Only rewrite unit files when outdated

index-import.cpp

@@ -103,9 +103,27 @@ static const FileEntry *getFileEntry(FileManager &fileMgr, StringRef path) {
   }
 }
 
-static IndexUnitWriter remapUnit(const std::unique_ptr<IndexUnitReader> &reader,
-                                 const Remapper &remapper, FileManager &fileMgr,
-                                 ModuleNameScope &moduleNames) {
+// Returns true if the Unit file of given output file already exists and is
+// older than the input file.
+static bool isUnitUpToDate(StringRef outputFile, StringRef inputFile,
+                           IndexUnitWriter &writer) {
+  std::string error;
+  auto isUpToDateOpt =
+      writer.isUnitUpToDateForOutputFile(outputFile, inputFile, error);
+  if (!isUpToDateOpt.hasValue()) {
+    errs() << "error: failed file status check:\n" << error << "\n";
+    return false;
+  }
+
+  return *isUpToDateOpt;
+}
+
+// Returns None if the Unit file is already up to date
+static Optional<IndexUnitWriter>
+remapUnit(StringRef inputUnitPath,
+          const std::unique_ptr<IndexUnitReader> &reader,
+          const Remapper &remapper, FileManager &fileMgr,
+          ModuleNameScope &moduleNames) {
   // The set of remapped paths.
   auto workingDir = remapper.remap(reader->getWorkingDirectory());
   auto outputFile = remapper.remap(reader->getOutputFile());
@@ -122,6 +140,18 @@ static IndexUnitWriter remapUnit(const std::unique_ptr<IndexUnitReader> &reader,
       reader->isModuleUnit(), reader->isDebugCompilation(), reader->getTarget(),
       sysrootPath, moduleNames.getModuleInfo);
 
+  // Check if the unit file is already up to date
+  SmallString<256> remappedOutputFilePath;
+  if (outputFile[0] != '/') {
+    // Convert outputFile to absolute path
+    path::append(remappedOutputFilePath, workingDir, outputFile);
+  } else {
+    remappedOutputFilePath = outputFile;
+  }
+  if (isUnitUpToDate(remappedOutputFilePath, inputUnitPath, writer)) {
+    return None;
+  }
+
   reader->foreachDependency([&](const IndexUnitReader::DependencyInfo &info) {
     const auto name = info.UnitOrRecordName;
     const auto moduleNameRef = moduleNames.getReference(info.ModuleName);
@@ -285,13 +315,15 @@ static bool remapIndex(const Remapper &remapper,
     }
 
     ModuleNameScope moduleNames;
-    auto writer = remapUnit(reader, remapper, fileMgr, moduleNames);
+    auto writer = remapUnit(unitPath, reader, remapper, fileMgr, moduleNames);
 
-    std::string unitWriteError;
-    if (writer.write(unitWriteError)) {
-      errs() << "error: failed to write index store; " << unitWriteError
-             << "\n";
-      success = false;
+    if (writer.hasValue()) {
+      std::string unitWriteError;
+      if (writer->write(unitWriteError)) {
+        errs() << "error: failed to write index store; " << unitWriteError
+               << "\n";
+        success = false;
+      }
     }
   }