[Introspection] Dont emit json if unchanged.

[njames93]

Summary

Saves running the generate inc script in the, somewhat common, case where the json file doesn't need changing.

[steveire]

Great, thanks!

clang/lib/Tooling/DumpTool/ASTSrcLocProcessor.cpp

 //===- ASTSrcLocProcessor.cpp --------------------------------*- C++ -*----===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "ASTSrcLocProcessor.h"
 
 #include "clang/Frontend/CompilerInstance.h"
 #include "llvm/Support/JSON.h"
+#include "llvm/Support/MemoryBuffer.h"
 
 using namespace clang::tooling;
 using namespace llvm;
 using namespace clang::ast_matchers;
 
 ASTSrcLocProcessor::ASTSrcLocProcessor(StringRef JsonPath)
     : JsonPath(JsonPath) {
 
void WriteJSON(StringRef JsonPath, llvm::json::Object &&ClassInheritance,
   llvm::json::Object JsonObj;
 
   using llvm::json::toJSON;
 
   JsonObj["classInheritance"] = std::move(ClassInheritance);
   JsonObj["classesInClade"] = std::move(ClassesInClade);
   JsonObj["classEntries"] = std::move(ClassEntries);
 
+  llvm::json::Value JsonVal(std::move(JsonObj));
+
+  bool WriteChange = false;
+  std::string OutString;
+  if (auto ExistingOrErr = MemoryBuffer::getFile(JsonPath, /*IsText=*/true)) {
+    raw_string_ostream Out(OutString);
+    Out << formatv("{0:2}", JsonVal);
+    if (ExistingOrErr.get()->getBuffer() == Out.str())
+      return;
+    WriteChange = true;
+  }
+
   std::error_code EC;
   llvm::raw_fd_ostream JsonOut(JsonPath, EC, llvm::sys::fs::F_Text);
   if (EC)
     return;
 
-  llvm::json::Value JsonVal(std::move(JsonObj));
+  if (WriteChange)
+    JsonOut << OutString;
+  else
     JsonOut << formatv("{0:2}", JsonVal);
 }
 
 void ASTSrcLocProcessor::generate() {
   WriteJSON(JsonPath, ::toJSON(ClassInheritance), ::toJSON(ClassesInClade),
             ::toJSON(ClassEntries));
 }
 
 void ASTSrcLocProcessor::generateEmpty() { WriteJSON(JsonPath, {}, {}, {}); }