From c03f25d8392641f56b58640f2c5ddd3fc31b8961 Mon Sep 17 00:00:00 2001
From: Gabriel Corona <gabriel.corona@loria.fr>
Date: Fri, 3 Jun 2016 12:20:56 +0200
Subject: [PATCH 1/1] Use a custom Result<T> value for simgrid::smix::kernel()

This avoids allocating a shared state (by std::promise) on the heap
for each such call.
---
 include/simgrid/simix.hpp |   8 +-
 include/xbt/future.hpp    | 166 +++++++++++++++++++++++++++++++++++++-
 2 files changed, 166 insertions(+), 8 deletions(-)

diff --git a/include/simgrid/simix.hpp b/include/simgrid/simix.hpp
index 1122d16cfb..1f88d9730b 100644
--- a/include/simgrid/simix.hpp
+++ b/include/simgrid/simix.hpp
@@ -41,16 +41,16 @@ typename std::result_of<F()>::type kernel(F&& code)
   if (SIMIX_is_maestro())
     return std::forward<F>(code)();
 
-  // If we are in the application, pass the code to the maestro which is
+  // If we are in the application, pass the code to the maestro which
   // executes it for us and reports the result. We use a std::future which
   // conveniently handles the success/failure value for us.
   typedef typename std::result_of<F()>::type R;
-  std::promise<R> promise;
+  simgrid::xbt::Result<R> result;
   simcall_run_kernel([&]{
     xbt_assert(SIMIX_is_maestro(), "Not in maestro");
-    simgrid::xbt::fulfillPromise(promise, std::forward<F>(code));
+    simgrid::xbt::fulfillPromise(result, std::forward<F>(code));
   });
-  return promise.get_future().get();
+  return result.get();
 }
 
 class Context;
diff --git a/include/xbt/future.hpp b/include/xbt/future.hpp
index d5a31a55ce..7fdfb0b807 100644
--- a/include/xbt/future.hpp
+++ b/include/xbt/future.hpp
@@ -7,16 +7,173 @@
 #ifndef XBT_FUTURE_HPP
 #define XBT_FUTURE_HPP
 
-#include <future>
+#include <cstddef>
+
 #include <utility>
 #include <exception>
 
 namespace simgrid {
 namespace xbt {
 
+/** A value or an exception
+ *
+ *  The API is similar to the one of future and promise.
+ **/
+template<class T>
+class Result {
+  enum class ResultStatus {
+    invalid,
+    value,
+    exception,
+  };
+public:
+  Result() {}
+  ~Result() { this->reset(); }
+
+  // Copy (if T is copyable) and move:
+  Result(Result const& that)
+  {
+    (*this) = that;
+  }
+  Result& operator=(Result const& that)
+  {
+    this->reset();
+    switch (that.status_) {
+      case ResultStatus::invalid:
+        break;
+      case ResultStatus::valid:
+        new (&value_) T(that.value);
+        break;
+      case ResultStatus::exception:
+        new (&exception_) T(that.exception);
+        break;
+    }
+    return *this;
+  }
+  Result(Result&& that)
+  {
+    *this = std::move(that);
+  }
+  Result& operator=(Result&& that)
+  {
+    this->reset();
+    switch (that.status_) {
+      case ResultStatus::invalid:
+        break;
+      case ResultStatus::valid:
+        new (&value_) T(std::move(that.value));
+        that.value.~T();
+        break;
+      case ResultStatus::exception:
+        new (&exception_) T(std::move(that.exception));
+        that.exception.~exception_ptr();
+        break;
+    }
+    that.status_ = ResultStatus::invalid;
+    return *this;
+  }
+
+  bool is_valid()
+  {
+    return status_ != ResultStatus::invalid;
+  }
+  void reset()
+  {
+    switch(status_) {
+      case ResultStatus::invalid:
+        break;
+      case ResultStatus::value:
+        value_.~T();
+        break;
+      case ResultStatus::exception:
+        exception_.~exception_ptr();
+        break;
+    }
+    status_ = ResultStatus::invalid;
+  }
+  void set_exception(std::exception_ptr e)
+  {
+    this->reset();
+    new (&exception_) std::exception_ptr(std::move(e));
+    status_ = ResultStatus::exception;
+  }
+  void set_value(T&& value)
+  {
+    this->reset();
+    new (&value_) T(std::move(value));
+    status_ = ResultStatus::value;
+  }
+  void set_value(T const& value)
+  {
+    this->reset();
+    new (&value_) T(value);
+    status_ = ResultStatus::value;
+  }
+
+  /** Extract the value from the future
+   *
+   *  After this the value is invalid.
+   **/
+  T get()
+  {
+    switch(status_) {
+      case ResultStatus::invalid:
+        throw std::logic_error("Invalid result");
+      case ResultStatus::value: {
+        T value = std::move(value_);
+        value_.~T();
+        status_ = ResultStatus::invalid;
+        return std::move(value);
+      }
+      case ResultStatus::exception: {
+        std::exception_ptr exception = std::move(exception_);
+        exception_.~exception_ptr();
+        status_ = ResultStatus::invalid;
+        std::rethrow_exception(std::move(exception));
+        break;
+      }
+    }
+  }
+private:
+  ResultStatus status_ = ResultStatus::invalid;
+  union {
+    T value_;
+    std::exception_ptr exception_;
+  };
+};
+
+template<>
+class Result<void> : public Result<nullptr_t>
+{
+public:
+  void set_value()
+  {
+    Result<std::nullptr_t>::set_value(nullptr);
+  }
+  void get()
+  {
+    Result<nullptr_t>::get();
+  }
+};
+
+template<class T>
+class Result<T&> : public Result<std::reference_wrapper<T>>
+{
+public:
+  void set_value(T& value)
+  {
+    Result<std::reference_wrapper<T>>::set_value(std::ref(value));
+  }
+  T& get()
+  {
+    return Result<std::reference_wrapper<T>>::get();
+  }
+};
+
 /** Fulfill a promise by executing a given code */
 template<class R, class F>
-void fulfillPromise(std::promise<R>& promise, F code)
+auto fulfillPromise(R& promise, F&& code)
+-> decltype(promise.set_value(code()))
 {
   try {
     promise.set_value(code());
@@ -31,8 +188,9 @@ void fulfillPromise(std::promise<R>& promise, F code)
  *  This is a special version for `std::promise<void>` because the default
  *  version does not compile in this case.
  */
-template<class F>
-void fulfillPromise(std::promise<void>& promise, F code)
+template<class P, class F>
+auto fulfillPromise(P& promise, F&& code)
+-> decltype(promise.set_value())
 {
   try {
     (code)();
-- 
2.20.1