[PATCH] Use std::async instead of our own parallel execution instrumentation.

[Rui Ueyama]

Hi Bigcheese,

Is there any reason to use our own library rather than the standard one?
Because we are already using C++11 threading library, I think I don't see
any reason not to use std::async().

http://llvm-reviews.chandlerc.com/D1904

Files:
  lib/Driver/Driver.cpp

Index: lib/Driver/Driver.cpp
===================================================================
--- lib/Driver/Driver.cpp
+++ lib/Driver/Driver.cpp
@@ -26,6 +26,7 @@
 #include "llvm/Support/raw_ostream.h"
 
 #include <mutex>
+#include <future>
 
 namespace lld {
 
@@ -45,39 +46,40 @@
   if (!inputGraph.size())
     return false;
 
-  bool fail = false;
-
   // Read inputs
   ScopedTask readTask(getDefaultDomain(), "Read Args");
-  TaskGroup tg;
   std::mutex diagnosticsMutex;
+  std::vector<std::future<bool> > tasks;
+  // First pass: submits tasks to the executor so that the files are parsed in
+  // paralell.
   for (auto &ie : inputGraph.inputElements()) {
-    tg.spawn([&] {
+    auto parseFuture = std::async(std::launch::async, [&]{
       // Writes to the same output stream is not guaranteed to be thread-safe.
       // We buffer the diagnostics output to a separate string-backed output
       // stream, acquire the lock, and then print it out.
       std::string buf;
       llvm::raw_string_ostream stream(buf);
 
-      if (error_code ec = ie->parse(context, stream)) {
+      error_code parseFailed = ie->parse(context, stream);
+      if (parseFailed) {
         FileNode *fileNode = llvm::dyn_cast<FileNode>(ie.get());
-        stream << fileNode->errStr(ec) << "\n";
-        fail = true;
+        stream << fileNode->errStr(parseFailed) << "\n";
       }
-
       stream.flush();
       if (!buf.empty()) {
         std::lock_guard<std::mutex> lock(diagnosticsMutex);
         diagnostics << buf;
       }
+      return !parseFailed;
     });
+    tasks.push_back(std::move(parseFuture));
   }
-  tg.sync();
+  // Second pass: collect parsing results. Exit from this function if error.
+  for (auto &task : tasks)
+    if (!task.get())
+      return false;
   readTask.end();
 
-  if (fail)
-    return false;
-
   std::unique_ptr<SimpleFileNode> fileNode(
       new SimpleFileNode("Internal Files"));
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