diff --git a/llvm/tools/llvm-cov/SourceCoverageView.h b/llvm/tools/llvm-cov/SourceCoverageView.h
--- a/llvm/tools/llvm-cov/SourceCoverageView.h
+++ b/llvm/tools/llvm-cov/SourceCoverageView.h
@@ -99,7 +99,7 @@
   CoveragePrinter(const CoverageViewOptions &Opts) : Opts(Opts) {}
 
   /// Return `OutputDir/ToplevelDir/Path.Extension`. If \p InToplevel is
-  /// false, skip the ToplevelDir component. If \p Relative is false, skip the
+  /// true, skip the ToplevelDir component. If \p Relative is true, skip the
   /// OutputDir component.
   std::string getOutputPath(StringRef Path, StringRef Extension,
                             bool InToplevel, bool Relative = true) const;
diff --git a/llvm/tools/llvm-cov/SourceCoverageView.cpp b/llvm/tools/llvm-cov/SourceCoverageView.cpp
--- a/llvm/tools/llvm-cov/SourceCoverageView.cpp
+++ b/llvm/tools/llvm-cov/SourceCoverageView.cpp
@@ -78,7 +78,7 @@
   case CoverageViewOptions::OutputFormat::Text:
     return std::make_unique<CoveragePrinterText>(Opts);
   case CoverageViewOptions::OutputFormat::HTML:
-    return std::make_unique<CoveragePrinterHTML>(Opts);
+    return std::make_unique<CoveragePrinterHTMLDirectory>(Opts);
   case CoverageViewOptions::OutputFormat::Lcov:
     // Unreachable because CodeCoverage.cpp should terminate with an error
     // before we get here.
diff --git a/llvm/tools/llvm-cov/SourceCoverageViewHTML.h b/llvm/tools/llvm-cov/SourceCoverageViewHTML.h
--- a/llvm/tools/llvm-cov/SourceCoverageViewHTML.h
+++ b/llvm/tools/llvm-cov/SourceCoverageViewHTML.h
@@ -19,6 +19,8 @@
 
 using namespace coverage;
 
+class ThreadPool;
+
 struct FileCoverageSummary;
 
 /// A coverage printer for html output.
@@ -44,6 +46,38 @@
                               const FileCoverageSummary &FCS) const;
 };
 
+/// A coverage printer for html output, but generates index files in every
+/// subdirectory to show a hierarchical view.
+class CoveragePrinterHTMLDirectory : public CoveragePrinterHTML {
+public:
+  using CoveragePrinterHTML::CoveragePrinterHTML;
+
+  Error createIndexFile(ArrayRef<std::string> SourceFiles,
+                        const coverage::CoverageMapping &Coverage,
+                        const CoverageFiltersMatchAll &Filters) override;
+
+private:
+  // Member variables for avoiding passing arguments repeatedly in recursive
+  // calls.
+  const coverage::CoverageMapping *CoverageNow;
+  const CoverageFiltersMatchAll *FiltersNow;
+
+  ThreadPool *PoolNow; // For calling CoverageReport::prepareSingleFileReport
+                       // asynchronously in createSubIndexFile.
+  unsigned RootLCPNow; // The common prefix length of all source files.
+
+  // Filter out files in LCP directory, group others by subdirectory and call
+  // recursively on them. The size of \p Files must be greater than 0. \p LCP
+  // is the common prefix length in chars/bytes. The FileCoverageSummary of
+  // this directory will be added to \p Totals.
+  Error createSubIndexFile(const ArrayRef<StringRef> &Files, const unsigned LCP,
+                           FileCoverageSummary *Totals) const;
+
+  std::string buildRelLinkToFile(StringRef RelPath, bool IsDir) const;
+
+  std::string buildNavLink(StringRef LCPath, unsigned LCP) const;
+};
+
 /// A code coverage view which supports html-based rendering.
 class SourceCoverageViewHTML : public SourceCoverageView {
   void renderViewHeader(raw_ostream &OS) override;
diff --git a/llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp b/llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp
--- a/llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp
+++ b/llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp
@@ -10,12 +10,15 @@
 ///
 //===----------------------------------------------------------------------===//
 
