[libc] Fix tablegen when using a runtimes build

[jhuber6]

Summary

When using LLVM_ENABLE_RUNTIMES=libc we need to perform a few extra
steps to include LLVM utilities similar to if we were performing a
standalone build. Libc depends on the tablegen utilities and the LLVM
libraries when performing a full build. When using an
LLVM_ENABLE_PROJECTS=libc build these are included as a part of the
greater LLVM build, but here we need to perform it maunally. This patch
should allow using LLVM_LIBC_FULL_BUILD=ON when building with
runtimes.

[sivachandra]

libc/CMakeLists.txt

17

IIRC, the only dependency we have outside of the libc directory is for tablegen. So, could we perhaps move this logic over to something more local to the places where tablegen is used? Also, we want to move away from using the LLVM_ prefix to libc's CMake variables - just use LIBC_ for the prefix. In this case, it can be more specific like LIBC_TABLEGEN_INCLUDE_DIRS.

[jhuber6]

libc/CMakeLists.txt

17

In the future we may need to have some more runtimes specific glue so I figured it was fine to put it up front similar to what most projects do for standalone builds. If you'd prefer it to be right next to where it's used that's fine. I can also change the name.

[sivachandra]

libc/CMakeLists.txt

17

Tablegen has been a nagging dependency which we want to stop using because it is not the most appropriate for our use case. So, at the least, having tablegen pieces confined to a smaller scope will help with future cleanups.

libc/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 # Default to C++17
 set(CMAKE_CXX_STANDARD 17)
 
 # Use old version of target_sources command which converts the source
 # file paths to full paths.
 cmake_policy(SET CMP0076 OLD)
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 # The top-level sourse and binary directories.
 set(LIBC_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBC_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 # The top-level directory in which libc is being built.
 set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
+# For a runtimes build we need to manually include tablgen and LLVM directories.
+if ("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
+  include(TableGen)
+  set(LLVM_LIBC_INCLUDE_DIRS ${LLVM_MAIN_INCLUDE_DIR} ${LLVM_BINARY_DIR}/include)
+endif()
+
 # Path libc/scripts directory.
 set(LIBC_BUILD_SCRIPTS_DIR "${LIBC_SOURCE_DIR}/utils/build_scripts")
 
 set(LIBC_TARGET_OS ${CMAKE_SYSTEM_NAME})
 string(TOLOWER ${LIBC_TARGET_OS} LIBC_TARGET_OS)
 
 # Defines LIBC_TARGET_ARCHITECTURE and associated macros.
 include(LLVMLibCArchitectures)

libc/utils/HdrGen/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS Support)
 
 add_tablegen(libc-hdrgen LIBC
   Command.h
   Command.cpp
   Generator.cpp
   Generator.h
   IncludeFileCommand.cpp
   IncludeFileCommand.h
   Main.cpp
   PublicAPICommand.cpp
   PublicAPICommand.h
 )
 
-target_include_directories(libc-hdrgen PRIVATE ${LIBC_SOURCE_DIR})
+target_include_directories(libc-hdrgen PRIVATE ${LIBC_SOURCE_DIR} ${LLVM_LIBC_INCLUDE_DIRS})
 target_link_libraries(libc-hdrgen PRIVATE LibcTableGenUtil)
 
 add_subdirectory(PrototypeTestGen)

libc/utils/LibcTableGenUtil/CMakeLists.txt

 add_llvm_library(
   LibcTableGenUtil
   APIIndexer.cpp
   APIIndexer.h
   LINK_COMPONENTS Support TableGen
 )
-target_include_directories(LibcTableGenUtil PUBLIC ${LIBC_SOURCE_DIR})
+target_include_directories(LibcTableGenUtil PUBLIC ${LIBC_SOURCE_DIR} ${LLVM_LIBC_INCLUDE_DIRS})