[CMake][Compiler-rt] Compute LLVM_MAIN_SRC_DIR assuming the monorepo

layout.

When doing a standalone compiler-rt build we currently rely on
getting information from the `llvm-config` binary. Previously
we would rely on calling `llvm-config --src-root` to find the
LLVM sources. Unfortunately the returned path could easily be wrong
if the sources were built on another machine.

Now that compiler-rt is part of a monorepo we can easily fix this
problem by finding the LLVM source tree next to `compiler-rt` in
the monorepo. We do this regardless of whether or not the `llvm-config`
binary is available which moves us one step closer to not requiring
`llvm-config` to be available.

To try avoid anyone breaking anyone who relies on the current behavior,
if the path assuming the monorepo layout doesn't exist we invoke
`llvm-config --src-root` to get the path. A deprecation warning is
emitted if this path is taken because we should remove this path
in the future given that other runtimes already assume the monorepo
layout.

We also now emit a warning if `LLVM_MAIN_SRC_DIR` does not exist.
The intention is that this should be a hard error in future but
to avoid breaking existing users we'll keep this as a warning
for now.

rdar://76016632

Differential Revision: https://reviews.llvm.org/D99620

(cherry picked from commit fa818bb)

Author: danliew-apple

compiler-rt/cmake/Modules/CompilerRTUtils.cmake

@@ -209,6 +209,57 @@ macro(detect_target_arch)
   endif()
 endmacro()
 
+function(get_compiler_rt_root_source_dir ROOT_DIR_VAR)
+  # Compute the path to the root of the Compiler-RT source tree
+  # regardless of how the project was configured.
+  #
+  # This function is useful because using `${CMAKE_SOURCE_DIR}`
+  # is error prone due to the numerous ways Compiler-RT can be
+  # configured.
+  #
+  # `ROOT_DIR_VAR` - the name of the variable to write the result to.
+  #
+  # TODO(dliew): When CMake min version is 3.17 or newer use
+  # `CMAKE_CURRENT_FUNCTION_LIST_DIR` instead.
+  if ("${ROOT_DIR_VAR}" STREQUAL "")
+    message(FATAL_ERROR "ROOT_DIR_VAR cannot be empty")
+  endif()
+
+  # Compiler-rt supports different source root paths.
+  # Handle each case here.
+  set(PATH_TO_COMPILER_RT_SOURCE_ROOT "")
+  if (DEFINED CompilerRTBuiltins_SOURCE_DIR)
+    # Compiler-RT Builtins standalone build.
+    # `llvm-project/compiler-rt/lib/builtins`
+    set(PATH_TO_COMPILER_RT_SOURCE_ROOT "${CompilerRTBuiltins_SOURCE_DIR}/../../")
+  elseif(DEFINED CompilerRT_SOURCE_DIR)
+    # Compiler-RT standalone build.
+    # `llvm-project/compiler-rt`
+    set(PATH_TO_COMPILER_RT_SOURCE_ROOT "${CompilerRT_SOURCE_DIR}")
+  elseif (EXISTS "${CMAKE_SOURCE_DIR}/../compiler-rt")
+    # In tree build with LLVM as the root project.
+    # See `llvm-project/projects/`.
+    # Assumes monorepo layout.
+    set(PATH_TO_COMPILER_RT_SOURCE_ROOT "${CMAKE_SOURCE_DIR}/../compiler-rt")
+  else()
+    message(FATAL_ERROR "Unhandled Compiler-RT source root configuration.")
+  endif()
+
+  get_filename_component(ROOT_DIR "${PATH_TO_COMPILER_RT_SOURCE_ROOT}" ABSOLUTE)
+  if (NOT EXISTS "${ROOT_DIR}")
+    message(FATAL_ERROR "Path \"${ROOT_DIR}\" doesn't exist")
+  endif()
+
+  # Sanity check: Make sure we can locate the current source file via the
+  # computed path.
+  set(PATH_TO_CURRENT_FILE "${ROOT_DIR}/cmake/Modules/CompilerRTUtils.cmake")
+  if (NOT EXISTS "${PATH_TO_CURRENT_FILE}")
+    message(FATAL_ERROR "Could not find \"${PATH_TO_CURRENT_FILE}\"")
+  endif()
+
+  set("${ROOT_DIR_VAR}" "${ROOT_DIR}" PARENT_SCOPE)
+endfunction()
+
 macro(load_llvm_config)
   if (NOT LLVM_CONFIG_PATH)
     find_program(LLVM_CONFIG_PATH "llvm-config"
@@ -219,6 +270,19 @@ macro(load_llvm_config)
                       "Reconfigure with -DLLVM_CONFIG_PATH=path/to/llvm-config.")
     endif()
   endif()
+
+  # Compute path to LLVM sources assuming the monorepo layout.
+  # We don't set `LLVM_MAIN_SRC_DIR` directly to avoid overriding a user provided
+  # CMake cache value.
+  get_compiler_rt_root_source_dir(COMPILER_RT_ROOT_SRC_PATH)
+  get_filename_component(LLVM_MAIN_SRC_DIR_DEFAULT "${COMPILER_RT_ROOT_SRC_PATH}/../llvm" ABSOLUTE)
+  if (NOT EXISTS "${LLVM_MAIN_SRC_DIR_DEFAULT}")
+    # TODO(dliew): Remove this legacy fallback path.
+    message(WARNING
+      "LLVM source tree not found at \"${LLVM_MAIN_SRC_DIR_DEFAULT}\". "
+      "You are not using the monorepo layout. This configuration is DEPRECATED.")
+  endif()
+
   if (LLVM_CONFIG_PATH)
     execute_process(
       COMMAND ${LLVM_CONFIG_PATH} "--obj-root" "--bindir" "--libdir" "--src-root" "--includedir"
@@ -236,10 +300,20 @@ macro(load_llvm_config)
 
