Revert rL349624 : Let TableGen write output only if it changed, instead of doing so in cmake, attempt 2

Differential Revision: https://reviews.llvm.org/D55842
-----------------
As discussed on PR43385 this is causing Visual Studio msbuilds to perpetually rebuild all tablegen generated files

llvm-svn: 373338

llvm/cmake/modules/TableGen.cmake

@@ -23,7 +23,7 @@ function(tablegen project ofn)
     file(RELATIVE_PATH ofn_rel
       ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_BINARY_DIR}/${ofn})
     set(additional_cmdline
-      -o ${ofn_rel}
+      -o ${ofn_rel}.tmp
       -d ${ofn_rel}.d
       WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
       DEPFILE ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.d
@@ -34,7 +34,7 @@ function(tablegen project ofn)
     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}
+      -o ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
       )
   endif()
 
@@ -67,7 +67,8 @@ function(tablegen project ofn)
   # 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}
+  add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
+    # Generate tablegen output in a temporary file.
     COMMAND ${${project}_TABLEGEN_EXE} ${ARGN} -I ${CMAKE_CURRENT_SOURCE_DIR}
     ${LLVM_TABLEGEN_FLAGS}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
@@ -80,9 +81,20 @@ function(tablegen project ofn)
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     COMMENT "Building ${ofn}..."
     )
+  add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
+    # 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}
+    DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
+    COMMENT "Updating ${ofn}..."
+    )
 
   # `make clean' must remove all those generated files:
-  set_property(DIRECTORY APPEND PROPERTY ADDITIONAL_MAKE_CLEAN_FILES ${ofn})
+  set_property(DIRECTORY APPEND
+    PROPERTY ADDITIONAL_MAKE_CLEAN_FILES ${ofn}.tmp ${ofn})
 
   set(TABLEGEN_OUTPUT ${TABLEGEN_OUTPUT} ${CMAKE_CURRENT_BINARY_DIR}/${ofn} PARENT_SCOPE)
   set_source_files_properties(${CMAKE_CURRENT_BINARY_DIR}/${ofn} PROPERTIES

llvm/lib/TableGen/Main.cpp

@@ -99,39 +99,23 @@ int llvm::TableGenMain(char *argv0, TableGenMainFn *MainFn) {
   if (Parser.ParseFile())
     return 1;
 
-  // Write output to memory.
-  std::string OutString;
-  raw_string_ostream Out(OutString);
-  if (MainFn(Out, Records))
-    return 1;
-
-  // Always write the depfile, even if the main output hasn't changed.
-  // If it's missing, Ninja considers the output dirty.  If this was below
-  // the early exit below and someone deleted the .inc.d file but not the .inc
-  // file, tablegen would never write the depfile.
+  std::error_code EC;
+  ToolOutputFile Out(OutputFilename, EC, sys::fs::OF_None);
+  if (EC)
+    return reportError(argv0, "error opening " + OutputFilename + ":" +
+                                  EC.message() + "\n");
   if (!DependFilename.empty()) {
     if (int Ret = createDependencyFile(Parser, argv0))
       return Ret;
   }
 
-  // Only updates the real output file if there are any differences.
-  // This prevents recompilation of all the files depending on it if there
-  // aren't any.
-  if (auto ExistingOrErr = MemoryBuffer::getFile(OutputFilename))
-    if (std::move(ExistingOrErr.get())->getBuffer() == Out.str())
-      return 0;
-
-  std::error_code EC;
-  ToolOutputFile OutFile(OutputFilename, EC, sys::fs::OF_None);
-  if (EC)
-    return reportError(argv0, "error opening " + OutputFilename + ":" +
-                                  EC.message() + "\n");
-  OutFile.os() << Out.str();
+  if (MainFn(Out.os(), Records))
+    return 1;
 
   if (ErrorsPrinted > 0)
     return reportError(argv0, Twine(ErrorsPrinted) + " errors.\n");
 
   // Declare success.
-  OutFile.keep();
+  Out.keep();
   return 0;
 }

llvm/utils/gn/secondary/llvm/utils/TableGen/tablegen.gni

@@ -64,6 +64,11 @@ template("tablegen") {
     depfile = "$gen_output.d"
     td_file = rebase_path(td_file, root_build_dir)
 
+    # FIXME: The cmake build lets tablegen write to a temp file and then copies
+    # it over the final output only if it has changed, for ninja's restat
+    # optimization. Instead of doing that in cmake, llvm-tblgen should do this
+    # itself. r330742 tried this, but it caused problems. Fix those and reland,
+    # so that the gn build has the optimization too.
     args = [
              rebase_path(tblgen_executable, root_build_dir),
              "-I",