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Optimize BDD Gc code slightly. #3752

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Merged
copybara-service merged 1 commit into from
Jan 28, 2026
+33 −23
Merged

Optimize BDD Gc code slightly. #3752

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56 changes: 33 additions & 23 deletions xls/data_structures/binary_decision_diagram.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -344,6 +344,14 @@ std::string BinaryDecisionDiagram::ToStringDnf(BddNodeIndex expr,
return result;
}

namespace {
#ifdef NDEBUG
constexpr bool kDebug = false;
#else
constexpr bool kDebug = true;
#endif
} // namespace

absl::StatusOr<std::vector<BddNodeIndex>> BinaryDecisionDiagram::GarbageCollect(
absl::Span<BddNodeIndex const> roots, double gc_threshold) {
if (nodes_size_ * gc_threshold < prev_nodes_size_) {
Expand Down Expand Up @@ -400,12 +408,12 @@ absl::StatusOr<std::vector<BddNodeIndex>> BinaryDecisionDiagram::GarbageCollect(
}
}
// build a new free-list prefix.
for (auto vs : iter::sliding_window(
iter::chain(dead, std::vector<BddNodeIndex>{BddNodeIndex(
free_node_head_.value())}),
2)) {
BddNodeIndex first = vs[0];
BddNodeIndex second = vs[1];
// Don't use iter::sliding_window because this is faster.
for (int i = 0; i < dead.size(); ++i) {
BddNodeIndex first = dead[i];
BddNodeIndex second = i + 1 < dead.size()
? dead[i + 1]
: BddNodeIndex(free_node_head_.value());
nodes_[first.value()] = FreeListNode(FreeListNode::Index(second.value()));
}
free_node_head_ = FreeListNode::Index(dead.front().value());
Expand Down Expand Up @@ -439,24 +447,26 @@ absl::StatusOr<std::vector<BddNodeIndex>> BinaryDecisionDiagram::GarbageCollect(
}
nodes_size_ = cnt_live;
prev_nodes_size_ = nodes_size_;
auto head = free_node_head_;
auto it = dead.begin();
InlineBitmap debug_seen(nodes_.size());
while (head.value() < nodes_.size() &&
head != FreeListNode::kNextIsConsecutive) {
CHECK(!debug_seen.Get(head.value()))
<< " loop in free list at " << head.value();
debug_seen.Set(head.value());
if (it != dead.end()) {
CHECK_EQ(head, FreeListNode::Index(it->value()));
++it;
}
CHECK(std::holds_alternative<FreeListNode>(nodes_[head.value()]));
auto cur = std::get<FreeListNode>(nodes_[head.value()]);
if (cur.raw_next() == FreeListNode::kNextIsConsecutive) {
break;
if constexpr (kDebug) {
auto head = free_node_head_;
auto it = dead.begin();
InlineBitmap debug_seen(nodes_.size());
while (head.value() < nodes_.size() &&
head != FreeListNode::kNextIsConsecutive) {
CHECK(!debug_seen.Get(head.value()))
<< " loop in free list at " << head.value();
debug_seen.Set(head.value());
if (it != dead.end()) {
CHECK_EQ(head, FreeListNode::Index(it->value()));
++it;
}
CHECK(std::holds_alternative<FreeListNode>(nodes_[head.value()]));
auto cur = std::get<FreeListNode>(nodes_[head.value()]);
if (cur.raw_next() == FreeListNode::kNextIsConsecutive) {
break;
}
head = cur.next_free(head);
}
head = cur.next_free(head);
}
return std::move(dead);
}
Expand Down
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