Merge pull request #7414 from ethereum/refactorSSA

Refactor SSA transform.

libyul/optimiser/SSATransform.cpp

@@ -32,45 +32,32 @@ using namespace dev;
 using namespace langutil;
 using namespace yul;
 
-void SSATransform::operator()(Identifier& _identifier)
+namespace
 {
-	if (m_currentVariableValues.count(_identifier.name))
-		_identifier.name = m_currentVariableValues[_identifier.name];
-}
 
-void SSATransform::operator()(ForLoop& _for)
+/**
+ * First step of SSA transform: Introduces new SSA variables for each assignment or
+ * declaration of a variable to be replaced.
+ */
+class IntroduceSSA: public ASTModifier
 {
-	// This will clear the current value in case of a reassignment inside the
-	// init part, although the new variable would still be in scope inside the whole loop.
-	// This small inefficiency is fine if we move the pre part of all for loops out
-	// of the for loop.
-	(*this)(_for.pre);
+public:
+	explicit IntroduceSSA(NameDispenser& _nameDispenser, set<YulString> const& _variablesToReplace):
+		m_nameDispenser(_nameDispenser), m_variablesToReplace(_variablesToReplace)
+	{ }
 
-	Assignments assignments;
-	assignments(_for.body);
-	assignments(_for.post);
-	for (auto const& var: assignments.names())
-		m_currentVariableValues.erase(var);
+	void operator()(Block& _block) override;
 
-	visit(*_for.condition);
-	(*this)(_for.body);
-	(*this)(_for.post);
-}
-
-
-void SSATransform::operator()(Block& _block)
-{
-	set<YulString> variablesToClearAtEnd;
-
-	// Creates a new variable and stores it in the current variable value map.
-	auto newVariable = [&](YulString _varName) -> YulString
-	{
-		YulString newName = m_nameDispenser.newName(_varName);
-		m_currentVariableValues[_varName] = newName;
-		variablesToClearAtEnd.emplace(_varName);
-		return newName;
+private:
+	NameDispenser& m_nameDispenser;
+	/// This is a set of all variables that are assigned to anywhere in the code.
+	/// Variables that are only declared but never re-assigned are not touched.
+	set<YulString> const& m_variablesToReplace;
 };
 
+
+void IntroduceSSA::operator()(Block& _block)
+{
 	iterateReplacing(
 		_block.statements,
 		[&](Statement& _s) -> boost::optional<vector<Statement>>
@@ -96,8 +83,8 @@ void SSATransform::operator()(Block& _block)
 				TypedNameList newVariables;
 				for (auto const& var: varDecl.variables)
 				{
-					YulString newName = newVariable(var.name);
 					YulString oldName = var.name;
+					YulString newName = m_nameDispenser.newName(oldName);
 					newVariables.emplace_back(TypedName{loc, newName, {}});
 					statements.emplace_back(VariableDeclaration{
 						loc,
@@ -123,8 +110,8 @@ void SSATransform::operator()(Block& _block)
 				TypedNameList newVariables;
 				for (auto const& var: assignment.variableNames)
 				{
-					YulString newName = newVariable(var.name);
 					YulString oldName = var.name;
+					YulString newName = m_nameDispenser.newName(oldName);
 					newVariables.emplace_back(TypedName{loc, newName, {}});
 					statements.emplace_back(Assignment{
 						loc,
@@ -140,14 +127,111 @@ void SSATransform::operator()(Block& _block)
 			return {};
 		}
 	);
-	for (auto const& var: variablesToClearAtEnd)
+}
+
+/**
+ * Second step of SSA transform: Replace the references to variables-to-be-replaced
+ * by their current values.
+ */
+class PropagateValues: public ASTModifier
+{
+public:
+	explicit PropagateValues(set<YulString> const& _variablesToReplace):
+		m_variablesToReplace(_variablesToReplace)
+	{ }
+
+	void operator()(Identifier& _identifier) override;
+	void operator()(VariableDeclaration& _varDecl) override;
+	void operator()(Assignment& _assignment) override;
+	void operator()(ForLoop& _for) override;
+	void operator()(Block& _block) override;
+
+private:
+	/// This is a set of all variables that are assigned to anywhere in the code.
+	/// Variables that are only declared but never re-assigned are not touched.
+	set<YulString> const& m_variablesToReplace;
+	map<YulString, YulString> m_currentVariableValues;
+	set<YulString> m_clearAtEndOfBlock;
+};
+
+void PropagateValues::operator()(Identifier& _identifier)
+{
+	if (m_currentVariableValues.count(_identifier.name))
+		_identifier.name = m_currentVariableValues[_identifier.name];
+}
+
+void PropagateValues::operator()(VariableDeclaration& _varDecl)
+{
+	ASTModifier::operator()(_varDecl);
+
+	if (_varDecl.variables.size() != 1)
+		return;
+	YulString name = _varDecl.variables.front().name;
+	if (!m_variablesToReplace.count(name))
+		return;
+
+	yulAssert(_varDecl.value->type() == typeid(Identifier), "");
+	m_currentVariableValues[name] = boost::get<Identifier>(*_varDecl.value).name;
+	m_clearAtEndOfBlock.insert(name);
+}
+
+
+void PropagateValues::operator()(Assignment& _assignment)
+{
+	visit(*_assignment.value);
+
+	if (_assignment.variableNames.size() != 1)
+		return;
+	YulString name = _assignment.variableNames.front().name;
+	if (!m_variablesToReplace.count(name))
+		return;
+
+	yulAssert(_assignment.value->type() == typeid(Identifier), "");
+	m_currentVariableValues[name] = boost::get<Identifier>(*_assignment.value).name;
+	m_clearAtEndOfBlock.insert(name);
+}
+
+void PropagateValues::operator()(ForLoop& _for)
+{
+	// This will clear the current value in case of a reassignment inside the
+	// init part, although the new variable would still be in scope inside the whole loop.
+	// This small inefficiency is fine if we move the pre part of all for loops out
+	// of the for loop.
+	(*this)(_for.pre);
+
+	Assignments assignments;
+	assignments(_for.body);
+	assignments(_for.post);
+
+	for (auto const& var: assignments.names())
 		m_currentVariableValues.erase(var);
+
+	visit(*_for.condition);
+	(*this)(_for.body);
+	(*this)(_for.post);
+}
+
+void PropagateValues::operator()(Block& _block)
+{
+	set<YulString> clearAtParentBlock = std::move(m_clearAtEndOfBlock);
+	m_clearAtEndOfBlock.clear();
+
+	ASTModifier::operator()(_block);
+
+	for (auto const& var: m_clearAtEndOfBlock)
+		m_currentVariableValues.erase(var);
+
+	m_clearAtEndOfBlock = std::move(clearAtParentBlock);
+}
+
 }
 
