[cmake] Question about target ${SPIRV_TOOLS}-shared and ${SPIRV_TOOLS}

[JackBoosY]

In the following code:

SPIRV-Tools/source/CMakeLists.txt

Lines 374 to 403 in 983ee23

	# Always build ${SPIRV_TOOLS}-shared. This is expected distro packages, and
	# unlike the other SPIRV_TOOLS target, defaults to hidden symbol visibility.
	add_library(${SPIRV_TOOLS}-shared SHARED ${SPIRV_SOURCES})
	spirv_tools_default_target_options(${SPIRV_TOOLS}-shared)
	set_target_properties(${SPIRV_TOOLS}-shared PROPERTIES CXX_VISIBILITY_PRESET hidden)
	target_compile_definitions(${SPIRV_TOOLS}-shared
	PRIVATE SPIRV_TOOLS_IMPLEMENTATION
	PUBLIC SPIRV_TOOLS_SHAREDLIB
	)
	if(SPIRV_TOOLS_BUILD_STATIC)
	add_library(${SPIRV_TOOLS}-static STATIC ${SPIRV_SOURCES})
	spirv_tools_default_target_options(${SPIRV_TOOLS}-static)
	# The static target does not have the '-static' suffix.
	set_target_properties(${SPIRV_TOOLS}-static PROPERTIES OUTPUT_NAME "${SPIRV_TOOLS}")
	# Create the "${SPIRV_TOOLS}" target as an alias to either "${SPIRV_TOOLS}-static"
	# or "${SPIRV_TOOLS}-shared" depending on the value of BUILD_SHARED_LIBS.
	if(BUILD_SHARED_LIBS)
	add_library(${SPIRV_TOOLS} ALIAS ${SPIRV_TOOLS}-shared)
	else()
	add_library(${SPIRV_TOOLS} ALIAS ${SPIRV_TOOLS}-static)
	endif()
	set(SPIRV_TOOLS_TARGETS ${SPIRV_TOOLS}-static ${SPIRV_TOOLS}-shared)
	else()
	add_library(${SPIRV_TOOLS} ${SPIRV_TOOLS_LIBRARY_TYPE} ${SPIRV_SOURCES})
	spirv_tools_default_target_options(${SPIRV_TOOLS})
	set(SPIRV_TOOLS_TARGETS ${SPIRV_TOOLS} ${SPIRV_TOOLS}-shared)
	endif()

There are two scenarios here:

1. If I pass -DSPIRV_TOOLS_BUILD_STATIC=ON, there will be 3 targets:

• ${SPIRV_TOOLS}-shared
• ${SPIRV_TOOLS}-static
• ${SPIRV_TOOLS}

${SPIRV_TOOLS} is point to ${SPIRV_TOOLS}-shared or ${SPIRV_TOOLS}-static according to BUILD_SHARED_LIBS. that's okay, but I don’t know why SPIRV_TOOLS_BUILD_STATIC and SPIRV_TOOLS_LIBRARY_TYPE and BUILD_SHARED_LIBS can exist at the same time.
If we use BUILD_SHARED_LIBS, the generated library should have the same expectation as the value of BUILD_SHARED_LIBS: generate dynamic library, otherwise generate static library.

2. If I pass -DSPIRV_TOOLS_BUILD_STATIC=OFF, there will be 2 targets:

• ${SPIRV_TOOLS}-shared
• ${SPIRV_TOOLS}

${SPIRV_TOOLS} uses the same source files with ${SPIRV_TOOLS}-shared.
And if BUILD_SHARED_LIBS is ON, there will be 2 dynamic libraries!
On Windows, ${SPIRV_TOOLS} has exported symbols, and ${SPIRV_TOOLS}-shared doesn't export any symbols. I think that's not make sence: how to use a dynamic library that does not export any symbols?
On non-Windows, you will see SPIRV_TOOLS is same with ${SPIRV_TOOLS}-shared.

So my question is:

1. Why is there such a design?
2. What is the point of always generating dynamic libraries?
3. Why not just use BUILD_SHARED_LIBS or SPIRV_TOOLS_BUILD_STATIC or SPIRV_TOOLS_LIBRARY_TYPE ?

Related downstrem PR: microsoft/vcpkg#19219