Refactor termination detection.

libevmasm/SemanticInformation.cpp

@@ -136,7 +136,13 @@ bool SemanticInformation::terminatesControlFlow(AssemblyItem const& _item)
 {
 	if (_item.type() != Operation)
 		return false;
-	switch (_item.instruction())
+	else
+		return terminatesControlFlow(_item.instruction());
+}
+
+bool SemanticInformation::terminatesControlFlow(Instruction _instruction)
+{
+	switch (_instruction)
 	{
 	case Instruction::RETURN:
 	case Instruction::SELFDESTRUCT:

libevmasm/SemanticInformation.h

@@ -48,6 +48,7 @@ struct SemanticInformation
 	static bool isJumpInstruction(AssemblyItem const& _item);
 	static bool altersControlFlow(AssemblyItem const& _item);
 	static bool terminatesControlFlow(AssemblyItem const& _item);
+	static bool terminatesControlFlow(Instruction _instruction);
 	/// @returns false if the value put on the stack by _item depends on anything else than
 	/// the information in the current block header, memory, storage or stack.
 	static bool isDeterministic(AssemblyItem const& _item);

libyul/optimiser/DeadCodeEliminator.cpp

@@ -19,6 +19,7 @@
  */
 
 #include <libyul/optimiser/DeadCodeEliminator.h>
+#include <libyul/optimiser/Semantics.h>
 #include <libyul/AsmData.h>
 
 #include <libevmasm/SemanticInformation.h>
@@ -30,42 +31,6 @@ using namespace std;
 using namespace dev;
 using namespace yul;
 
-namespace
-{
-bool isTerminating(yul::ExpressionStatement const& _exprStmnt)
-{
-	if (_exprStmnt.expression.type() != typeid(FunctionalInstruction))
-		return false;
-
-	auto const& funcInstr = boost::get<FunctionalInstruction>(_exprStmnt.expression);
-
-	return eth::SemanticInformation::terminatesControlFlow(funcInstr.instruction);
-}
-
-/// Returns an iterator to the first terminating statement or
-/// `_block.statements.end()()` when none was found
-auto findFirstTerminatingStatement(Block& _block)
-{
-	return find_if(
-		_block.statements.begin(),
-		_block.statements.end(),
-		[](Statement const& _stmnt)
-		{
-			if (
-				_stmnt.type() == typeid(ExpressionStatement) &&
-				isTerminating(boost::get<ExpressionStatement>(_stmnt))
-			)
-				return true;
-			else if (_stmnt.type() == typeid(Break))
-				return true;
-			else if (_stmnt.type() == typeid(Continue))
-				return true;
-
-			return false;
-		}
-	);
-}
-}
 
 void DeadCodeEliminator::operator()(ForLoop& _for)
 {
@@ -75,24 +40,19 @@ void DeadCodeEliminator::operator()(ForLoop& _for)
 
 void DeadCodeEliminator::operator()(Block& _block)
 {
-	auto& statements = _block.statements;
+	TerminationFinder::ControlFlow controlFlowChange;
+	size_t index;
+	tie(controlFlowChange, index) = TerminationFinder::firstUnconditionalControlFlowChange(_block.statements);
 
-	auto firstTerminatingStatment = findFirstTerminatingStatement(_block);
-
-	if (
-		firstTerminatingStatment != statements.end() &&
-		firstTerminatingStatment + 1 != statements.end()
-	)
-		statements.erase(
-			std::remove_if(
-				firstTerminatingStatment + 1,
-				statements.end(),
-				[] (Statement const& _s)
-				{
-					return _s.type() != typeid(yul::FunctionDefinition);
-				}
+	// Erase everything after the terminating statement that is not a function definition.
+	if (controlFlowChange != TerminationFinder::ControlFlow::FlowOut && index != size_t(-1))
+		_block.statements.erase(
+			remove_if(
+				_block.statements.begin() + index + 1,
+				_block.statements.end(),
+				[] (Statement const& _s) { return _s.type() != typeid(yul::FunctionDefinition); }
 			),
-			statements.end()
+			_block.statements.end()
 		);
 
 	ASTModifier::operator()(_block);

libyul/optimiser/DeadCodeEliminator.h

@@ -41,7 +41,7 @@ namespace yul
  * Because variables declared in a for loop's init block have their scope extended to the loop body,
  * we require ForLoopInitRewriter to run before this step.
  *
- * Prerequisite: ForLoopInitRewriter
+ * Prerequisite: ForLoopInitRewriter, Function Hoister, Function Grouper
  */
 class DeadCodeEliminator: public ASTModifier
 {

libyul/optimiser/Semantics.cpp

@@ -72,3 +72,41 @@ void MovableChecker::visit(Statement const&)
 {
 	assertThrow(false, OptimizerException, "Movability for statement requested.");
 }
+
+pair<TerminationFinder::ControlFlow, size_t> TerminationFinder::firstUnconditionalControlFlowChange(
+	vector<Statement> const& _statements
+)
+{
+	for (size_t i = 0; i < _statements.size(); ++i)
+	{
+		ControlFlow controlFlow = controlFlowKind(_statements[i]);
+		if (controlFlow != ControlFlow::FlowOut)
+			return {controlFlow, i};
+	}
+	return {ControlFlow::FlowOut, size_t(-1)};
+}
+
+TerminationFinder::ControlFlow TerminationFinder::controlFlowKind(Statement const& _statement)
+{
+	if (
+		_statement.type() == typeid(ExpressionStatement) &&
+		isTerminatingBuiltin(boost::get<ExpressionStatement>(_statement))
+	)
+		return ControlFlow::Terminate;
+	else if (_statement.type() == typeid(Break))
+		return ControlFlow::Break;
+	else if (_statement.type() == typeid(Continue))
+		return ControlFlow::Continue;
+	else
+		return ControlFlow::FlowOut;
+}
+
+bool TerminationFinder::isTerminatingBuiltin(ExpressionStatement const& _exprStmnt)
+{
+	if (_exprStmnt.expression.type() != typeid(FunctionalInstruction))
+		return false;
+
+	return eth::SemanticInformation::terminatesControlFlow(
+		boost::get<FunctionalInstruction>(_exprStmnt.expression).instruction
+	);
+}

libyul/optimiser/Semantics.h

@@ -56,4 +56,34 @@ private:
 	bool m_movable = true;
 };
 
+/**
+ * Helper class to find "irregular" control flow.
+ * This includes termination, break and continue.
+ */
+class TerminationFinder
+{
+public:
+	enum class ControlFlow { FlowOut, Break, Continue, Terminate };
+
+	/// @returns the index of the first statement in the provided sequence
+	/// that is an unconditional ``break``, ``continue`` or a
+	/// call to a terminating builtin function.
+	/// If control flow can continue at the end of the list,
+	/// returns `FlowOut` and ``size_t(-1)``.
+	/// The function might return ``FlowOut`` even though control
+	/// flow cannot actually continue.
+	static std::pair<ControlFlow, size_t> firstUnconditionalControlFlowChange(
+		std::vector<Statement> const& _statements
+	);
+
+	/// @returns the control flow type of the given statement.
+	/// This function could return FlowOut even if control flow never continues.
+	static ControlFlow controlFlowKind(Statement const& _statement);
+
+	/// @returns true if the expression statement is a direct
+	/// call to a builtin terminating function like
+	/// ``stop``, ``revert`` or ``return``.
+	static bool isTerminatingBuiltin(ExpressionStatement const& _exprStmnt);
+};
+
 }