linux build error: error: calling a constexpr __host__ function("ceil_div") from a __global__ function("nms_kernel") is not allowed.

[qizhen816]

When I build the latest torchvision with cuda 9.2, errors occur:

-- Caffe2: CUDA detected: 9.2
-- Caffe2: CUDA nvcc is: /usr/local/cuda/bin/nvcc
-- Caffe2: CUDA toolkit directory: /usr/local/cuda
-- Caffe2: Header version is: 9.2
-- Found cuDNN: v7.6.3  (include: /usr/include, library: /usr/lib/x86_64-linux-gnu/libcudnn.so)
-- Autodetected CUDA architecture(s):  6.1 6.1 6.1 6.1
-- Added CUDA NVCC flags for: -gencode;arch=compute_61,code=sm_61
-- Configuring done
-- Generating done
-- Build files have been written to: /mnt/cxx_restful/vision/build
[  4%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/models/inception.cpp.o
[  8%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/models/mnasnet.cpp.o
[ 12%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/models/mobilenet.cpp.o
[ 16%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/models/resnet.cpp.o
[ 20%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/models/shufflenetv2.cpp.o
[ 25%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/models/squeezenet.cpp.o
[ 29%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/models/vgg.cpp.o
[ 33%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/cpu/DeformConv_cpu.cpp.o
[ 37%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/cpu/PSROIAlign_cpu.cpp.o
[ 41%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/cpu/PSROIPool_cpu.cpp.o
[ 45%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/cpu/ROIAlign_cpu.cpp.o
[ 50%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/cpu/ROIPool_cpu.cpp.o
[ 54%] Building CXX object CMakeFiles/torchvision.dir/torchvision/csrc/cpu/nms_cpu.cpp.o
[ 58%] Building CUDA object CMakeFiles/torchvision.dir/torchvision/csrc/cuda/DeformConv_cuda.cu.o
[ 62%] Building CUDA object CMakeFiles/torchvision.dir/torchvision/csrc/cuda/PSROIAlign_cuda.cu.o
[ 66%] Building CUDA object CMakeFiles/torchvision.dir/torchvision/csrc/cuda/PSROIPool_cuda.cu.o
[ 70%] Building CUDA object CMakeFiles/torchvision.dir/torchvision/csrc/cuda/ROIAlign_cuda.cu.o
[ 75%] Building CUDA object CMakeFiles/torchvision.dir/torchvision/csrc/cuda/ROIPool_cuda.cu.o
[ 79%] Building CUDA object CMakeFiles/torchvision.dir/torchvision/csrc/cuda/nms_cuda.cu.o
/mnt/cxx_restful/vision/torchvision/csrc/cuda/nms_cuda.cu(66): error: calling a constexpr __host__ function("ceil_div") from a __global__ function("nms_kernel") is not allowed. The experimental flag '--expt-relaxed-constexpr' can be used to allow this.

1 error detected in the compilation of "/tmp/tmpxft_00008508_00000000-6_nms_cuda.cpp1.ii".
CMakeFiles/torchvision.dir/build.make:338: recipe for target 'CMakeFiles/torchvision.dir/torchvision/csrc/cuda/nms_cuda.cu.o' failed
make[2]: *** [CMakeFiles/torchvision.dir/torchvision/csrc/cuda/nms_cuda.cu.o] Error 1
CMakeFiles/Makefile2:78: recipe for target 'CMakeFiles/torchvision.dir/all' failed
make[1]: *** [CMakeFiles/torchvision.dir/all] Error 2
Makefile:132: recipe for target 'all' failed
make: *** [all] Error 2

I checked cuda.cmake in libtorch and there is list(APPEND CUDA_NVCC_FLAGS "--expt-relaxed-constexpr"), apparently it didn't work. Any clue how to fix this?
CMakeLists.txt

[fmassa]

This is related to #2127

@zasdfgbnm could you send a PR fixing this?

[zasdfgbnm]

It is strange that why it is not compiling with --expt-relaxed-constexpr, but we can just make this function __host__ __device__ so that we no longer rely on --expt-relaxed-constexpr. See #2217