diff --git a/compiler-rt/cmake/Modules/CompilerRTUtils.cmake b/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
--- a/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
+++ b/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
@@ -219,6 +219,18 @@
                       "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_filename_component(LLVM_MAIN_SRC_DIR_DEFAULT "${CMAKE_CURRENT_SOURCE_DIR}/../llvm" ABSOLUTE)
+  if (NOT EXISTS "${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 +248,20 @@
 
     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 +327,20 @@
     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)