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Remove a loop that no longer loops #67

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Merged
neboat merged 1 commit into from
Jan 20, 2026
Merged

Remove a loop that no longer loops #67

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59 changes: 28 additions & 31 deletions runtime/scheduler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -377,43 +377,35 @@ static Closure *Closure_return(__cilkrts_worker *const w, worker_id self,
/* If in the future the worker's map is not created lazily,
assert it is not null here. */

/* need a loop as multiple siblings can return while we
are performing reductions */
// Multiple siblings can return while we are performing reductions.
// The following code used to be in a loop. Now this function
// is called multiple times if necessary to finish reduction.

// always lock from top to bottom
parent->lock(self);
child->lock(self);

// Deal with reducers.
while (true) {
// invariant: a closure cannot unlink itself w/out lock on parent
// so what this points to cannot change while we have lock on parent

hyper_table *rht = child->right_ht;
child->right_ht = nullptr;

// Get the "left" hypermap, which either belongs to a left sibling, if
// it exists, or the parent, otherwise.
hyper_table **lht_ptr;
Closure *const left_sib = child->left_sib;
if (left_sib != nullptr) {
lht_ptr = &left_sib->right_ht;
} else {
lht_ptr = &parent->child_ht;
}
hyper_table *lht = *lht_ptr;
*lht_ptr = nullptr;

// If we have no hypermaps on either the left or right, deposit the
// active hypermap and break from the loop.
if (lht == nullptr && rht == nullptr) {
// Deposit the current active hypermap
hyper_table *active_ht = w->hyper_table;
w->hyper_table = nullptr;
*lht_ptr = active_ht;
break;
}
// invariant: a closure cannot unlink itself w/out lock on parent
// so what this points to cannot change while we have lock on parent

hyper_table *rht = child->right_ht;
child->right_ht = nullptr;

// Get the "left" hypermap, which either belongs to a left sibling, if
// it exists, or the parent, otherwise.
hyper_table **lht_ptr;
Closure *const left_sib = child->left_sib;
if (left_sib != nullptr) {
lht_ptr = &left_sib->right_ht;
} else {
lht_ptr = &parent->child_ht;
}
hyper_table *lht = *lht_ptr;
*lht_ptr = nullptr;

// If we have hypermaps on either the left or right, arrange
// for them to be merged.
if (lht != nullptr || rht != nullptr) {
child->set_status(CLOSURE_RUNNING);

child->unlock(self);
Expand All @@ -430,6 +422,11 @@ static Closure *Closure_return(__cilkrts_worker *const w, worker_id self,
return child;
}

// Deposit the current active hypermap
hyper_table *active_ht = w->hyper_table;
w->hyper_table = nullptr;
*lht_ptr = active_ht;

// Cilk_exception_handler ended up pushing a stack frame onto child, to do
// reductions. Because there are no reductions to do, pop that frame.
__cilkrts_leave_frame(child->frame);
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