[Concurrency] Implement basic runtime support for task futures.

include/swift/ABI/MetadataKind.def

@@ -85,8 +85,8 @@ METADATAKIND(HeapGenericLocalVariable,
 METADATAKIND(ErrorObject,
              1 | MetadataKindIsNonType | MetadataKindIsRuntimePrivate)
 
-/// A heap-allocated simple task.
-METADATAKIND(SimpleTask,
+/// A heap-allocated task.
+METADATAKIND(Task,
              2 | MetadataKindIsNonType | MetadataKindIsRuntimePrivate)
 
 // getEnumeratedMetadataKind assumes that all the enumerated values here

include/swift/ABI/Task.h

@@ -19,6 +19,7 @@
 
 #include "swift/Basic/RelativePointer.h"
 #include "swift/ABI/HeapObject.h"
+#include "swift/ABI/Metadata.h"
 #include "swift/ABI/MetadataValues.h"
 #include "swift/Runtime/Config.h"
 #include "swift/Basic/STLExtras.h"
@@ -29,6 +30,8 @@ class AsyncTask;
 class AsyncContext;
 class Executor;
 class Job;
+struct OpaqueValue;
+struct SwiftError;
 class TaskStatusRecord;
 
 /// An ExecutorRef isn't necessarily just a pointer to an executor
@@ -86,6 +89,13 @@ class AsyncFunctionPointer {
 
 /// A schedulable job.
 class alignas(2 * alignof(void*)) Job {
+protected:
+  // Indices into SchedulerPrivate, for use by the runtime.
+  enum {
+    /// The next waiting task link, an AsyncTask that is waiting on a future.
+    NextWaitingTaskIndex = 0,
+  };
+
 public:
   // Reserved for the use of the scheduler.
   void *SchedulerPrivate[2];
@@ -230,19 +240,142 @@ class AsyncTask : public HeapObject, public Job {
     }
   };
 
-  bool isFuture() const { return Flags.task_isFuture(); }
-
   bool hasChildFragment() const { return Flags.task_isChildTask(); }
   ChildFragment *childFragment() {
     assert(hasChildFragment());
     return reinterpret_cast<ChildFragment*>(this + 1);
   }
 
-  // TODO: Future fragment
+  class FutureFragment {
+  public:
+    /// Describes the status of the future.
+    ///
+    /// Futures always begin in the "Executing" state, and will always
+    /// make a single state change to either Success or Error.
+    enum class Status : uintptr_t {
+      /// The future is executing or ready to execute. The storage
+      /// is not accessible.
+      Executing = 0,
+
+      /// The future has completed with result (of type \c resultType).
+      Success,
+
+      /// The future has completed by throwing an error (an \c Error
+      /// existential).
+      Error,
+    };
+
+    /// An item within the wait queue, which includes the status and the
+    /// head of the list of tasks.
+    struct WaitQueueItem {
+      /// Mask used for the low status bits in a wait queue item.
+      static const uintptr_t statusMask = 0x03;
+
+      uintptr_t storage;
+
+      Status getStatus() const {
+        return static_cast<Status>(storage & statusMask);
+      }
+
+      AsyncTask *getTask() const {
+        return reinterpret_cast<AsyncTask *>(storage & ~statusMask);
+      }
+
+      static WaitQueueItem get(Status status, AsyncTask *task) {
+        return WaitQueueItem{
+          reinterpret_cast<uintptr_t>(task) | static_cast<uintptr_t>(status)};
+      }
+    };
+
+  private:
+    /// Queue containing all of the tasks that are waiting in `get()`.
+    ///
+    /// The low bits contain the status, the rest of the pointer is the
+    /// AsyncTask.
+    std::atomic<WaitQueueItem> waitQueue;
+
+    /// The type of the result that will be produced by the future.
+    const Metadata *resultType;
+
+    // Trailing storage for the result itself. The storage will be uninitialized,
+    // contain an instance of \c resultType, or contaon an an \c Error.
+
+    friend class AsyncTask;
+
+  public:
+    explicit FutureFragment(const Metadata *resultType)
+      : waitQueue(WaitQueueItem::get(Status::Executing, nullptr)),
+        resultType(resultType) { }
+
+    /// Destroy the storage associated with the future.
+    void destroy();
+
+    /// Retrieve a pointer to the storage of result.
+    OpaqueValue *getStoragePtr() {
+      return reinterpret_cast<OpaqueValue *>(
+          reinterpret_cast<char *>(this) + storageOffset(resultType));
+    }
+
+    /// Retrieve the error.
+    SwiftError *&getError() {
+      return *reinterpret_cast<SwiftError **>(
+           reinterpret_cast<char *>(this) + storageOffset(resultType));
+    }
+
+    /// Compute the offset of the storage from the base of the future
+    /// fragment.
+    static size_t storageOffset(const Metadata *resultType)  {
+      size_t offset = sizeof(FutureFragment);
+      size_t alignment =
+          std::max(resultType->vw_alignment(), alignof(SwiftError *));
+      return (offset + alignment - 1) & ~(alignment - 1);
+    }
+
+    /// Determine the size of the future fragment given a particular future
+    /// result type.
+    static size_t fragmentSize(const Metadata *resultType) {
+      return storageOffset(resultType) +
+          std::max(resultType->vw_size(), sizeof(SwiftError *));
+    }
+  };
+
+  bool isFuture() const { return Flags.task_isFuture(); }
+
+  FutureFragment *futureFragment() {
+    assert(isFuture());
+    if (hasChildFragment()) {
+      return reinterpret_cast<FutureFragment *>(
+          reinterpret_cast<ChildFragment*>(this + 1) + 1);
+    }
+
+    return reinterpret_cast<FutureFragment *>(this + 1);
+  }
+
+  /// Wait for this future to complete.
+  ///
+  /// \returns the status of the future. If this result is
+  /// \c Executing, then \c waitingTask has been added to the
+  /// wait queue and will be scheduled when the future completes. Otherwise,
+  /// the future has completed and can be queried.
+  FutureFragment::Status waitFuture(AsyncTask *waitingTask);
+
+  /// Complete this future.
+  ///
+  /// Upon completion, any waiting tasks will be scheduled on the given
+  /// executor.
+  void completeFuture(AsyncContext *context, ExecutorRef executor);
 
