection CompileTaskflowWithCUDADifferentLinkers Link Objects Using Different Linkers

You can choose to use a compiler other than @c nvcc for the final link step.
Since your CPU compiler does not know how to link CUDA device code, 
you have to add a step in your build to have @c nvcc link the CUDA device code,
using the option @c -dlink:

@code{.bash}
~$ nvcc -o gpuCode.o -dlink main.o cudaflow.o
@endcode

This step links all the <i>device object code</i> and places it into @c gpuCode.o.

@attention
Note that this step does not link the CPU object code
and discards the CPU object code in @c main.o and @c cudaflow.o.


To complete the link to an executable, you can use, for example, @c ld or @c g++.

@code{.bash}
# replace /usr/local/cuda/lib64 with your own CUDA library installation path
~$ g++ -pthread -L /usr/local/cuda/lib64/ -lcudart \
   gpuCode.o main.o cudaflow.o -o main

# run the main program
~$ ./main
main.cpp!
cudaflow.cpp!
@endcode

We give @c g++ all of the objects again because it needs the CPU object code, 
which is not in @c gpuCode.o. 
The device code stored in the original objects, @c main.o and @c cudaflow.o,
does not conflict with the code in @c gpuCode.o. 
@c g++ ignores device code because it does not know how to link it, 
and the device code in @c gpuCode.o is already linked and ready to go. 

@attention
This intentional ignorance is extremely useful in large builds 
where intermediate objects may have both CPU and GPU code. 
In this case, we just let the GPU and CPU linkers each do its own job, 
noting that the CPU linker is always the last one we run. 
The CUDA Runtime API library is automatically linked when we use @c nvcc for linking, 
but we must explicitly link it (@c -lcudart) when using another linker.


*/


}


