Merge pull request #5649 from ethereum/returnsAreZero

[Yul] Consider return variables to be zero initially.

3 files changed, 15 insertions, 2 deletions

diff --git a/libyul/optimiser/DataFlowAnalyzer.cpp b/libyul/optimiser/DataFlowAnalyzer.cpp
index 7bff2c89..c8d236dc 100644
--- a/libyul/optimiser/DataFlowAnalyzer.cpp
+++ b/libyul/optimiser/DataFlowAnalyzer.cpp
@@ -96,7 +96,10 @@ void DataFlowAnalyzer::operator()(FunctionDefinition& _fun)
 	for (auto const& parameter: _fun.parameters)
 		m_variableScopes.back().variables.emplace(parameter.name);
 	for (auto const& var: _fun.returnVariables)
+	{
 		m_variableScopes.back().variables.emplace(var.name);
+		handleAssignment({var.name}, nullptr);
+	}
 	ASTModifier::operator()(_fun);
 
 	popScope();
diff --git a/libyul/optimiser/SSAValueTracker.cpp b/libyul/optimiser/SSAValueTracker.cpp
index a0e44b2b..23eb9ec2 100644
--- a/libyul/optimiser/SSAValueTracker.cpp
+++ b/libyul/optimiser/SSAValueTracker.cpp
@@ -33,12 +33,18 @@ void SSAValueTracker::operator()(Assignment const& _assignment)
 		m_values.erase(var.name);
 }
 
+void SSAValueTracker::operator()(FunctionDefinition const& _funDef)
+{
+	for (auto const& var: _funDef.returnVariables)
+		setValue(var.name, nullptr);
+	ASTWalker::operator()(_funDef);
+}
+
 void SSAValueTracker::operator()(VariableDeclaration const& _varDecl)
 {
-	static Expression const zero{Literal{{}, LiteralKind::Number, YulString{"0"}, {}}};
 	if (!_varDecl.value)
 		for (auto const& var: _varDecl.variables)
-			setValue(var.name, &zero);
+			setValue(var.name, nullptr);
 	else if (_varDecl.variables.size() == 1)
 		setValue(_varDecl.variables.front().name, _varDecl.value.get());
 }
@@ -50,5 +56,8 @@ void SSAValueTracker::setValue(YulString _name, Expression const* _value)
 		OptimizerException,
 		"Source needs to be disambiguated."
 	);
+	static Expression const zero{Literal{{}, LiteralKind::Number, YulString{"0"}, {}}};
+	if (!_value)
+		_value = &zero;
 	m_values[_name] = _value;
 }
diff --git a/libyul/optimiser/SSAValueTracker.h b/libyul/optimiser/SSAValueTracker.h
index 0a6cde80..0680485f 100644
--- a/libyul/optimiser/SSAValueTracker.h
+++ b/libyul/optimiser/SSAValueTracker.h
@@ -41,6 +41,7 @@ class SSAValueTracker: public ASTWalker
 {
 public:
 	using ASTWalker::operator();
+	void operator()(FunctionDefinition const& _funDef) override;
 	void operator()(VariableDeclaration const& _varDecl) override;
 	void operator()(Assignment const& _assignment) override;