-#include "CoverageReport.h"
 #include "SourceCoverageViewHTML.h"
+#include "CoverageReport.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/ThreadPool.h"
+#include <forward_list>
+#include <map>
 #include <optional>
 
 using namespace llvm;
@@ -65,7 +68,7 @@
 }
 
 const char *BeginHeader =
-  "<head>"
+    "<head>"
     "<meta name='viewport' content='width=device-width,initial-scale=1'>"
     "<meta charset='UTF-8'>";
 
@@ -272,11 +275,96 @@
   OS << EndHeader << "<body>";
 }
 
+void emitTableRow(raw_ostream &OS, const CoverageViewOptions &Opts,
+                  const std::string &FirstCol, const FileCoverageSummary &FCS,
+                  bool IsTotals) {
+  SmallVector<std::string, 8> Columns;
+
+  // Format a coverage triple and add the result to the list of columns.
+  auto AddCoverageTripleToColumn =
+      [&Columns, &Opts](unsigned Hit, unsigned Total, float Pctg) {
+        std::string S;
+        {
+          raw_string_ostream RSO{S};
+          if (Total)
+            RSO << format("%*.2f", 7, Pctg) << "% ";
+          else
+            RSO << "- ";
+          RSO << '(' << Hit << '/' << Total << ')';
+        }
+        const char *CellClass = "column-entry-yellow";
+        if (Pctg >= Opts.HighCovWatermark)
+          CellClass = "column-entry-green";
+        else if (Pctg < Opts.LowCovWatermark)
+          CellClass = "column-entry-red";
+        Columns.emplace_back(tag("td", tag("pre", S), CellClass));
+      };
+
+  Columns.emplace_back(tag("td", tag("pre", FirstCol)));
+  AddCoverageTripleToColumn(FCS.FunctionCoverage.getExecuted(),
+                            FCS.FunctionCoverage.getNumFunctions(),
+                            FCS.FunctionCoverage.getPercentCovered());
+  if (Opts.ShowInstantiationSummary)
+    AddCoverageTripleToColumn(FCS.InstantiationCoverage.getExecuted(),
+                              FCS.InstantiationCoverage.getNumFunctions(),
+                              FCS.InstantiationCoverage.getPercentCovered());
+  AddCoverageTripleToColumn(FCS.LineCoverage.getCovered(),
+                            FCS.LineCoverage.getNumLines(),
+                            FCS.LineCoverage.getPercentCovered());
+  if (Opts.ShowRegionSummary)
+    AddCoverageTripleToColumn(FCS.RegionCoverage.getCovered(),
+                              FCS.RegionCoverage.getNumRegions(),
+                              FCS.RegionCoverage.getPercentCovered());
+  if (Opts.ShowBranchSummary)
+    AddCoverageTripleToColumn(FCS.BranchCoverage.getCovered(),
+                              FCS.BranchCoverage.getNumBranches(),
+                              FCS.BranchCoverage.getPercentCovered());
+
+  if (IsTotals)
+    OS << tag("tr", join(Columns.begin(), Columns.end(), ""), "light-row-bold");
+  else
+    OS << tag("tr", join(Columns.begin(), Columns.end(), ""), "light-row");
+}
+
 void emitEpilog(raw_ostream &OS) {
   OS << "</body>"
      << "</html>";
 }
 
