diff --git a/llvm/cmake/modules/TableGen.cmake b/llvm/cmake/modules/TableGen.cmake
--- a/llvm/cmake/modules/TableGen.cmake
+++ b/llvm/cmake/modules/TableGen.cmake
@@ -14,6 +14,8 @@
     message(FATAL_ERROR "${project}_TABLEGEN_EXE not set")
   endif()
 
+  set(output_path)
+
   # Use depfile instead of globbing arbitrary *.td(s)
   # DEPFILE is available for Ninja Generator with CMake>=3.7.
   if(CMAKE_GENERATOR STREQUAL "Ninja" AND NOT CMAKE_VERSION VERSION_LESS 3.7)
@@ -24,8 +26,8 @@
     # Note that tblgen is executed on ${CMAKE_BINARY_DIR} as working directory.
     file(RELATIVE_PATH ofn_rel
       ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_BINARY_DIR}/${ofn})
+    set(output_path ${ofn_rel})
     set(additional_cmdline
-      -o ${ofn_rel}
       -d ${ofn_rel}.d
       WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
       DEPFILE ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.d
@@ -35,9 +37,8 @@
   else()
     file(GLOB local_tds "*.td")
     file(GLOB_RECURSE global_tds "${LLVM_MAIN_INCLUDE_DIR}/llvm/*.td")
-    set(additional_cmdline
-      -o ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
-      )
+    set(output_path ${CMAKE_CURRENT_BINARY_DIR}/${ofn})
+    set(additiona_cmdline)
   endif()
 
   if (IS_ABSOLUTE ${LLVM_TARGET_DEFINITIONS})
@@ -70,10 +71,18 @@
   # ("${${project}_TABLEGEN_TARGET}" STREQUAL "${${project}_TABLEGEN_EXE}")
   # but lets us having smaller and cleaner code here.
   add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
+    # Because we don't want to affect modification time of tablegen targets
+    # that have not changed, emit the tablegen output to a temp file and only
+    # overwrite an existing file if the output has changed.
     COMMAND ${${project}_TABLEGEN_EXE} ${ARGN} -I ${CMAKE_CURRENT_SOURCE_DIR}
-    ${LLVM_TABLEGEN_FLAGS}
-    ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
-    ${additional_cmdline}
+      -o ${output_path}.tmp
+      ${LLVM_TABLEGEN_FLAGS}
+      ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
+      ${additional_cmdline}
+    COMMAND
+      ${CMAKE_COMMAND} -E copy_if_different ${output_path}.tmp ${output_path}
+    COMMAND
+      ${CMAKE_COMMAND} -E remove ${output_path}.tmp
     # The file in LLVM_TARGET_DEFINITIONS may be not in the current
     # directory and local_tds may not contain it, so we must
     # explicitly list it here: