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[SYCL] Fix handling of discarded events in preview lib #19005

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Jun 17, 2025
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3 changes: 3 additions & 0 deletions sycl/include/sycl/handler.hpp
Original file line number Diff line number Diff line change
@@ -571,6 +571,9 @@ class __SYCL_EXPORT handler {
/// object destruction.
///
/// \return a SYCL event object representing the command group
///
/// Note: in preview mode, handler.finalize() is expected to return
/// nullptr if the event is not needed (discarded).
#ifdef __INTEL_PREVIEW_BREAKING_CHANGES
detail::EventImplPtr finalize();
#else
6 changes: 5 additions & 1 deletion sycl/source/detail/queue_impl.hpp
Original file line number Diff line number Diff line change
@@ -709,7 +709,11 @@ class queue_impl : public std::enable_shared_from_this<queue_impl> {
}

#ifdef __INTEL_PREVIEW_BREAKING_CHANGES
#define parseEvent(arg) (arg)
inline const detail::EventImplPtr &
parseEvent(const detail::EventImplPtr &Event) {
assert(!Event || !Event->isDiscarded());
return Event;
}
#else
inline detail::EventImplPtr parseEvent(const event &Event) {
const detail::EventImplPtr &EventImpl = getSyclObjImpl(Event);
19 changes: 7 additions & 12 deletions sycl/source/detail/scheduler/commands.cpp
Original file line number Diff line number Diff line change
@@ -3132,22 +3132,17 @@ ur_result_t ExecCGCommand::enqueueImpQueue() {
auto RawEvents = getUrEvents(EventImpls);
flushCrossQueueDeps(EventImpls, MWorkerQueue);

// We can omit creating a UR event and create a "discarded" event if the
// command has been explicitly marked as not needing an event, e.g. if the
// user did not ask for one, and there are no requirements.
bool DiscardUrEvent = MQueue && !MEventNeeded &&
MQueue->supportsDiscardingPiEvents() &&
MCommandGroup->getRequirements().size() == 0;

ur_event_handle_t UREvent = nullptr;
ur_event_handle_t *Event = DiscardUrEvent ? nullptr : &UREvent;
detail::event_impl *EventImpl = DiscardUrEvent ? nullptr : MEvent.get();
ur_event_handle_t *Event = !MEventNeeded ? nullptr : &UREvent;
detail::event_impl *EventImpl = !MEventNeeded ? nullptr : MEvent.get();

auto SetEventHandleOrDiscard = [&]() {
if (Event)
if (!MEventNeeded) {
assert(MEvent->isDiscarded());
} else {
assert(!MEvent->isDiscarded());
MEvent->setHandle(*Event);
else
MEvent->setStateDiscarded();
}
};

switch (MCommandGroup->getType()) {
8 changes: 8 additions & 0 deletions sycl/source/detail/scheduler/scheduler.cpp
Original file line number Diff line number Diff line change
@@ -133,6 +133,14 @@ EventImplPtr Scheduler::addCG(
}
NewEvent = NewCmd->getEvent();
NewEvent->setSubmissionTime();

// This is the last moment we can mark the event as discarded.
// Doing this during command execution would lead to incorrect
// event handling (as event would change it's state from non-discarded
// to discarded).
if (!EventNeeded) {
NewEvent->setStateDiscarded();
}
}

enqueueCommandForCG(NewEvent, AuxiliaryCmds);
3 changes: 2 additions & 1 deletion sycl/source/detail/scheduler/scheduler.hpp
Original file line number Diff line number Diff line change
@@ -376,7 +376,8 @@ class Scheduler {
/// directly to the queue.
/// \param Dependencies Optional list of dependency
/// sync points when enqueuing to a command buffer.
/// \return an event object to wait on for command group completion.
/// \return an event object to wait on for command group completion. It can
/// be a discarded event.
EventImplPtr addCG(
std::unique_ptr<detail::CG> CommandGroup, const QueueImplPtr &Queue,
bool EventNeeded, ur_exp_command_buffer_handle_t CommandBuffer = nullptr,
8 changes: 6 additions & 2 deletions sycl/source/handler.cpp
Original file line number Diff line number Diff line change
@@ -895,11 +895,15 @@ event handler::finalize() {
#endif
}

bool DiscardEvent = !impl->MEventNeeded && Queue &&
Queue->supportsDiscardingPiEvents() &&
CommandGroup->getRequirements().size() == 0;

detail::EventImplPtr Event = detail::Scheduler::getInstance().addCG(
std::move(CommandGroup), Queue->shared_from_this(), impl->MEventNeeded);
std::move(CommandGroup), Queue->shared_from_this(), !DiscardEvent);

#ifdef __INTEL_PREVIEW_BREAKING_CHANGES
MLastEvent = Event;
MLastEvent = DiscardEvent ? nullptr : Event;
#else
MLastEvent = detail::createSyclObjFromImpl<event>(Event);
#endif