+/// Determine the length of the longest redundant prefix of the substrs starts
+/// from \p LCP in \p Paths. \p Paths can't be empty. If there's only one path
+/// in \p Paths, the length of the path is returned. Note this is differnet from
+/// the behavior for the function of the same name in CoverageReport.cpp .
+unsigned getRedundantPrefixLen(ArrayRef<StringRef> Paths, unsigned LCP = 0) {
+  assert(!Paths.empty() && "Paths must have at least one element");
+
+  auto Iter = Paths.begin();
+  auto IterE = Paths.end();
+  auto First = Iter->substr(LCP);
+  SmallVector<StringRef, 16> CommonComponents{sys::path::begin(First),
+                                              sys::path::end(First)};
+  unsigned Len = First.size() - LCP;
+  while (++Iter != IterE) {
+    auto Substr = Iter->substr(LCP);
+    auto I = sys::path::begin(Substr);
+    auto E = sys::path::end(Substr);
+
+    unsigned Index = 0;
+    while (I != E) {
+      if (Index >= CommonComponents.size())
+        break;
+      if (CommonComponents[Index] != *I) {
+        CommonComponents.resize(Index);
+        Len = I - sys::path::begin(Substr);
+        break;
+      }
+      ++Index, ++I;
+    }
+  }
+
+  return Len;
+}
+
 } // anonymous namespace
 
 Expected<CoveragePrinter::OwnedStream>
@@ -334,29 +422,7 @@
 /// false, link the summary to \p SF.
 void CoveragePrinterHTML::emitFileSummary(raw_ostream &OS, StringRef SF,
                                           const FileCoverageSummary &FCS,
-                                          bool IsTotals) const {
-  SmallVector<std::string, 8> Columns;
-
-  // Format a coverage triple and add the result to the list of columns.
-  auto AddCoverageTripleToColumn =
-      [&Columns, this](unsigned Hit, unsigned Total, float Pctg) {
-        std::string S;
-        {
-          raw_string_ostream RSO{S};
-          if (Total)
-            RSO << format("%*.2f", 7, Pctg) << "% ";
-          else
-            RSO << "- ";
-          RSO << '(' << Hit << '/' << Total << ')';
-        }
-        const char *CellClass = "column-entry-yellow";
-        if (Pctg >= Opts.HighCovWatermark)
-          CellClass = "column-entry-green";
-        else if (Pctg < Opts.LowCovWatermark)
-          CellClass = "column-entry-red";
-        Columns.emplace_back(tag("td", tag("pre", S), CellClass));
-      };
-
+                                          bool IsTotals) const {  
   // Simplify the display file path, and wrap it in a link if requested.
   std::string Filename;
   if (IsTotals) {
@@ -365,30 +431,7 @@
     Filename = buildLinkToFile(SF, FCS);
   }
 
-  Columns.emplace_back(tag("td", tag("pre", Filename)));
-  AddCoverageTripleToColumn(FCS.FunctionCoverage.getExecuted(),
-                            FCS.FunctionCoverage.getNumFunctions(),
-                            FCS.FunctionCoverage.getPercentCovered());
-  if (Opts.ShowInstantiationSummary)
-    AddCoverageTripleToColumn(FCS.InstantiationCoverage.getExecuted(),
-                              FCS.InstantiationCoverage.getNumFunctions(),
-                              FCS.InstantiationCoverage.getPercentCovered());
-  AddCoverageTripleToColumn(FCS.LineCoverage.getCovered(),
-                            FCS.LineCoverage.getNumLines(),
-                            FCS.LineCoverage.getPercentCovered());
-  if (Opts.ShowRegionSummary)
-    AddCoverageTripleToColumn(FCS.RegionCoverage.getCovered(),
-                              FCS.RegionCoverage.getNumRegions(),
-                              FCS.RegionCoverage.getPercentCovered());
-  if (Opts.ShowBranchSummary)
-    AddCoverageTripleToColumn(FCS.BranchCoverage.getCovered(),
-                              FCS.BranchCoverage.getNumBranches(),
-                              FCS.BranchCoverage.getPercentCovered());
-
-  if (IsTotals)
-    OS << tag("tr", join(Columns.begin(), Columns.end(), ""), "light-row-bold");
-  else
-    OS << tag("tr", join(Columns.begin(), Columns.end(), ""), "light-row");
+  emitTableRow(OS, Opts, Filename, FCS, IsTotals);
 }
 
