CUDA: Add CMAKE_CUDA_SEPARABLE_COMPILATION variable

Use its value to initialize the `CUDA_SEPARABLE_COMPILATION` target
property when targets are created.

Fixes: #17478

Help/manual/cmake-variables.7.rst

@@ -290,6 +290,7 @@ Variables that Control the Build
    /variable/CMAKE_COMPILE_PDB_OUTPUT_DIRECTORY
    /variable/CMAKE_COMPILE_PDB_OUTPUT_DIRECTORY_CONFIG
    /variable/CMAKE_CONFIG_POSTFIX
+   /variable/CMAKE_CUDA_SEPARABLE_COMPILATION
    /variable/CMAKE_DEBUG_POSTFIX
    /variable/CMAKE_ENABLE_EXPORTS
    /variable/CMAKE_EXE_LINKER_FLAGS

Help/prop_tgt/CUDA_SEPARABLE_COMPILATION.rst

@@ -11,3 +11,7 @@ For instance:
 .. code-block:: cmake
 
   set_property(TARGET myexe PROPERTY CUDA_SEPARABLE_COMPILATION ON)
+
+This property is initialized by the value of the
+:variable:`CMAKE_CUDA_SEPARABLE_COMPILATION` variable if it is set when a
+target is created.

Help/release/dev/cuda-sep-comp-var.rst

@@ -0,0 +1,6 @@
+cuda-sep-comp-var
+-----------------
+
+* A :variable:`CMAKE_CUDA_SEPARABLE_COMPILATION` variable was added to
+  initialize the :prop_tgt:`CUDA_SEPARABLE_COMPILATION` target property
+  on targets when they are created.

Help/variable/CMAKE_CUDA_SEPARABLE_COMPILATION.rst

@@ -0,0 +1,6 @@
+CMAKE_CUDA_SEPARABLE_COMPILATION
+--------------------------------
+
+Default value for :prop_tgt:`CUDA_SEPARABLE_COMPILATION` target property.
+This variable is used to initialize the property on each target as it is
+created.

Source/cmTarget.cxx

@@ -279,6 +279,7 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
     this->SetPropertyDefault("CUDA_STANDARD_REQUIRED", nullptr);
     this->SetPropertyDefault("CUDA_EXTENSIONS", nullptr);
     this->SetPropertyDefault("CUDA_COMPILER_LAUNCHER", nullptr);
+    this->SetPropertyDefault("CUDA_SEPARABLE_COMPILATION", nullptr);
     this->SetPropertyDefault("LINK_SEARCH_START_STATIC", nullptr);
     this->SetPropertyDefault("LINK_SEARCH_END_STATIC", nullptr);
   }

Tests/CudaOnly/SeparateCompilation/CMakeLists.txt

@@ -13,7 +13,13 @@ string(APPEND CMAKE_CUDA_FLAGS " -gencode arch=compute_30,code=compute_30")
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CUDA_STANDARD 11)
 
+set(CMAKE_CUDA_SEPARABLE_COMPILATION ON)
 add_library(CUDASeparateLibA STATIC file1.cu file2.cu file3.cu)
+get_property(sep_comp TARGET CUDASeparateLibA PROPERTY CUDA_SEPARABLE_COMPILATION)
+if(NOT sep_comp)
+  message(FATAL_ERROR "CUDA_SEPARABLE_COMPILATION not initialized")
+endif()
+unset(CMAKE_CUDA_SEPARABLE_COMPILATION)
 
 if(CMAKE_CUDA_SIMULATE_ID STREQUAL "MSVC")
   # Test adding a flag that is not in our CUDA flag table for VS.