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

Removes one subprocess and one temp file from the build for each tablegen
invocation.

No intended behavior change.

https://reviews.llvm.org/D45899

llvm-svn: 330742

1 files changed, 16 insertions, 5 deletions

diff --git a/llvm/lib/TableGen/Main.cpp b/llvm/lib/TableGen/Main.cpp
index be35f89..d230300 100644
--- a/llvm/lib/TableGen/Main.cpp
+++ b/llvm/lib/TableGen/Main.cpp
@@ -96,8 +96,21 @@ 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 Out(OutputFilename, EC, sys::fs::F_Text);
+  ToolOutputFile OutFile(OutputFilename, EC, sys::fs::F_Text);
   if (EC)
     return reportError(argv0, "error opening " + OutputFilename + ":" +
                                   EC.message() + "\n");
@@ -105,14 +118,12 @@ int llvm::TableGenMain(char *argv0, TableGenMainFn *MainFn) {
     if (int Ret = createDependencyFile(Parser, argv0))
       return Ret;
   }
-
-  if (MainFn(Out.os(), Records))
-    return 1;
+  OutFile.os() << Out.str();
 
   if (ErrorsPrinted > 0)
     return reportError(argv0, Twine(ErrorsPrinted) + " errors.\n");
 
   // Declare success.
-  Out.keep();
+  OutFile.keep();
   return 0;
 }