Teach CMake to only touch tablegen-generated files if anything

actually changed.

Patch by Frits van Bommel!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117102 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/cmake/modules/TableGen.cmake b/cmake/modules/TableGen.cmake
index cf7cd1f62e52215da84b8632f6ba9735ebdf8b3d..bb8fc13c547ef6bed3bc10dd3289e2f8d5da7a7d 100644 (file)
--- a/cmake/modules/TableGen.cmake
+++ b/cmake/modules/TableGen.cmake
@@ -13,10 +13,19 @@ macro(tablegen ofn)
       ${CMAKE_CURRENT_SOURCE_DIR}/${LLVM_TARGET_DEFINITIONS})
   endif()
   add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
+    # Generate tablegen output in a temporary file.
     COMMAND ${LLVM_TABLEGEN_EXE} ${ARGN} -I ${CMAKE_CURRENT_SOURCE_DIR}
     -I ${LLVM_MAIN_SRC_DIR}/lib/Target -I ${LLVM_MAIN_INCLUDE_DIR}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE} 
-    -o ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
+    -o ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
+    # Only update the real output file if there are any differences.
+    # This prevents recompilation of all the files depending on it if there
+    # aren't any.
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different
+        ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
+        ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
+    # Either way, we no longer need the temporary file.
+    COMMAND ${CMAKE_COMMAND} -E remove ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
     DEPENDS tblgen ${local_tds} ${global_tds}
     COMMENT "Building ${ofn}..."
     )