[Runtime] Remove FIXMEs and hacks for tail calls.

Reverts hack from 9b4f7d6.

Author: varungandhi-apple

include/swift/ABI/Task.h

@@ -113,13 +113,15 @@ class alignas(2 * alignof(void*)) Job :
   /// Given that we've fully established the job context in the current
   /// thread, actually start running this job.  To establish the context
   /// correctly, call swift_job_run or runJobInExecutorContext.
+  SWIFT_CC(swiftasync)
   void runInFullyEstablishedContext();
 
   /// Given that we've fully established the job context in the
   /// current thread, and that the job is a simple (non-task) job,
   /// actually start running this job.
+  SWIFT_CC(swiftasync)
   void runSimpleInFullyEstablishedContext() {
-    RunJob(this);
+    return RunJob(this); // 'return' forces tail call
   }
 };
 
@@ -260,8 +262,9 @@ class AsyncTask : public Job {
   /// in the current thread, start running this task.  To establish
   /// the job context correctly, call swift_job_run or
   /// runInExecutorContext.
+  SWIFT_CC(swiftasync)
   void runInFullyEstablishedContext() {
-    ResumeTask(ResumeContext);
+    return ResumeTask(ResumeContext); // 'return' forces tail call
   }
 
   /// Check whether this task has been cancelled.
@@ -534,11 +537,12 @@ static_assert(alignof(AsyncTask) == 2 * alignof(void*),
 static_assert(offsetof(AsyncTask, Id) == 4 * sizeof(void *) + 4,
               "AsyncTask::Id offset is wrong");
 
+SWIFT_CC(swiftasync)
 inline void Job::runInFullyEstablishedContext() {
   if (auto task = dyn_cast<AsyncTask>(this))
-    task->runInFullyEstablishedContext();
+    return task->runInFullyEstablishedContext(); // 'return' forces tail call
   else
-    runSimpleInFullyEstablishedContext();
+    return runSimpleInFullyEstablishedContext(); // 'return' forces tail call
 }
 
 /// An asynchronous context within a task.  Generally contexts are

stdlib/public/Concurrency/Actor.cpp

@@ -1483,6 +1483,7 @@ static void swift_job_runImpl(Job *job, ExecutorRef executor) {
 ///                     the actor lives for the duration of job execution.
 ///                     Note that this may conflict with the retain/release
 ///                     design in the DefaultActorImpl, but it does fix bugs!
+SWIFT_CC(swiftasync)
 static void processDefaultActor(DefaultActorImpl *currentActor,
                                 RunningJobInfo runner) {
 #if SWIFT_TASK_PRINTF_DEBUG
@@ -1557,6 +1558,7 @@ static void processDefaultActor(DefaultActorImpl *currentActor,
   swift_release(actor);
 }
 
+SWIFT_CC(swiftasync)
 void ProcessInlineJob::process(Job *job) {
   DefaultActorImpl *actor = DefaultActorImpl::fromInlineJob(job);
 
@@ -1565,11 +1567,11 @@ void ProcessInlineJob::process(Job *job) {
   auto targetPriority = job->getPriority();
   auto runner = RunningJobInfo::forInline(targetPriority);
 
-  // FIXME: force tail call
   swift_retain(actor);
-  return processDefaultActor(actor, runner);
+  return processDefaultActor(actor, runner); // 'return' forces tail call
 }
 
+SWIFT_CC(swiftasync)
 void ProcessOutOfLineJob::process(Job *job) {
   auto self = cast<ProcessOutOfLineJob>(job);
   DefaultActorImpl *actor = self->Actor;
@@ -1581,21 +1583,20 @@ void ProcessOutOfLineJob::process(Job *job) {
 
   delete self;
 
-  // FIXME: force tail call
   swift_retain(actor);
-  return processDefaultActor(actor, runner);
+  return processDefaultActor(actor, runner); // 'return' forces tail call
 }
 
+SWIFT_CC(swiftasync)
 void ProcessOverrideJob::process(Job *job) {
   auto self = cast<ProcessOverrideJob>(job);
 
   // Pull the actor and priority out of the job.
   auto actor = self->Actor;
   auto runner = RunningJobInfo::forOverride(self);
 
-  // FIXME: force tail call
   swift_retain(actor);
-  return processDefaultActor(actor, runner);
+  return processDefaultActor(actor, runner); // 'return' forces tail call
 }
 
 void DefaultActorImpl::enqueue(Job *job) {
@@ -1800,13 +1801,6 @@ static bool tryAssumeThreadForSwitch(ExecutorRef newExecutor,
   return false;
 }
 
-__attribute__((noinline))
-SWIFT_CC(swiftasync)
-static void force_tail_call_hack(AsyncTask *task) {
-  // This *should* be executed as a tail call.
-  return task->runInFullyEstablishedContext();
-}
-
 /// Given that we've assumed control of an executor on this thread,
 /// continue to run the given task on it.
 SWIFT_CC(swiftasync)
@@ -1819,10 +1813,7 @@ static void runOnAssumedThread(AsyncTask *task, ExecutorRef executor,
   if (oldTracking) {
     oldTracking->setActiveExecutor(executor);
 
-    // FIXME: force tail call
-    // return task->runInFullyEstablishedContext();
-    // This hack "ensures" that this call gets executed as a tail call.
-    return force_tail_call_hack(task);
+    return task->runInFullyEstablishedContext(); // 'return' forces tail call
   }
 
   // Otherwise, set up tracking info.
@@ -1865,8 +1856,7 @@ static void swift_task_switchImpl(SWIFT_ASYNC_CONTEXT AsyncContext *resumeContex
   // we can just immediately continue running with the resume function
   // we were passed in.
   if (!currentExecutor.mustSwitchToRun(newExecutor)) {
-    // FIXME: force tail call
-    return resumeFunction(resumeContext);
+    return resumeFunction(resumeContext); // 'return' forces tail call
   }
 
   auto task = swift_task_getCurrent();
@@ -1890,7 +1880,7 @@ static void swift_task_switchImpl(SWIFT_ASYNC_CONTEXT AsyncContext *resumeContex
     fprintf(stderr, "[%p] switch succeeded, task %p assumed thread for executor %p\n", pthread_self(), task, newExecutor.getIdentity());
 #endif
     giveUpThreadForSwitch(currentExecutor, runner);
-    // FIXME: force tail call
+    // 'return' forces tail call
     return runOnAssumedThread(task, newExecutor, trackingInfo, runner);
   }