     set(LLVM_BINARY_DIR ${BINARY_DIR} CACHE PATH "Path to LLVM build tree")
     set(LLVM_LIBRARY_DIR ${LIBRARY_DIR} CACHE PATH "Path to llvm/lib")
-    set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
     set(LLVM_TOOLS_BINARY_DIR ${TOOLS_BINARY_DIR} CACHE PATH "Path to llvm/bin")
     set(LLVM_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Paths to LLVM headers")
 
+    if (NOT EXISTS "${LLVM_MAIN_SRC_DIR_DEFAULT}")
+      # TODO(dliew): Remove this legacy fallback path.
+      message(WARNING
+        "Consulting llvm-config for the LLVM source path "
+        "as a fallback. This behavior will be removed in the future.")
+      # We don't set `LLVM_MAIN_SRC_DIR` directly to avoid overriding a user
+      # provided CMake cache value.
+      set(LLVM_MAIN_SRC_DIR_DEFAULT "${MAIN_SRC_DIR}")
+      message(STATUS "Using LLVM source path (${LLVM_MAIN_SRC_DIR_DEFAULT}) from llvm-config")
+    endif()
+
     # Detect if we have the LLVMXRay and TestingSupport library installed and
     # available from llvm-config.
     execute_process(
@@ -305,6 +379,20 @@ macro(load_llvm_config)
     set(LLVM_LIBRARY_OUTPUT_INTDIR
       ${LLVM_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib${LLVM_LIBDIR_SUFFIX})
   endif()
+
+  # Finally set the cache variable now that `llvm-config` has also had a chance
+  # to set `LLVM_MAIN_SRC_DIR_DEFAULT`.
+  set(LLVM_MAIN_SRC_DIR "${LLVM_MAIN_SRC_DIR_DEFAULT}" CACHE PATH "Path to LLVM source tree")
+  message(STATUS "LLVM_MAIN_SRC_DIR: \"${LLVM_MAIN_SRC_DIR}\"")
+  if (NOT EXISTS "${LLVM_MAIN_SRC_DIR}")
+    # TODO(dliew): Make this a hard error
+    message(WARNING "LLVM_MAIN_SRC_DIR (${LLVM_MAIN_SRC_DIR}) does not exist. "
+                    "You can override the inferred path by adding "
+                    "`-DLLVM_MAIN_SRC_DIR=<path_to_llvm_src>` to your CMake invocation "
+                    "where `<path_to_llvm_src>` is the path to the `llvm` directory in "
+                    "the `llvm-project` repo. "
+                    "This will be treated as error in the future.")
+  endif()
 endmacro()
 
 macro(construct_compiler_rt_default_triple)