 void SSATransform::run(Block& _ast, NameDispenser& _nameDispenser)
 {
 	Assignments assignments;
 	assignments(_ast);
-	SSATransform{_nameDispenser, assignments.names()}(_ast);
+	IntroduceSSA{_nameDispenser, assignments.names()}(_ast);
+	PropagateValues{assignments.names()}(_ast);
 }
 
+

libyul/optimiser/SSATransform.h

@@ -75,22 +75,7 @@ class NameDispenser;
 class SSATransform: public ASTModifier
 {
 public:
-	void operator()(Identifier&) override;
-	void operator()(ForLoop&) override;
-	void operator()(Block& _block) override;
-
 	static void run(Block& _ast, NameDispenser& _nameDispenser);
-
-private:
-	explicit SSATransform(NameDispenser& _nameDispenser, std::set<YulString> const& _variablesToReplace):
-		m_nameDispenser(_nameDispenser), m_variablesToReplace(_variablesToReplace)
-	{ }
-
-	NameDispenser& m_nameDispenser;
-	/// This is a set of all variables that are assigned to anywhere in the code.
-	/// Variables that are only declared but never re-assigned are not touched.
-	std::set<YulString> const& m_variablesToReplace;
-	std::map<YulString, YulString> m_currentVariableValues;
 };
 
 }

test/libyul/yulOptimizerTests/ssaAndBack/for_loop.yul

@@ -23,8 +23,8 @@
 //     let b := mload(1)
 //     for { } lt(mload(a), mload(b)) { a := mload(b) }
 //     {
-//         let b_3 := mload(a)
-//         let a_6 := mload(b_3)
-//         b := mload(a_6)
+//         let b_4 := mload(a)
+//         let a_7 := mload(b_4)
+//         b := mload(a_7)
 //     }
 // }

test/libyul/yulOptimizerTests/ssaTransform/for_simple.yul

@@ -36,12 +36,12 @@
 //     }
 //     a
 //     {
-//         let a_7 := add(a, 6)
-//         a := a_7
+//         let a_6 := add(a, 6)
+//         a := a_6
 //     }
 //     {
-//         let a_6 := add(a, 12)
-//         a := a_6
+//         let a_7 := add(a, 12)
+//         a := a_7
 //     }
 //     let a_8 := add(a, 8)
 //     a := a_8