Index: clang-tools-extra/clang-doc/tool/ClangDocMain.cpp
===================================================================
--- clang-tools-extra/clang-doc/tool/ClangDocMain.cpp
+++ clang-tools-extra/clang-doc/tool/ClangDocMain.cpp
@@ -38,6 +38,7 @@
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Signals.h"
+#include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/raw_ostream.h"
 #include <string>
 
@@ -67,6 +68,12 @@
     llvm::cl::desc("CSS stylesheets to extend the default styles."),
     llvm::cl::cat(ClangDocCategory));
 
+static llvm::cl::opt<int> ThreadCount(
+    "thread-count",
+    llvm::cl::desc("Threads to use for collecting and reducing infos."),
+    llvm::cl::init(llvm::hardware_concurrency()),
+    llvm::cl::cat(ClangDocCategory));
+
 enum OutputFormatTy {
   md,
   yaml,
@@ -153,30 +160,6 @@
   return Path;
 }
 
-// Iterate through tool results and build string map of info vectors from the
-// encoded bitstreams.
-bool bitcodeResultsToInfos(
-    tooling::ToolResults &Results,
-    llvm::StringMap<std::vector<std::unique_ptr<doc::Info>>> &Output) {
-  bool Err = false;
-  Results.forEachResult([&](StringRef Key, StringRef Value) {
-    llvm::BitstreamCursor Stream(Value);
-    doc::ClangDocBitcodeReader Reader(Stream);
-    auto Infos = Reader.readBitcode();
-    if (!Infos) {
-      llvm::errs() << toString(Infos.takeError()) << "\n";
-      Err = true;
-      return;
-    }
-    for (auto &I : Infos.get()) {
-      auto R =
-          Output.try_emplace(Key, std::vector<std::unique_ptr<doc::Info>>());
-      R.first->second.emplace_back(std::move(I));
-    }
-  });
-  return Err;
-}
-
 int main(int argc, const char **argv) {
   llvm::sys::PrintStackTraceOnErrorSignal(argv[0]);
   std::error_code OK;
@@ -237,37 +220,67 @@
   // In ToolResults, the Key is the hashed USR and the value is the
   // bitcode-encoded representation of the Info object.
   llvm::outs() << "Collecting infos...\n";
-  llvm::StringMap<std::vector<std::unique_ptr<doc::Info>>> USRToInfos;
-  if (bitcodeResultsToInfos(*Exec->get()->getToolResults(), USRToInfos))
-    return 1;
+  llvm::StringMap<std::vector<StringRef>> USRToBitcode;
+  Exec->get()->getToolResults()->forEachResult(
+      [&](StringRef Key, StringRef Value) {
+        auto R = USRToBitcode.try_emplace(Key, std::vector<StringRef>());
+        R.first->second.emplace_back(Value);
+      });
 
   // First reducing phase (reduce all decls into one info per decl).
-  llvm::outs() << "Reducing " << USRToInfos.size() << " infos...\n";
-  for (auto &Group : USRToInfos) {
-    auto Reduced = doc::mergeInfos(Group.getValue());
-    if (!Reduced) {
-      llvm::errs() << llvm::toString(Reduced.takeError());
-      continue;
-    }
+  llvm::outs() << "Reducing " << USRToBitcode.size() << " infos...\n";
+  bool Error = false;
+  llvm::ThreadPool Pool(ThreadCount);
+  for (auto &Group : USRToBitcode) {
+    Pool.async([&]() {
+      std::vector<std::unique_ptr<doc::Info>> Infos;
 
-    doc::Info *I = Reduced.get().get();
-    auto InfoPath = getInfoOutputFile(OutDirectory, I->Path, I->extractName(),
-                                      "." + Format);
-    if (!InfoPath) {
-      llvm::errs() << toString(InfoPath.takeError()) << "\n";
-      return 1;
-    }
-    std::error_code FileErr;
-    llvm::raw_fd_ostream InfoOS(InfoPath.get(), FileErr, llvm::sys::fs::F_None);
-    if (FileErr != OK) {
-      llvm::errs() << "Error opening info file: " << FileErr.message() << "\n";
-      continue;
-    }
+      for (auto &Bitcode : Group.getValue()) {
+        llvm::BitstreamCursor Stream(Bitcode);
+        doc::ClangDocBitcodeReader Reader(Stream);
+        auto ReadInfos = Reader.readBitcode();
+        if (!ReadInfos) {
+          llvm::errs() << toString(ReadInfos.takeError()) << "\n";
+          Error = true;
+          return;
+        }
+        std::move(ReadInfos->begin(), ReadInfos->end(),
+                  std::back_inserter(Infos));
+      }
+
+      auto Reduced = doc::mergeInfos(Infos);
+      if (!Reduced) {
+        llvm::errs() << llvm::toString(Reduced.takeError());
+        return;
+      }
+
+      doc::Info *I = Reduced.get().get();
+      auto InfoPath = getInfoOutputFile(OutDirectory, I->Path, I->extractName(),
+                                        "." + Format);
+      if (!InfoPath) {
+        llvm::errs() << toString(InfoPath.takeError()) << "\n";
+        Error = true;
+        return;
+      }
+      std::error_code FileErr;
+      llvm::raw_fd_ostream InfoOS(InfoPath.get(), FileErr,
+                                  llvm::sys::fs::F_None);
+      if (FileErr != OK) {
+        llvm::errs() << "Error opening info file: " << FileErr.message()
+                     << "\n";
+        return;
+      }
 
-    if (auto Err = G->get()->generateDocForInfo(I, InfoOS, CDCtx))
-      llvm::errs() << toString(std::move(Err)) << "\n";
+      if (auto Err = G->get()->generateDocForInfo(I, InfoOS, CDCtx))
+        llvm::errs() << toString(std::move(Err)) << "\n";
+    });
   }
 
+  Pool.wait();
+
+  if (Error)
+    return 1;
+
   if (!G->get()->createResources(CDCtx))
     return 1;