[build] Only rewrite TableGen files if they've changed

[harlanhaskins]

Summary

Previously, re-running CMake caused all the TableGen files to be rebuilt, which
invalidated headers that used them. Instead, only overwrite files if they've
changed.

llvm/cmake/modules/TableGen.cmake

 # LLVM_TARGET_DEFINITIONS must contain the name of the .td file to process.
 # Extra parameters for `tblgen' may come after `ofn' parameter.
 # Adds the name of the generated file to TABLEGEN_OUTPUT.
 
 include(LLVMExternalProjectUtils)
 
 if(LLVM_MAIN_INCLUDE_DIR)
   set(LLVM_TABLEGEN_FLAGS -I ${LLVM_MAIN_INCLUDE_DIR})
 endif()
 
 function(tablegen project ofn)
   # Validate calling context.
   if(NOT ${project}_TABLEGEN_EXE)
     message(FATAL_ERROR "${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)
     # Make output path relative to build.ninja, assuming located on
     # ${CMAKE_BINARY_DIR}.
     # CMake emits build targets as relative paths but Ninja doesn't identify
     # absolute path (in *.d) as relative path (in build.ninja)
     # 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
       )
     set(local_tds)
     set(global_tds)
   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})
     set(LLVM_TARGET_DEFINITIONS_ABSOLUTE ${LLVM_TARGET_DEFINITIONS})
   else()
     set(LLVM_TARGET_DEFINITIONS_ABSOLUTE
       ${CMAKE_CURRENT_SOURCE_DIR}/${LLVM_TARGET_DEFINITIONS})
   endif()
function(tablegen project ofn)
   # a tablegen change, as cmake does not propagate file-level dependencies
   # of custom targets. See the following ticket for more information:
   # https://cmake.org/Bug/view.php?id=15858
   # The dependency on both, the target and the file, produces the same
   # dependency twice in the result file when
   # ("${${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}
+      -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:
     DEPENDS ${${project}_TABLEGEN_TARGET} ${${project}_TABLEGEN_EXE}
       ${local_tds} ${global_tds}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     COMMENT "Building ${ofn}..."
     )