[cmake] Allow optionally to build `tablegen` in `Release` and use it in `Debug` for the `Ninja Multi-Config` generator

[akyrtzi]

Summary

If Ninja Multi-Config is used and LLVM_OPTIMIZED_TABLEGEN=YES is set
tablegen binaries will be built in Release, even when building for Debug configuration,
without needing a separate configured build directory just for tablegen binaries.
To get such a setup you'd configure like this:

cmake -G 'Ninja Multi-Config' /path/to/llvm-project/llvm \
     -DCMAKE_CONFIGURATION_TYPES="Debug;Release" -DCMAKE_CROSS_CONFIGS="Release" \
     -DLLVM_OPTIMIZED_TABLEGEN=YES                       \
   ....

Also note the other benefit of Ninja Multi-Config is saving configure time, particularly when
switching between Debug and Release during development.

[compnerd]

llvm/cmake/modules/TableGen.cmake

103

This can actually break the dependency tracking; I think that we should figure out how to get the target setup and use that. At that point, rather than computing the path manually, we should be able to use the generator expression $<TARGET_FILE:target> to get the path.

[akyrtzi]

llvm/cmake/modules/TableGen.cmake

103

Could you suggest a specific change? Unfortunately my CMake-foo is rather weak..😅

llvm/CMakeLists.txt

else()
   option(LLVM_BUILD_LLVM_DYLIB "Build libllvm dynamic library" ${LLVM_BUILD_LLVM_DYLIB_default})
 endif()
 
 if (LLVM_LINK_LLVM_DYLIB AND BUILD_SHARED_LIBS)
   message(FATAL_ERROR "Cannot enable BUILD_SHARED_LIBS with LLVM_LINK_LLVM_DYLIB.  We recommend disabling BUILD_SHARED_LIBS.")
 endif()
 
 option(LLVM_OPTIMIZED_TABLEGEN "Force TableGen to be built with optimization" OFF)
-if(CMAKE_CROSSCOMPILING OR (LLVM_OPTIMIZED_TABLEGEN AND (LLVM_ENABLE_ASSERTIONS OR CMAKE_CONFIGURATION_TYPES)))
+if (LLVM_OPTIMIZED_TABLEGEN AND NOT CMAKE_CROSSCOMPILING AND CMAKE_GENERATOR STREQUAL "Ninja Multi-Config" AND "Release" IN_LIST CMAKE_CROSS_CONFIGS)
+  # "Ninja Multi-Config" generator allows using release tablegen binaries
+  # without needing a separate configured build directory for them.
+  set(LLVM_OPTIMIZED_TABLEGEN_VIA_NINJA_MULTI_CONFIG TRUE)
+endif()
+if(CMAKE_CROSSCOMPILING OR (LLVM_OPTIMIZED_TABLEGEN AND NOT LLVM_OPTIMIZED_TABLEGEN_VIA_NINJA_MULTI_CONFIG AND (LLVM_ENABLE_ASSERTIONS OR CMAKE_CONFIGURATION_TYPES)))
   set(LLVM_USE_HOST_TOOLS ON)
 endif()
 
 option(LLVM_OMIT_DAGISEL_COMMENTS "Do not add comments to DAG ISel" ON)
 if (CMAKE_BUILD_TYPE AND uppercase_CMAKE_BUILD_TYPE MATCHES "^(RELWITHDEBINFO|DEBUG)$")
   set(LLVM_OMIT_DAGISEL_COMMENTS OFF)
 endif()
 

llvm/cmake/modules/TableGen.cmake

function(tablegen project ofn)
   # ("${${project}_TABLEGEN_TARGET}" STREQUAL "${${project}_TABLEGEN_EXE}")
   # but lets us having smaller and cleaner code here.
   get_directory_property(tblgen_includes INCLUDE_DIRECTORIES)
   list(TRANSFORM tblgen_includes PREPEND -I)
 
   set(tablegen_exe ${${project}_TABLEGEN_EXE})
   set(tablegen_depends ${${project}_TABLEGEN_TARGET} ${tablegen_exe})
 
+  if (LLVM_OPTIMIZED_TABLEGEN_VIA_NINJA_MULTI_CONFIG)
+    set(tablegen_exe "${CMAKE_BINARY_DIR}/Release/bin/${${project}_TABLEGEN_EXE}")
+    # Only depend on the release executable; if we also depend on the target
+    # we'll build for both release and debug.
+    set(tablegen_depends ${tablegen_exe})
+  endif()
+
   add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
     COMMAND ${tablegen_exe} ${ARGN} -I ${CMAKE_CURRENT_SOURCE_DIR}
     ${tblgen_includes}
     ${LLVM_TABLEGEN_FLAGS}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     ${tblgen_change_flag}
     ${additional_cmdline}
     # The file in LLVM_TARGET_DEFINITIONS may be not in the current