Index: tools/llvm-cov/CodeCoverage.cpp
===================================================================
--- tools/llvm-cov/CodeCoverage.cpp
+++ tools/llvm-cov/CodeCoverage.cpp
@@ -39,6 +39,7 @@
 
 #include <functional>
 #include <map>
+#include <sstream>
 #include <system_error>
 
 using namespace llvm;
@@ -536,6 +537,8 @@
 }
 
 int CodeCoverageTool::run(Command Cmd, int argc, const char **argv) {
+  errs() << "Started: ";
+  printTimestamp();
   cl::opt<std::string> CovFilename(
       cl::Positional, cl::desc("Covered executable or object file."));
 
@@ -769,6 +772,8 @@
   case Export:
     return doExport(argc, argv, commandLineParser);
   }
+  errs() << "Done: ";
+  printTimestamp();
   return 0;
 }
 
Index: tools/llvm-cov/CoverageExporterJson.h
===================================================================
--- tools/llvm-cov/CoverageExporterJson.h
+++ tools/llvm-cov/CoverageExporterJson.h
@@ -77,6 +77,10 @@
   void renderFile(const coverage::CoverageData &FileCoverage,
                   const FileCoverageSummary &FileReport);
 
+  /// \brief Render summary for a single file.
+  void renderFileSummary(const std::string &Filename,
+                         const FileCoverageSummary &FileReport);
+
   /// \brief Render a CoverageSegment.
   void renderSegment(const coverage::CoverageSegment &Segment);
 
Index: tools/llvm-cov/CoverageExporterJson.cpp
===================================================================
--- tools/llvm-cov/CoverageExporterJson.cpp
+++ tools/llvm-cov/CoverageExporterJson.cpp
@@ -118,6 +118,8 @@
 }
 
 void CoverageExporterJson::renderRoot() {
+  errs() << "renderRoot(), getting unique source files: ";
+  printTimestamp();
   std::vector<std::string> SourceFiles;
   for (StringRef SF : Coverage.getUniqueSourceFiles())
     SourceFiles.emplace_back(SF);
@@ -126,6 +128,8 @@
 
 void CoverageExporterJson::renderRoot(
     const std::vector<std::string> &SourceFiles) {
+  errs() << "renderRoot(sourceFiles): ";
+  printTimestamp();
   // Start Root of JSON object.
   emitDictStart();
 
@@ -142,9 +146,15 @@
   emitDictKey("files");
 
   FileCoverageSummary Totals = FileCoverageSummary("Totals");
+  errs() << "prepareFileReports: ";
+  printTimestamp();
   auto FileReports = CoverageReport::prepareFileReports(Coverage, Totals,
                                                         SourceFiles, Options);
+  errs() << "renderFiles: ";
+  printTimestamp();
   renderFiles(SourceFiles, FileReports);
+  errs() << "render leftovers: ";
+  printTimestamp();
 
   // Skip functions-level information for summary-only export mode.
   if (!Options.ExportSummaryOnly) {
@@ -210,8 +220,13 @@
 
   for (unsigned I = 0, E = SourceFiles.size(); I < E; ++I) {
     // Render the file.
-    auto FileCoverage = Coverage.getCoverageForFile(SourceFiles[I]);
-    renderFile(FileCoverage, FileReports[I]);
+    if (Options.ExportSummaryOnly) {
+      // Skip segments and expansions for summary-only export mode.
+      renderFileSummary(SourceFiles[I], FileReports[I]);
+    } else {
+      auto FileCoverage = Coverage.getCoverageForFile(SourceFiles[I]);
+      renderFile(FileCoverage, FileReports[I]);
+    }
   }
 
   // End List of Files.
@@ -226,30 +241,42 @@
 
   emitDictElement("filename", FileCoverage.getFilename());
 
-  // Skip segments and expansions for summary-only export mode.
-  if (!Options.ExportSummaryOnly) {
-    emitDictKey("segments");
+  emitDictKey("segments");
 
-    // Start List of Segments.
-    emitArrayStart();
+  // Start List of Segments.
+  emitArrayStart();
 
-    for (const auto &Segment : FileCoverage)
-      renderSegment(Segment);
+  for (const auto &Segment : FileCoverage)
+    renderSegment(Segment);
 
-    // End List of Segments.
-    emitArrayEnd();
+  // End List of Segments.
+  emitArrayEnd();
 
-    emitDictKey("expansions");
+  emitDictKey("expansions");
 
-    // Start List of Expansions.
-    emitArrayStart();
+  // Start List of Expansions.
+  emitArrayStart();
 
-    for (const auto &Expansion : FileCoverage.getExpansions())
-      renderExpansion(Expansion);
+  for (const auto &Expansion : FileCoverage.getExpansions())
+    renderExpansion(Expansion);
 
-    // End List of Expansions.
-    emitArrayEnd();
-  }
+  // End List of Expansions.
+  emitArrayEnd();
+
+  emitDictKey("summary");
+  renderSummary(FileReport);
+
+  // End File.
+  emitDictEnd();
+}
+
+void CoverageExporterJson::renderFileSummary(
+    const std::string &Filename,
+    const FileCoverageSummary &FileReport) {
+  // Start File.
+  emitDictStart();
+
+  emitDictElement("filename", Filename);
 
   emitDictKey("summary");
   renderSummary(FileReport);
Index: tools/llvm-cov/CoverageReport.h
===================================================================
--- tools/llvm-cov/CoverageReport.h
+++ tools/llvm-cov/CoverageReport.h
@@ -64,6 +64,8 @@
                          const CoverageFiltersMatchAll &Filters) const;
 };
 
+void printTimestamp();
+
 } // end namespace llvm
 
 #endif // LLVM_COV_COVERAGEREPORT_H
Index: tools/llvm-cov/CoverageReport.cpp
===================================================================
--- tools/llvm-cov/CoverageReport.cpp
+++ tools/llvm-cov/CoverageReport.cpp
@@ -20,6 +20,10 @@
 #include "llvm/Support/Threading.h"
 #include <numeric>
 
+#include <chrono>
+#include <ctime>
+#include <iostream>
+
 using namespace llvm;
 
 namespace {
@@ -443,4 +447,11 @@
   render(Totals, OS);
 }
 
+void printTimestamp() {
+  using namespace std::chrono;
+  system_clock::time_point today = system_clock::now();
+  time_t tt = system_clock::to_time_t ( today );
+  std::cerr << ctime(&tt) << "\n";
+}
+
 } // end namespace llvm