From 8a116e0b3efdd18f65e17bc7133a94a008c17235 Mon Sep 17 00:00:00 2001
From: Oscar Fuentes <ofv@wanadoo.es>
Date: Fri, 22 Oct 2010 16:20:10 +0000
Subject: 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
---
 cmake/modules/TableGen.cmake | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

(limited to 'cmake')

diff --git a/cmake/modules/TableGen.cmake b/cmake/modules/TableGen.cmake
index cf7cd1f..bb8fc13 100644
--- 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}..."
     )
-- 
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