 Error CoveragePrinterHTML::createIndexFile(
@@ -465,6 +508,242 @@
   return Error::success();
 }
 
+Error CoveragePrinterHTMLDirectory::createIndexFile(
+    ArrayRef<std::string> SourceFiles, const CoverageMapping &Coverage,
+    const CoverageFiltersMatchAll &Filters) {
+  // The createSubIndexFile function only works when SourceFiles is
+  // not empty. So we fallback to CoveragePrinterHTML when it is.
+  if (SourceFiles.empty())
+    return CoveragePrinterHTML::createIndexFile(SourceFiles, Coverage, Filters);
+
+  // Emit the default stylesheet.
+  auto CSSOrErr = createOutputStream("style", "css", /*InToplevel=*/true);
+  if (Error E = CSSOrErr.takeError())
+    return E;
+
+  OwnedStream CSS = std::move(CSSOrErr.get());
+  CSS->operator<<(CSSForCoverage);
+
+  // Emit index files in every subdirectory.
+  std::vector<StringRef> Files(SourceFiles.begin(), SourceFiles.end());
+  ThreadPoolStrategy PoolS = hardware_concurrency(Opts.NumThreads);
+  if (Opts.NumThreads == 0) {
+    PoolS = heavyweight_hardware_concurrency(Files.size());
+    PoolS.Limit = true;
+  }
+  ThreadPool Pool(PoolS); // The above initialization code was copied from
+                          // CoverageReport::prepareFileReports.
+
+  if (SourceFiles.size() == 1)
+    RootLCPNow = sys::path::parent_path(SourceFiles.front()).size() + 1;
+    // We handle this case specially. If there's only one file in total,
+    // its parent directory is considered as the LCP.
+  else
+    RootLCPNow = getRedundantPrefixLen(Files);
+  auto LCPath = SourceFiles.front().substr(0, RootLCPNow);
+
+  CoverageNow = &Coverage;
+  FiltersNow = &Filters;
+  PoolNow = &Pool;
+  FileCoverageSummary RootTotal({}); // The root total is useless and ignored.
+  if (auto E = createSubIndexFile(Files, RootLCPNow, &RootTotal))
+    return E;
+
+  // Emit the top level index file. Top level index file is just a redirection
+  // to the index file in the LCP directory.
+  auto OSOrErr = createOutputStream("index", "html", /*InToplevel=*/true);
+  if (auto E = OSOrErr.takeError())
+    return E;
+  auto OS = std::move(OSOrErr.get());
+  auto LCPIndexFilePath =
+      getOutputPath(LCPath + "index", "html", /*InToplevel=*/false);
+  *OS.get() << R"(<!DOCTYPE html>
+  <html>
+    <head>
+      <meta http-equiv="Refresh" content="0; url=')"
+            << LCPIndexFilePath << R"('" />
+    </head>
+    <body></body>
+  </html>
+  )";
+
+  return Error::success();
+}
+
+Error CoveragePrinterHTMLDirectory::createSubIndexFile(
+    const ArrayRef<StringRef> &Files, const unsigned LCP,
+    FileCoverageSummary *Totals) const {
+  assert(!Files.empty() && "Only works when Files is not empty");
+
+  auto LCPath = Files.front().substr(0, LCP).str();
+  auto OSOrErr =
+      createOutputStream(LCPath + "index", "html", /*InToplevel=*/false);
+  if (auto E = OSOrErr.takeError())
+    return E;
+  auto OS = std::move(OSOrErr.get());
+  raw_ostream &OSRef = *OS.get();
+
+  assert(Opts.hasOutputDirectory() && "No output directory for index file");
+  auto IndexHtmlPath = getOutputPath(LCPath + "index", "html",
+                                     /*InToplevel=*/false, /*Relative=*/false);
+  emitPrelude(OSRef, Opts, getPathToStyle(IndexHtmlPath));
+
+  // Emit some basic information about the coverage report.
