Introduce side-effect-free as relaxed version of movable.

libyul/Dialect.h

@@ -49,6 +49,8 @@ struct BuiltinFunction
 	/// This means the function cannot depend on storage or memory, cannot have any side-effects,
 	/// but it can depend on state that is constant across an EVM-call.
 	bool movable = false;
+	/// If true, a call to this function can be omitted without changing semantics.
+	bool sideEffectFree = false;
 	/// If true, can only accept literals as arguments and they cannot be moved to variables.
 	bool literalArguments = false;
 };

libyul/backends/evm/EVMDialect.cpp

@@ -42,7 +42,7 @@ EVMDialect::EVMDialect(AsmFlavour _flavour, bool _objectAccess, langutil::EVMVer
 	if (!m_objectAccess)
 		return;
 
-	addFunction("datasize", 1, 1, true, true, [this](
+	addFunction("datasize", 1, 1, true, true, true, [this](
 		FunctionCall const& _call,
 		AbstractAssembly& _assembly,
 		std::function<void()>
@@ -59,7 +59,7 @@ EVMDialect::EVMDialect(AsmFlavour _flavour, bool _objectAccess, langutil::EVMVer
 			_assembly.appendDataSize(m_subIDs.at(dataName));
 		}
 	});
-	addFunction("dataoffset", 1, 1, true, true, [this](
+	addFunction("dataoffset", 1, 1, true, true, true, [this](
 		FunctionCall const& _call,
 		AbstractAssembly& _assembly,
 		std::function<void()>
@@ -76,7 +76,7 @@ EVMDialect::EVMDialect(AsmFlavour _flavour, bool _objectAccess, langutil::EVMVer
 			_assembly.appendDataOffset(m_subIDs.at(dataName));
 		}
 	});
-	addFunction("datacopy", 3, 0, false, false, [](
+	addFunction("datacopy", 3, 0, false, false, false, [](
 		FunctionCall const&,
 		AbstractAssembly& _assembly,
 		std::function<void()> _visitArguments
@@ -132,6 +132,7 @@ void EVMDialect::addFunction(
 	size_t _params,
 	size_t _returns,
 	bool _movable,
+	bool _sideEffectFree,
 	bool _literalArguments,
 	std::function<void(FunctionCall const&, AbstractAssembly&, std::function<void()>)> _generateCode
 )
@@ -142,6 +143,7 @@ void EVMDialect::addFunction(
 	f.parameters.resize(_params);
 	f.returns.resize(_returns);
 	f.movable = _movable;
+	f.sideEffectFree = _sideEffectFree;
 	f.literalArguments = _literalArguments;
 	f.generateCode = std::move(_generateCode);
 }

libyul/backends/evm/EVMDialect.h

@@ -75,6 +75,7 @@ protected:
 		size_t _params,
 		size_t _returns,
 		bool _movable,
+		bool _sideEffectFree,
 		bool _literalArguments,
 		std::function<void(FunctionCall const&, AbstractAssembly&, std::function<void()>)> _generateCode
 	);

libyul/backends/wasm/WasmDialect.cpp

@@ -71,5 +71,6 @@ void WasmDialect::addFunction(string _name, size_t _params, size_t _returns)
 	f.parameters.resize(_params);
 	f.returns.resize(_returns);
 	f.movable = false;
+	f.sideEffectFree = false;
 	f.literalArguments = false;
 }

libyul/optimiser/Semantics.cpp

@@ -51,21 +51,30 @@ void MovableChecker::operator()(Identifier const& _identifier)
 
 void MovableChecker::operator()(FunctionalInstruction const& _instr)
 {
+	ASTWalker::operator()(_instr);
+
 	if (!eth::SemanticInformation::movable(_instr.instruction))
 		m_movable = false;
-	else
-		ASTWalker::operator()(_instr);
+	if (!eth::SemanticInformation::sideEffectFree(_instr.instruction))
+		m_sideEffectFree = false;
 }
 
 void MovableChecker::operator()(FunctionCall const& _functionCall)
 {
+	ASTWalker::operator()(_functionCall);
+
 	if (BuiltinFunction const* f = m_dialect.builtin(_functionCall.functionName.name))
-		if (f->movable)
-		{
-			ASTWalker::operator()(_functionCall);
-			return;
-		}
-	m_movable = false;
+	{
+		if (!f->movable)
+			m_movable = false;
+		if (!f->sideEffectFree)
+			m_sideEffectFree = false;
+	}
+	else
+	{
+		m_movable = false;
+		m_sideEffectFree = false;
+	}
 }
 
 void MovableChecker::visit(Statement const&)

libyul/optimiser/Semantics.h

@@ -46,6 +46,8 @@ public:
 	using ASTWalker::visit;
 
 	bool movable() const { return m_movable; }
+	bool sideEffectFree() const { return m_sideEffectFree; }
+
 	std::set<YulString> const& referencedVariables() const { return m_variableReferences; }
 
 private:
@@ -54,6 +56,9 @@ private:
 	std::set<YulString> m_variableReferences;
 	/// Is the current expression movable or not.
 	bool m_movable = true;
+	/// Is the current expression side-effect free, i.e. can be removed
+	/// without changing the semantics.
+	bool m_sideEffectFree = true;
 };
 
 /**