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

1 files changed, 8 insertions, 24 deletions

diff --git a/llvm/lib/TableGen/Main.cpp b/llvm/lib/TableGen/Main.cpp
index 34195c31a8b..bf97e37f324 100644
--- a/llvm/lib/TableGen/Main.cpp
+++ b/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;
 }