[PATCH] D60051: [build] Only rewrite TableGen files if they've changed

[Harlan Haskins via Phabricator via llvm-commits]

harlanhaskins created this revision.
harlanhaskins added a reviewer: beanz.
Herald added subscribers: llvm-commits, mgorny.
Herald added a project: LLVM.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D60051

Files:
  llvm/cmake/modules/TableGen.cmake


Index: llvm/cmake/modules/TableGen.cmake
===================================================================
--- llvm/cmake/modules/TableGen.cmake
+++ 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:


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D60051.193030.patch
Type: text/x-patch
Size: 2311 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20190331/e5e13112/attachment.bin>