Provide max iterations for stack compressor as parameter.

libyul/optimiser/StackCompressor.cpp

@@ -132,13 +132,18 @@ void eliminateVariables(shared_ptr<Dialect> const& _dialect, ASTNode& _node, siz
 
 }
 
-bool StackCompressor::run(shared_ptr<Dialect> const& _dialect, Block& _ast, bool _optimizeStackAllocation)
+bool StackCompressor::run(
+	shared_ptr<Dialect> const& _dialect,
+	Block& _ast,
+	bool _optimizeStackAllocation,
+	size_t _maxIterations
+)
 {
 	yulAssert(
 		_ast.statements.size() > 0 && _ast.statements.at(0).type() == typeid(Block),
 		"Need to run the function grouper before the stack compressor."
 	);
-	for (size_t iterations = 0; iterations < 6; iterations++)
+	for (size_t iterations = 0; iterations < _maxIterations; iterations++)
 	{
 		map<YulString, int> stackSurplus = CompilabilityChecker::run(_dialect, _ast, _optimizeStackAllocation);
 		if (stackSurplus.empty())

libyul/optimiser/StackCompressor.h

@@ -41,7 +41,12 @@ class StackCompressor
 public:
 	/// Try to remove local variables until the AST is compilable.
 	/// @returns true if it was successful.
-	static bool run(std::shared_ptr<Dialect> const& _dialect, Block& _ast, bool _optimizeStackAllocation);
+	static bool run(
+		std::shared_ptr<Dialect> const& _dialect,
+		Block& _ast,
+		bool _optimizeStackAllocation,
+		size_t _maxIterations
+	);
 };
 
 }

libyul/optimiser/Suite.cpp

@@ -185,8 +185,12 @@ void OptimiserSuite::run(
 	Rematerialiser::run(*_dialect, ast);
 	UnusedPruner::runUntilStabilised(*_dialect, ast, reservedIdentifiers);
 
+	// This is a tuning parameter, but actually just prevents infinite loops.
+	size_t stackCompressorMaxIterations = 16;
 	FunctionGrouper{}(ast);
-	solAssert(StackCompressor::run(_dialect, ast, _optimizeStackAllocation), "");
+	// We ignore the return value because we will get a much better error
+	// message once we perform code generation.
+	StackCompressor::run(_dialect, ast, _optimizeStackAllocation, stackCompressorMaxIterations);
 	BlockFlattener{}(ast);
 
 	VarNameCleaner{ast, *_dialect, reservedIdentifiers}(ast);

test/libyul/YulOptimizerTest.cpp

@@ -247,7 +247,8 @@ bool YulOptimizerTest::run(ostream& _stream, string const& _linePrefix, bool con
 	{
 		disambiguate();
 		(FunctionGrouper{})(*m_ast);
-		StackCompressor::run(m_dialect, *m_ast, true);
+		size_t maxIterations = 16;
+		StackCompressor::run(m_dialect, *m_ast, true, maxIterations);
 		(BlockFlattener{})(*m_ast);
 	}
 	else if (m_optimizerStep == "fullSuite")

test/tools/yulopti.cpp

@@ -198,7 +198,7 @@ public:
 				SSAReverser::run(*m_ast);
 				break;
 			case 'p':
-				StackCompressor::run(m_dialect, *m_ast, true);
+				StackCompressor::run(m_dialect, *m_ast, true, 16);
 				break;
 			default:
 				cout << "Unknown option." << endl;