[cmake] Add support for explicit STATIC libraries even when building mostly shared libraries.

[stephenneuendorffer]

Summary

By default when BUILD_SHARED_LIBS=ON, all libraries are forced to be
shared. However, with circular dependencies between libraries this
becomes a problem. This change allows libraries to be explicitly
marked as static in CMAKE to solve circular dependencies while getting
most of the build-time advantages from BUILD_SHARED_LIBS=ON

[tstellar]

Are there currently libraries in tree which have circular dependencies?

[stephenneuendorffer]

@tstellar: Not that I know of, but MLIR has some circular dependencies and I've used this to solve them. This should be NFC for existing usage.

[stephenneuendorffer]

Now that MLIR is in tree, are there any specific objections to accepting this patch? We intend to eliminate the circular library dependence as well, however, there seems to be little reason to disallow this usage, as far as I can see.

[stephenneuendorffer]

We solved this in MLIR by breaking the circular dependence.

llvm/cmake/modules/AddLLVM.cmake

function(add_llvm_install_targets target)
   if(target_dependencies)
     add_dependencies(${target} ${target_dependencies})
     add_dependencies(${target}-stripped ${target_dependencies})
   endif()
 endfunction()
 
 macro(add_llvm_library name)
   cmake_parse_arguments(ARG
-    "SHARED;BUILDTREE_ONLY;MODULE;INSTALL_WITH_TOOLCHAIN"
+    "SHARED;STATIC;BUILDTREE_ONLY;MODULE;INSTALL_WITH_TOOLCHAIN"
     ""
     ""
     ${ARGN})
   if(ARG_MODULE)
     llvm_add_library(${name} MODULE ${ARG_UNPARSED_ARGUMENTS})
+  elseif(ARG_STATIC)
+    llvm_add_library(${name} STATIC ${ARG_UNPARSED_ARGUMENTS})
   elseif( BUILD_SHARED_LIBS OR ARG_SHARED )
     llvm_add_library(${name} SHARED ${ARG_UNPARSED_ARGUMENTS})
   else()
     llvm_add_library(${name} ${ARG_UNPARSED_ARGUMENTS})
   endif()
 
   # Libraries that are meant to only be exposed via the build tree only are
   # never installed and are only exported as a target in the special build tree