diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -596,6 +596,19 @@
     llvm_externalize_debuginfo(${name})
     llvm_codesign(${name} ENTITLEMENTS ${ARG_ENTITLEMENTS})
   endif()
+  # clang and ninja use high-resolutions timestamps, but libtool
+  # doesn't, so the archive will often get an older timestamp than the
+  # last object that was added or updated.  To fix this, we add a
+  # custom command to touch archive after it's been built so that
+  # ninja won't rebuild it unnecessarily the next time it's run.
+  if(ARG_STATIC AND
+      CMAKE_GENERATOR STREQUAL "Ninja" AND
+      NOT CMAKE_SYSTEM_NAME MATCHES "Windows")
+    add_custom_command(TARGET ${name}
+      POST_BUILD
+      COMMAND touch ${LLVM_LIBRARY_DIR}/${CMAKE_STATIC_LIBRARY_PREFIX}${name}${CMAKE_STATIC_LIBRARY_SUFFIX}
+      )
+  endif()
 endfunction()
 
 function(add_llvm_install_targets target)