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
@@ -240,6 +240,21 @@
     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")
 
+    # `--src-root` might report a path does not exist (e.g. built on another machine).
+    # In this case we fallback on the LLVM sources living next to `compiler-rt` which
+    # is true in the compiler-rt is checked out as part of the monorepo.
+    if (NOT EXISTS "${LLVM_MAIN_SRC_DIR}")
+      # TODO(dliew): We should make this behavior the only behavior rather than
+      # a fallback once we know nobody relies on the old behavior.
+      message(WARNING "LLVM source directory (${LLVM_MAIN_SRC_DIR}) reported by llvm-config does not exist")
+      get_filename_component(LLVM_MAIN_SRC_DIR_REL "${CMAKE_CURRENT_SOURCE_DIR}/../llvm" ABSOLUTE)
+      if (NOT EXISTS "${LLVM_MAIN_SRC_DIR_REL}")
+        message(FATAL_ERROR "LLVM source directory (${LLVM_MAIN_SRC_DIR_REL}) does not exist")
+      endif()
+      set(LLVM_MAIN_SRC_DIR "${LLVM_MAIN_SRC_DIR_REL}")
+    endif()
+    message(STATUS "LLVM source directory: \"${LLVM_MAIN_SRC_DIR}\"")
+
     # Detect if we have the LLVMXRay and TestingSupport library installed and
     # available from llvm-config.
     execute_process(