   static bool classof(const Job *job) {
     return job->isAsyncTask();
   }
+
+private:
+  /// Access the next waiting task, which establishes a singly linked list of
+  /// tasks that are waiting on a future.
+  AsyncTask *&getNextWaitingTask() {
+    return reinterpret_cast<AsyncTask *&>(
+        SchedulerPrivate[NextWaitingTaskIndex]);
+  }
 };
 
 // The compiler will eventually assume these.
@@ -327,6 +460,20 @@ class YieldingAsyncContext : public AsyncContext {
   }
 };
 
+/// An asynchronous context within a task that describes a general "Future".
+/// task.
+///
+/// This type matches the ABI of a function `<T> () async throws -> T`, which
+/// is the type used by `Task.runDetached` and `Task.group.add` to create
+/// futures.
+class FutureAsyncContext : public AsyncContext {
+public:
+  SwiftError *errorResult = nullptr;
+  OpaqueValue *indirectResult;
+
+  using AsyncContext::AsyncContext;
+};
+
 } // end namespace swift
 
 #endif

include/swift/Runtime/Concurrency.h

@@ -49,6 +49,31 @@ AsyncTaskAndContext swift_task_create_f(JobFlags flags,
                                         AsyncFunctionType<void()> *function,
                                         size_t initialContextSize);
 
+/// Create a task object with a future which will run the given
+/// function.
+///
+/// The task is not yet scheduled.
+///
+/// If a parent task is provided, flags.task_hasChildFragment() must
+/// be true, and this must be called synchronously with the parent.
+/// The parent is responsible for creating a ChildTaskStatusRecord.
+/// TODO: should we have a single runtime function for creating a task
+/// and doing this child task status record management?
+///
+/// flags.task_isFuture must be set. \c futureResultType is the type
+///
+SWIFT_EXPORT_FROM(swift_Concurrency) SWIFT_CC(swift)
+AsyncTaskAndContext swift_task_create_future(
+    JobFlags flags, AsyncTask *parent, const Metadata *futureResultType,
+    const AsyncFunctionPointer<void()> *function);
+
+/// Create a task object with a future which will run the given
+/// function.
+SWIFT_EXPORT_FROM(swift_Concurrency) SWIFT_CC(swift)
+AsyncTaskAndContext swift_task_create_future_f(
+    JobFlags flags, AsyncTask *parent, const Metadata *futureResultType,
+    AsyncFunctionType<void()> *function, size_t initialContextSize);
+
 /// Allocate memory in a task.
 ///
 /// This must be called synchronously with the task.
@@ -83,6 +108,34 @@ SWIFT_EXPORT_FROM(swift_Concurrency) SWIFT_CC(swift)
 JobPriority
 swift_task_escalate(AsyncTask *task, JobPriority newPriority);
 
+/// The result of waiting for a task future.
+struct TaskFutureWaitResult {
+  enum Kind : uintptr_t {
+    /// The waiting task has been added to the future's wait queue, and will
+    /// be scheduled once the future has completed.
+    Waiting,
+
+    /// The future succeeded and produced a result value. \c storage points
+    /// at that value.
+    Success,
+
+    /// The future finished by throwing an error. \c storage is that error
+    /// existential.
+    Error,
+  };
+
+  Kind kind;
+  OpaqueValue *storage;
+};
+
+/// Wait for a future task to complete.
+///
+/// This can be called from any thread.
+///
+SWIFT_EXPORT_FROM(swift_Concurrency) SWIFT_CC(swift)
+TaskFutureWaitResult
+swift_task_future_wait(AsyncTask *task, AsyncTask *waitingTask);
+
 /// Add a status record to a task.  The record should not be
 /// modified while it is registered with a task.
 ///