# UNDF: UNDF-2026-000000293
--- a/spirv_cfg.hpp
+++ b/spirv_cfg.hpp
@@ -78,6 +78,7 @@ private:
 	std::vector<uint32_t> post_order;
 	std::unordered_map<uint32_t, VisitOrder> visit_order;
 	SmallVector<uint32_t> visit_stack;
+	std::unordered_set<uint32_t> visit_stack_set;  // O(1) membership mirror of visit_stack
 	uint32_t last_visited_size = 0;

 	void post_order_visit_entry(uint32_t block);
--- a/spirv_cfg.cpp
+++ b/spirv_cfg.cpp
@@ -99,6 +99,7 @@ void CFG::post_order_visit_entry(uint32_t block)
 {
 	visit_stack.push_back(block);
+	visit_stack_set.insert(block);

 	while (!visit_stack.empty())
 	{
@@ -118,6 +119,7 @@ void CFG::post_order_visit_entry(uint32_t block)
 		while (!visit_stack.empty() && visit_order[visit_stack.back()].visited_branches)
 		{
 			post_order_visit_resolve(visit_stack.back());
+			visit_stack_set.erase(visit_stack.back());
 			visit_stack.pop_back();
 		}
 	}
@@ -126,8 +128,7 @@ void CFG::post_order_visit_entry(uint32_t block)
 void CFG::visit_branch(uint32_t block_id)
 {
 	// Prune obvious duplicates.
-	if (std::find(visit_stack.begin() + last_visited_size, visit_stack.end(), block_id) == visit_stack.end() &&
-	    !has_visited_branch(block_id))
+	if (visit_stack_set.find(block_id) == visit_stack_set.end() &&
+	    !has_visited_branch(block_id))
 	{
 		visit_stack.push_back(block_id);
+		visit_stack_set.insert(block_id);
 	}
 }