[CMake] Question regarding CUDA support in CMake 3.8.0-rc2

[Robert Maynard]

Hi,

The purpose of the cmake_device_link.o is to resolve device side
symbols when doing separable compilation. We do this device linking
step for all CUDA enabled targets as it makes the internal CMake to
logic significantly easier ( no need to propagate another property
through the graph, etc ). Updating CMake to only do device linking
where needed is theoretical possible, but opens up a new question on
consistency [1].

What you have pointed out is actually two issues. The first is that
target_compile_options by design does not propagate any flags to the
linker, which is a design decision made by CMake. The second being
that for CUDA arch/code flags in general need to be passed to both the
compiler and the linker.

So currently CUDA projects should specify arch/code flags through
CMAKE_CUDA_FLAGS/CMAKE_CUDA_FLAGS_<CONFIG> to have them propagate to
the linker.

[1] - If we only do device linking for targets that need separable
compilation, we introduce a split in CUDA targets where some are
satisfied by add_compile_options, and others require
CMAKE_CUDA_FLAGS/CMAKE_CUDA_FLAGS_<CONFIG>. To complicate this
situation you can't provide -arch/code in both as that will generally
result in a collection of invalid flags.


On Wed, Mar 15, 2017 at 8:22 PM, Nick Henderson
<nick.henderson at gmail.com> wrote:
> Hello!
>
> I am testing out the CUDA support in CMake 3.8.0-rc2.
>
> When running `make VERBOSE=1` in the build directory, I get a warning
> generated related to the GPU architecture flags for nvcc:
>
> ```
> [ 80%] Linking CUDA device code
> /home/nwh/git/foobar/build/exec/CMakeFiles/exec.dir/cmake_device_link.o
> cd /home/nwh/git/foobar/build/exec && /usr/local/bin/cmake -E
> cmake_link_script CMakeFiles/exec.dir/dlink.txt --verbose=1
> /usr/local/cuda/bin/nvcc   -Xcompiler=-fPIC -shared -dlink
> CMakeFiles/exec.dir/exec.cc.o -o CMakeFiles/exec.dir/cmake_device_link.o
> ../libfoobar.a
> nvcc warning : The 'compute_20', 'sm_20', and 'sm_21' architectures are
> deprecated, and may be removed in a future release (Use
> -Wno-deprecated-gpu-targets to suppress warning).
> ```
>
> I don't get the warning when the executable source is being compiled because
> an up-to-date architecture is specified:
>
> ```
> [ 60%] Building CUDA object exec/CMakeFiles/exec.dir/exec.cc.o
> cd /home/nwh/git/foobar/build/exec && /usr/local/cuda/bin/nvcc
> -I/home/nwh/git/foobar  -arch compute_30 -std=c++11 -x cu -c
> /home/nwh/git/foobar/exec/exec.cc -o CMakeFiles/exec.dir/exec.cc.o
> ```
>
> Link to sample project: https://github.com/nwh/foobar
>
> Questions:
>
> * Is this a problem?
> * What is the purpose of cmake_device_link.o?
>
> Thanks,
> Nick
>
>
>
> --
> View this message in context: http://cmake.3232098.n2.nabble.com/Question-regarding-CUDA-support-in-CMake-3-8-0-rc2-tp7595171.html
> Sent from the CMake mailing list archive at Nabble.com.
> --
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