+  if (Opts.hasProjectTitle())
+    OSRef << tag(ProjectTitleTag, escape(Opts.ProjectTitle, Opts));
+  auto NavLink = buildNavLink(LCPath, LCP);
+  OSRef << tag(ReportTitleTag, "Coverage Report (" + NavLink + ")");
+  if (Opts.hasCreatedTime())
+    OSRef << tag(CreatedTimeTag, escape(Opts.CreatedTimeStr, Opts));
+
+  // Emit a link to some documentation.
+  OSRef << tag("p", "Click " +
+                        a("http://clang.llvm.org/docs/"
+                          "SourceBasedCodeCoverage.html#interpreting-reports",
+                          "here") +
+                        " for information about interpreting this report.");
+
+  // Emit a table containing links to reports for each file in the covmapping.
+  // Exclude files which don't contain any regions.
+  OSRef << BeginCenteredDiv << BeginTable;
+  emitColumnLabelsForIndex(OSRef, Opts);
+
+  // Filter out files in current directory and group others by subdiretory.
+  // std::map is used to keep entries in order.
+  std::map<StringRef, FileCoverageSummary> FilesHere;
+  std::map<StringRef, std::vector<StringRef>> DirsHere;
+  for (auto &&File : Files) {
+    auto SubPath = File.substr(LCPath.size());
+
+    auto I = sys::path::begin(SubPath);
+    auto E = sys::path::end(SubPath);
+    assert(I != E && "Such case should have been filtered out in the caller");
+
+    auto Name = *I;
+    if (++I == E) {
+      auto Iter = FilesHere.emplace(Name, File).first;
+      PoolNow->async(&CoverageReport::prepareSingleFileReport, File,
+                     CoverageNow, Opts, LCP, &Iter->second, FiltersNow);
+    }
+
+    else
+      DirsHere[Name].push_back(File);
+  }
+
+  // Emit subdirectories first to make them at the top of the table.
+  FileCoverageSummary TotalsHere(LCPath);
+  for (auto &&Dir : DirsHere) {
+    if (Dir.second.size() == 1) {
+      // If there's only one file in that subdirectory, we don't bother to
+      // create index file for it. Instead, we show the file report directly at
+      // this level.
+      auto RelPath = Dir.second.front().substr(LCP);
+      FileCoverageSummary FCS(RelPath);
+      CoverageReport::prepareSingleFileReport(Dir.second[0], CoverageNow, Opts,
+                                              LCP, &FCS, FiltersNow);
+      emitTableRow(OSRef, Opts, buildRelLinkToFile(RelPath, /*IsDir=*/false),
+                   FCS, /*IsTotals=*/false);
+      TotalsHere += FCS;
+    }
+
+    else {
+      auto SubDirLCP = getRedundantPrefixLen(Dir.second, LCP);
+      auto RelPath = Dir.second.front().substr(LCP, SubDirLCP);
+      FileCoverageSummary FCS(RelPath);
+      if (auto E =
+              createSubIndexFile(Dir.second, LCP + SubDirLCP, &FCS))
+        return E;
+      emitTableRow(OSRef, Opts, buildRelLinkToFile(RelPath, /*IsDir=*/true),
+                   FCS, /*IsTotals=*/false);
+      TotalsHere += FCS;
+    }
+  }
+
+  // Then emit the files in current directory.
+  PoolNow->wait(); // Makes files reporting overlap with subdir reporting.
+  std::vector<decltype(FilesHere)::const_iterator> EmptyFiles;
+  for (auto I = FilesHere.begin(), E = FilesHere.end(); I != E; ++I) {
+    if (I->second.FunctionCoverage.getNumFunctions())
+      emitTableRow(OSRef, Opts, buildRelLinkToFile(I->first, /*IsDir=*/false),
