[CMake] Add option LLVM_FORCE_CREATE_SYMLINKS

[thieta]

Summary

On Windows we don't create symlinks for the binaries (clang++, clang-cl)
since the support requires special setup (group policy settings and
you need to know exactly our distribution story). But if you know
about these things and have a controlled environment there is a lot
of storage to be saved, so let's add a manual opt-in for using symlinks
on Windows with LLVM_FORCE_CREATE_SYMLINKS=ON.

[phosek]

I'd prefer to have an option like LLVM_USE_SYMLINKS and default to ON when CMAKE_HOST_UNIX, otherwise OFF.

[thieta]

Changed the option to LLVM_USE_SYMLINKS

[thieta]

In D135578#3847826, @phosek wrote:

I'd prefer to have an option like LLVM_USE_SYMLINKS and default to ON when CMAKE_HOST_UNIX, otherwise OFF.

Yeah sounds good. And I also fixed the relative symlinks on Windows.

[mstorsjo]

This looks reasonable to me (although I haven't dug through the code to see if there are more cases that would need to be set in sync). Is it kosher to add new end-user options like this in a nested cmake module file?

I'll leave it open for @phosek to comment on in any case.

[thieta]

In D135578#3848706, @mstorsjo wrote:

This looks reasonable to me (although I haven't dug through the code to see if there are more cases that would need to be set in sync). Is it kosher to add new end-user options like this in a nested cmake module file?

I'll leave it open for @phosek to comment on in any case.

There are other places where symlinks are used and some similar logic is being applied - like here:
https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/builtins/CMakeLists.txt#L533

I am not sure it's worth to fix this at the same time since that has to do with symlinking the files when building rather then the finished distribution.

Regarding option() in this cmakefile - I had the same idea, but I saw several option() calls in this file already, so I figured it was a pattern already.

[phosek]

In D135578#3848714, @thieta wrote:

In D135578#3848706, @mstorsjo wrote:

This looks reasonable to me (although I haven't dug through the code to see if there are more cases that would need to be set in sync). Is it kosher to add new end-user options like this in a nested cmake module file?

I'll leave it open for @phosek to comment on in any case.

There are other places where symlinks are used and some similar logic is being applied - like here:
https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/builtins/CMakeLists.txt#L533

I am not sure it's worth to fix this at the same time since that has to do with symlinking the files when building rather then the finished distribution.

This would be a separate option since we don't use AddLLVM.cmake module in that context so I think it's better to do it as a separate change.

Regarding option() in this cmakefile - I had the same idea, but I saw several option() calls in this file already, so I figured it was a pattern already.

Those are only defined in functions, there are no top-level options in this module. I'd move this option to llvm/CMakeLists.txt.

[thieta]

Moved option to llvm/CMakeLists.txt

llvm/CMakeLists.txt

 set(CMAKE_EXPORT_COMPILE_COMMANDS 1)
 
 option(LLVM_INSTALL_BINUTILS_SYMLINKS
   "Install symlinks from the binutils tool names to the corresponding LLVM tools." OFF)
 
 option(LLVM_INSTALL_CCTOOLS_SYMLINKS
   "Install symlinks from the cctools tool names to the corresponding LLVM tools." OFF)
 
+# By default we use symlinks on Unix platforms and copy binaries on Windows
+# If you have the correct setup on Windows you can use this option to enable
+# symlinks and save a lot of diskspace.
+option(LLVM_USE_SYMLINKS "Use symlinks instead of copying binaries" ${CMAKE_HOST_UNIX})
+
 option(LLVM_INSTALL_UTILS "Include utility binaries in the 'install' target." OFF)
 
 option(LLVM_INSTALL_TOOLCHAIN_ONLY "Only include toolchain files in the 'install' target." OFF)
 
 # Unfortunatly Clang is too eager to search directories for module maps, which can cause the
 # installed version of the maps to be found when building LLVM from source. Therefore we turn off
 # the installation by default. See llvm.org/PR31905.
 option(LLVM_INSTALL_MODULEMAPS "Install the modulemap files in the 'install' target." OFF)

llvm/cmake/modules/AddLLVM.cmake

function(llvm_add_tool_symlink project link_name target)
   # expression. To reconstruct the platform-agnostic path we have to do some
   # magic. First we grab one of the types, and a type-specific path. Then from
   # the type-specific path we find the last occurrence of the type in the path,
   # and replace it with CMAKE_CFG_INTDIR. This allows the build step to be type
   # agnostic again.
   if(NOT ARG_OUTPUT_DIR)
     # If you're not overriding the OUTPUT_DIR, we can make the link relative in
     # the same directory.
-    if(CMAKE_HOST_UNIX)
+    if(LLVM_USE_SYMLINKS)
       set(dest_binary "$<TARGET_FILE_NAME:${target}>")
     endif()
     if(CMAKE_CONFIGURATION_TYPES)
       list(GET CMAKE_CONFIGURATION_TYPES 0 first_type)
       string(TOUPPER ${first_type} first_type_upper)
       set(first_type_suffix _${first_type_upper})
     endif()
     get_target_property(target_type ${target} TYPE)
if(CMAKE_CONFIGURATION_TYPES)
       string(SUBSTRING "${ARG_OUTPUT_DIR}" 0 ${type_start} path_prefix)
       string(SUBSTRING "${ARG_OUTPUT_DIR}" ${type_start} -1 path_suffix)
       string(REPLACE "/${first_type}/" "/${CMAKE_CFG_INTDIR}/"
              path_suffix ${path_suffix})
       set(ARG_OUTPUT_DIR ${path_prefix}${path_suffix})
     endif()
   endif()
 
-  if(CMAKE_HOST_UNIX)
+  if(LLVM_USE_SYMLINKS)
     set(LLVM_LINK_OR_COPY create_symlink)
   else()
     set(LLVM_LINK_OR_COPY copy)
   endif()
 
   set(output_path "${ARG_OUTPUT_DIR}/${link_name}${CMAKE_EXECUTABLE_SUFFIX}")
 
   set(target_name ${link_name})