Fix pathological complexity explosion for certain shaders.

Certain shaders where functions have a *ton* of merging control flow
will end up with exponential time complexity to figure out parameter
preservation semantics.

The trivial fix to make it O(1) again is to terminate recursive traversal early if we've seen
the path before. Simple oversight :(

Author: HansKristian-Work

spirv_cross.cpp

@@ -2829,7 +2829,8 @@ const SPIRConstant &Compiler::get_constant(ConstantID id) const
 	return get<SPIRConstant>(id);
 }
 
-static bool exists_unaccessed_path_to_return(const CFG &cfg, uint32_t block, const unordered_set<uint32_t> &blocks)
+static bool exists_unaccessed_path_to_return(const CFG &cfg, uint32_t block, const unordered_set<uint32_t> &blocks,
+                                             unordered_set<uint32_t> &visit_cache)
 {
 	// This block accesses the variable.
 	if (blocks.find(block) != end(blocks))
@@ -2841,8 +2842,14 @@ static bool exists_unaccessed_path_to_return(const CFG &cfg, uint32_t block, con
 
 	// If any of our successors have a path to the end, there exists a path from block.
 	for (auto &succ : cfg.get_succeeding_edges(block))
-		if (exists_unaccessed_path_to_return(cfg, succ, blocks))
-			return true;
+	{
+		if (visit_cache.count(succ) == 0)
+		{
+			if (exists_unaccessed_path_to_return(cfg, succ, blocks, visit_cache))
+				return true;
+			visit_cache.insert(succ);
+		}
+	}
 
 	return false;
 }
@@ -2899,7 +2906,8 @@ void Compiler::analyze_parameter_preservation(
 		// void foo(int &var) { if (cond) var = 10; }
 		// Using read/write counts, we will think it's just an out variable, but it really needs to be inout,
 		// because if we don't write anything whatever we put into the function must return back to the caller.
-		if (exists_unaccessed_path_to_return(cfg, entry.entry_block, itr->second))
+		unordered_set<uint32_t> visit_cache;
+		if (exists_unaccessed_path_to_return(cfg, entry.entry_block, itr->second, visit_cache))
 			arg.read_count++;
 	}
 }