diff --git a/flang-rt/CMakeLists.txt b/flang-rt/CMakeLists.txt
index 50b8e83..319c496 100644
--- a/flang-rt/CMakeLists.txt
+++ b/flang-rt/CMakeLists.txt
@@ -138,6 +138,8 @@ cmake_path(NORMAL_PATH FLANG_RT_INSTALL_RESOURCE_LIB_PATH)
 # with this prefix will be forwarded in bootstrap builds.
 
 option(FLANG_RT_INCLUDE_TESTS "Generate build targets for the flang-rt unit and regression-tests." "${LLVM_INCLUDE_TESTS}")
+set(FLANG_RT_BUILTINS_LIBRARY "" CACHE FILEPATH
+    "Path to an existing compiler-rt builtins library to use when Flang-RT needs compiler builtins.")
 
 # Provide an interface to link against the LLVM libc/libc++ projects directly.
 set(FLANG_RT_SUPPORTED_PROVIDERS system llvm)
@@ -236,7 +238,9 @@ check_cxx_source_compiles(
 
 # Search for clang_rt.builtins library. Need in addition to msvcrt.
 if (WIN32)
-  find_compiler_rt_library(builtins FLANG_RT_BUILTINS_LIBRARY)
+  if (NOT FLANG_RT_BUILTINS_LIBRARY)
+    find_compiler_rt_library(builtins FLANG_RT_BUILTINS_LIBRARY)
+  endif ()
 endif ()
 
 # Build with _XOPEN_SOURCE on AIX to avoid errors caused by _ALL_SOURCE.
diff --git a/flang-rt/README.md b/flang-rt/README.md
index 4fe66a8..d56fbe1 100644
--- a/flang-rt/README.md
+++ b/flang-rt/README.md
@@ -138,6 +138,12 @@ CMake itself provide.
    When `OFF`, does not add any tests and unittests. The `check-flang-rt`
    build target will do nothing.
 
+ * `FLANG_RT_BUILTINS_LIBRARY` (path; default: `""`)
+
+   Path to an existing compiler-rt builtins library. When left empty, Flang-RT
+   uses the compiler to find the builtins library on platforms where one is
+   needed.
+
  * `FLANG_RUNTIME_F128_MATH_LIB` (default: `""`)
 
    Determines the implementation of `REAL(16)` math functions. If set to
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index fcbfed4..77e4e41 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -654,11 +654,19 @@ if(LLVM_EXPERIMENTAL_TARGETS_TO_BUILD STREQUAL "all")
   set(LLVM_EXPERIMENTAL_TARGETS_TO_BUILD ${LLVM_ALL_EXPERIMENTAL_TARGETS})
 endif()
 
+option(FLANG_USE_SYSTEM_COMPILER_RT
+  "Use an installed compiler-rt instead of adding compiler-rt to LLVM_ENABLE_RUNTIMES for Flang."
+  OFF)
+
 if("flang" IN_LIST LLVM_ENABLE_PROJECTS AND
    "AArch64" IN_LIST LLVM_TARGETS_TO_BUILD AND
    NOT ("compiler-rt" IN_LIST LLVM_ENABLE_RUNTIMES OR "compiler-rt" IN_LIST LLVM_ENABLE_PROJECTS))
-  message(STATUS "Enabling compiler-rt as a dependency of Flang")
-  list(APPEND LLVM_ENABLE_RUNTIMES "compiler-rt")
+  if(FLANG_USE_SYSTEM_COMPILER_RT)
+    message(STATUS "Using system compiler-rt for Flang")
+  else()
+    message(STATUS "Enabling compiler-rt as a dependency of Flang")
+    list(APPEND LLVM_ENABLE_RUNTIMES "compiler-rt")
+  endif()
 endif()
 
 set(LLVM_TARGETS_TO_BUILD