@cudawarped What do you think?

@cudawarped What do you think?

This removes the fix added in #25941. Additionaly if we wanted to remove that fix there would be no reason to have the section after setting the language because it doesn't do anything @tony19053000?

if(NOT CMAKE_CUDA_ARCHITECTURES)
  set(CMAKE_CUDA_ARCHITECTURES "")
endif()

As per the original issue (#25920) and PR (#25941) when the minimum version of CMake is 3.18 CMAKE_CUDA_ARCHITECTURES is automatically initialized to the default architecture chosen by the compiler. This means without that fix (and with this PR) we would have to ignore CMAKE_CUDA_ARCHITECTURES or always use it (we couldn't use CUDA_ARCH_BIN/CUDA_ARCH_PTX).

The reason for this is that if CMAKE_CUDA_ARCHITECTURES is empty enabling CUDA as a first class language (enable_language(CUDA)) initializes CMAKE_CUDA_ARCHITECTURES so it contains a valid value. #25941 prevents that so we can fall back to OpenCV's internal logic which uses CUDA_ARCH_BIN/CUDA_ARCH_PTX. e.g.

Behaviour with #25941

CMAKE_CUDA_ARCHITECTURES not set

• CMAKE_CUDA_ARCHITECTURES= (in)
• enable_language(CUDA)
• CMAKE_CUDA_ARCHITECTURES=52 (default architecture chosen by the compiler)
• set(CMAKE_CUDA_ARCHITECTURES "")
• CMAKE_CUDA_ARCHITECTURES= (out)

CMAKE_CUDA_ARCHITECTURES set

• CMAKE_CUDA_ARCHITECTURES=75 (in)
• USER_DEFINED_CMAKE_CUDA_ARCHITECTURES=TRUE
• enable_language(CUDA)
• CMAKE_CUDA_ARCHITECTURES=75 (out)

Behaviour with this PR (previous behaviour)

CMAKE_CUDA_ARCHITECTURES not set

• CMAKE_CUDA_ARCHITECTURES= (in)
• enable_language(CUDA)
• CMAKE_CUDA_ARCHITECTURES=52 (default architecture chosen by the compiler)
• CMAKE_CUDA_ARCHITECTURES=52 (out)

I think the current code essentially just simplifies to this, or does it not?

      if(NOT CMAKE_CUDA_ARCHITECTURES)
        set(CMAKE_CUDA_ARCHITECTURES "")
      endif()
      enable_language(CUDA)

Also as a related note. I just tried to build the latest opencv 4.x main (cloned 3 days ago), and from a completely empty build directory/cache, I've initialised CMake with -DCMAKE_CUDA_ARCHITECTURES:STRING="86;89;90;100;120". However, my CUDA_ARCH_BIN in the cmake cache was then still set to the "generic" default values of "5.0; ...". So it seems initialising the CUDA architecture with CMAKE_CUDA_ARCHITECTURES (which is the standard) is currently not working?

I think the current code essentially just simplifies to this, or does it not?

      if(NOT CMAKE_CUDA_ARCHITECTURES)
        set(CMAKE_CUDA_ARCHITECTURES "")
      endif()
      enable_language(CUDA)

That results in the following error for me

CMake Error at C:/Program Files/CMake/share/cmake-3.29/Modules/Internal/CMakeCUDAArchitecturesValidate.cmake:7 (message):
CMAKE_CUDA_ARCHITECTURES must be non-empty if set.
Call Stack (most recent call first):
C:/Program Files/CMake/share/cmake-3.29/Modules/CMakeDetermineCUDACompiler.cmake:112 (cmake_cuda_architectures_validate)
CMakeLists.txt:693 (enable_language)

Try configuring with -DENABLE_CUDA_FIRST_CLASS_LANGUAGE=ON -DCMAKE_CUDA_ARCHITECTURES=86 -DMIN_VER_CMAKE=3.18 with and without your suggestions.

Also as a related note. I just tried to build the latest opencv 4.x main (cloned 3 days ago), and from a completely empty build directory/cache, I've initialised CMake with -DCMAKE_CUDA_ARCHITECTURES:STRING="86;89;90;100;120". However, my CUDA_ARCH_BIN in the cmake cache was then still set to the "generic" default values of "5.0; ...". So it seems initialising the CUDA architecture with CMAKE_CUDA_ARCHITECTURES (which is the standard) is currently not working?

I just tried and its working for me have you set CUDA a first class language (-DENABLE_CUDA_FIRST_CLASS_LANGUAGE=ON)?