+              I->second, /*IsTotals=*/false);
+    else
+      EmptyFiles.push_back(I);
+    TotalsHere += I->second;
+  }
+
+  // Emit the totals row.
+  emitTableRow(OSRef, Opts, "Totals", TotalsHere, /*IsTotals=*/false);
+  OSRef << EndTable << EndCenteredDiv;
+
+  // Emit links to files which don't contain any functions. These are normally
+  // not very useful, but could be relevant for code which abuses the
+  // preprocessor.
+  if (!EmptyFiles.empty() && FiltersNow->empty()) {
+    OSRef << tag("p", "Files which contain no functions. (These "
+                      "files contain code pulled into other files "
+                      "by the preprocessor.)\n");
+    OSRef << BeginCenteredDiv << BeginTable;
+    for (auto I : EmptyFiles) {
+      auto Link = buildRelLinkToFile(I->first, /*IsDir=*/false);
+      OSRef << tag("tr", tag("td", tag("pre", Link)), "light-row") << '\n';
+    }
+    OSRef << EndTable << EndCenteredDiv;
+  }
+
+  // Emit epilog.
+  OSRef << tag("h5", escape(Opts.getLLVMVersionString(), Opts));
+  emitEpilog(OSRef);
+
+  *Totals += TotalsHere;
+  return Error::success();
+}
+
+std::string CoveragePrinterHTMLDirectory::buildRelLinkToFile(StringRef RelPath,
+                                                             bool IsDir) const {
+  SmallString<128> LinkTextStr(RelPath);
+  sys::path::remove_dots(LinkTextStr, /*remove_dot_dot=*/true);
+  sys::path::native(LinkTextStr);
+
+  std::string LinkTargetStr;
+  if (IsDir) {
+    LinkTextStr += sys::path::get_separator();
+    LinkTargetStr = (RelPath + "index.html").str();
+  } else
+    LinkTargetStr = (RelPath + ".html").str();
+  
+  auto LinkText = escape(LinkTextStr, Opts);
+  auto LinkTarget = escape(LinkTargetStr, Opts);
+  return a(LinkTarget, LinkText);
+}
+
+std::string CoveragePrinterHTMLDirectory::buildNavLink(StringRef LCPath,
+                                                       unsigned LCP) const {
+  assert(LCP >= RootLCPNow);
+
+  StringRef RootPath, SubPath;
+  if (RootLCPNow > 0)
+    RootPath = LCPath.substr(0, RootLCPNow - 1); // Remove trailing slash.
+  if (LCP > RootLCPNow)
+    SubPath = LCPath.substr(RootLCPNow, LCP - RootLCPNow - 1);
+  std::vector<StringRef> Components{RootPath};
+  Components.insert(Components.end(), sys::path::begin(SubPath),
+                    sys::path::end(SubPath));
+
+  std::string S;
+  for (int I = 0, E = Components.size(); I < E; ++I) {
+    std::string Link(".");
+    Link += sys::path::get_separator();
+    for (int J = Components.size() - I; --J > 0;) {
+      Link += std::string("..");
+      Link += sys::path::get_separator();
+    }
+    S += a(Link, Components[I].str());
+    S += sys::path::get_separator();
+  }
+
+  return S;
+}
+
 void SourceCoverageViewHTML::renderViewHeader(raw_ostream &OS) {
   OS << BeginCenteredDiv << BeginTable;
 }
@@ -593,8 +872,9 @@
         continue;
 
       Snippets[I + 1] =
-          tag("div", Snippets[I + 1] + tag("span", formatCount(CurSeg->Count),
-                                           "tooltip-content"),
+          tag("div",
+              Snippets[I + 1] +
+                  tag("span", formatCount(CurSeg->Count), "tooltip-content"),
               "tooltip");
 
       if (getOptions().Debug)