Revert r330742: Let TableGen write output only if it changed, instead of doing so in cmake.

This change causes us to re-run tablegen for every single target on
every single build. This is much, much worse than the problem being
fixed AFAICT.

On my system, it makes a clean rebuild of `llc` with nothing changed go
from .5s to over 8s. On systems with less parallelism, slower file
systems, or high process startup overhead this will be even more
extreme.

The only way I see this could be a win is in clean builds where we churn
the filesystem. But I think incremental rebuild is more important, and
so if we want to re-instate this, it needs to be done in a way that
doesn't trigger constant re-runs of tablegen.

llvm-svn: 331702

1 files changed, 5 insertions, 16 deletions

diff --git a/llvm/lib/TableGen/Main.cpp b/llvm/lib/TableGen/Main.cpp
index bcd7dce95a8..3a070162608 100644
--- a/llvm/lib/TableGen/Main.cpp
+++ b/llvm/lib/TableGen/Main.cpp
@@ -96,21 +96,8 @@ 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;
-
-  // 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::F_Text);
+  ToolOutputFile Out(OutputFilename, EC, sys::fs::F_Text);
   if (EC)
     return reportError(argv0, "error opening " + OutputFilename + ":" +
                                   EC.message() + "\n");
@@ -118,12 +105,14 @@ int llvm::TableGenMain(char *argv0, TableGenMainFn *MainFn) {
     if (int Ret = createDependencyFile(Parser, argv0))
       return Ret;
   }
-  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;
 }