cmake: Copy files to dependent targets (specifically for use in OpenCL) - c++

Question

I am fiddling around with OpenCL, where files are typically compiled at runtime. The typical structure of a project looks like this:
foo/
cl/foo.cl
inc/foo.hpp
src/foo.cpp
CMakeLists.txt
where foo.cpp will have a command to build the OpenCL kernel. Something like:
buildKernelFromFile('cl/foo.cl'); // Note the relative path
To facilitate this, I have been setting up the CMakeLists.txt file to copy the .cl files to a cl/ folder in the binary directory:
set(kernels
foo.cl)
foreach (kernel IN ITEMS ${kernels})
configure_file(${CMAKE_CURRENT_LIST_DIR}/cl/${kernel} ${CMAKE_CURRENT_BINARY_DIR}/cl/${kernel} COPYONLY)
endforeach ()
The issue is that when the foo project is an executable, then this approach is fine since the .cl files will be copied to the same location as the executable. However, when the foo project is a library, this approach no longer works because the relative path is to the binary directory of the library, not the binary directory of the executable using the library.
So what I would like is a way to somehow tell CMake to copy the .cl files to the binary directory of any dependent targets. Specifically, if I build the OpenCL project like this
add_library(foo foo.cpp)
and then in a separate CMakeLists.txt file, link to that project:
add_executable(something main.cpp)
target_link_library(something PRIVATE foo)
I need to copy my .cl files to the build folder CMAKE_CURRENT_BINARY_DIR of the something target.
One thing I would like to avoid is having to save the .cl files as string literals and then using them like an include. (Ignore this comment if it does not make sense to you).
EDIT:
In the meantime, in the OpenCL library's CMakeLists.txt file (foo/CMakeLists.txt), I simply declared a function
# Copy the OpenCL kernels to the build folder
set(KERNEL_DIR ${CMAKE_CURRENT_LIST_DIR}/cl PARENT_SCOPE)
function(copy_kernels)
set(kernels
foo.cl)
foreach (kernel IN ITEMS ${kernels})
configure_file(${KERNEL_DIR}/${kernel} ${CMAKE_CURRENT_BINARY_DIR}/cl/${kernel} COPYONLY)
endforeach ()
endfunction()
which should be called by dependent projects. Then in the dependent project, in addition to linking to the foo target, you need to call this function
copy_kernels()
target_link_library(something PRIVATE foo)
I would like to figure out how to get rid of that copy_kernels() call, but I don't know cmake well enough to do that.

Answer 1

Frustratingly there isn't a neat way of doing this in CMake. As you say, configure_file should be exactly what we need, but it's not tied to a target and runs at configuration time (so you'd have to re-run cmake every time a kernel changed). The best I've been able to manage is this:
file(GLOB KERNELS cl/*.cl)
foreach(CurrentKernel IN LISTS KERNELS)
cmake_path(GET CurrentKernel FILENAME FileName)
add_custom_command(
TARGET foo PRE_BUILD
COMMAND ${CMAKE_COMMAND} -E copy_if_different ${CurrentKernel} ${CMAKE_CURRENT_BINARY_DIR}/cl/${FileName}
COMMENT "Copying kernel: ${CurrenKernel}")
endforeach()