Merge pull request #5267 from ethereum/ssatransform

SSA transform - first step.

2 files changed, 228 insertions, 0 deletions

diff --git a/libyul/optimiser/SSATransform.cpp b/libyul/optimiser/SSATransform.cpp
new file mode 100644
index 00000000..9db7bd03
--- /dev/null
+++ b/libyul/optimiser/SSATransform.cpp
@@ -0,0 +1,130 @@
+/*
+	This file is part of solidity.
+
+	solidity is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	solidity is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+/**
+ * Optimiser component that turns subsequent assignments to variable declarations
+ * and assignments.
+ */
+
+#include <libyul/optimiser/SSATransform.h>
+
+#include <libyul/optimiser/NameCollector.h>
+#include <libyul/optimiser/NameDispenser.h>
+
+#include <libsolidity/inlineasm/AsmData.h>
+
+#include <libdevcore/CommonData.h>
+
+
+using namespace std;
+using namespace dev;
+using namespace dev::yul;
+using namespace dev::solidity;
+
+void SSATransform::operator()(Identifier& _identifier)
+{
+	if (m_currentVariableValues.count(_identifier.name))
+		_identifier.name = m_currentVariableValues[_identifier.name];
+}
+
+void SSATransform::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 SSATransform::operator()(Block& _block)
+{
+	set<string> variablesToClearAtEnd;
+
+	// Creates a new variable (and returns its declaration) with value _value
+	// and replaces _value by a reference to that new variable.
+	auto replaceByNew = [&](SourceLocation _loc, string _varName, string _type, shared_ptr<Expression>& _value) -> VariableDeclaration
+	{
+		string newName = m_nameDispenser.newName(_varName);
+		m_currentVariableValues[_varName] = newName;
+		variablesToClearAtEnd.insert(_varName);
+		shared_ptr<Expression> v = make_shared<Expression>(Identifier{_loc, newName});
+		_value.swap(v);
+		return VariableDeclaration{_loc, {TypedName{_loc, std::move(newName), std::move(_type)}}, std::move(v)};
+	};
+
+	iterateReplacing(
+		_block.statements,
+		[&](Statement& _s) -> boost::optional<vector<Statement>>
+		{
+			if (_s.type() == typeid(VariableDeclaration))
+			{
+				VariableDeclaration& varDecl = boost::get<VariableDeclaration>(_s);
+				if (varDecl.value)
+					visit(*varDecl.value);
+				if (varDecl.variables.size() != 1 || !m_variablesToReplace.count(varDecl.variables.front().name))
+					return {};
+				// Replace "let a := v" by "let a_1 := v  let a := v"
+				VariableDeclaration newVarDecl = replaceByNew(
+					varDecl.location,
+					varDecl.variables.front().name,
+					varDecl.variables.front().type,
+					varDecl.value
+				);
+				return vector<Statement>{std::move(newVarDecl), std::move(varDecl)};
+			}
+			else if (_s.type() == typeid(Assignment))
+			{
+				Assignment& assignment = boost::get<Assignment>(_s);
+				visit(*assignment.value);
+				if (assignment.variableNames.size() != 1)
+					return {};
+				assertThrow(m_variablesToReplace.count(assignment.variableNames.front().name), OptimizerException, "");
+				// Replace "a := v" by "let a_1 := v  a := v"
+				VariableDeclaration newVarDecl = replaceByNew(
+					assignment.location,
+					assignment.variableNames.front().name,
+					{}, // TODO determine type
+					assignment.value
+				);
+				return vector<Statement>{std::move(newVarDecl), std::move(assignment)};
+			}
+			else
+				visit(_s);
+			return {};
+		}
+	);
+	for (auto const& var: variablesToClearAtEnd)
+		m_currentVariableValues.erase(var);
+}
+
+void SSATransform::run(Block& _ast, NameDispenser& _nameDispenser)
+{
+	Assignments assignments;
+	assignments(_ast);
+	SSATransform{_nameDispenser, assignments.names()}(_ast);
+}
+
diff --git a/libyul/optimiser/SSATransform.h b/libyul/optimiser/SSATransform.h
new file mode 100644
index 00000000..2adc657d
--- /dev/null
+++ b/libyul/optimiser/SSATransform.h
@@ -0,0 +1,98 @@
+/*
+	This file is part of solidity.
+
+	solidity is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	solidity is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+/**
+ * Optimiser component that turns subsequent assignments to variable declarations
+ * and assignments.
+ */
+#pragma once
+
+#include <libyul/ASTDataForward.h>
+
+#include <libyul/optimiser/ASTWalker.h>
+
+#include <vector>
+
+namespace dev
+{
+namespace yul
+{
+
+class NameDispenser;
+
+/**
+ * Optimizer stage that tries to replace repeated assignments to
+ * existing variables by declarations of new variables as much as
+ * possible.
+ * The reassignments are still there, but all references to the
+ * reassigned variables are replaced by the newly declared variables.
+ *
+ * Example:
+ * {
+ *   let a := 1
+ *   mstore(a, 2)
+ *   a := 3
+ * }
+ * is transformed to
+ * {
+ *   let a_1 := 1
+ *   let a := a_1
+ *   mstore(a_1, 2)
+ *   let a_3 := 3
+ *   a := a_3
+ * }
+ *
+ * Exact semantics:
+ *
+ * For any variable a that is assigned to somewhere in the code (assignment with
+ * declaration does not count) perform the following transforms:
+ *  - replace "let a := v" by "let a_1 := v   let a := a_1"
+ *  - replace "a := v" by "let a_1 := v   a := a_1"
+ * Furthermore, always note the current variable/value assigned to a and replace each
+ * reference to a by this variable.
+ * The current value mapping is cleared for a variable a at the end of each block
+ * in which it was assigned and just after the for loop init block if it is assigned
+ * inside the for loop.
+ *
+ * After this stage, redundantAssignmentRemover is recommended to remove the unnecessary
+ * intermediate assignments.
+ *
+ * This stage provides best results if CSE is run right before it, because
+ * then it does not generate excessive amounts of variables.
+ *
+ * TODO Which transforms are required to keep this idempotent?
+ */
+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<std::string> const& _variablesToReplace):
+		m_nameDispenser(_nameDispenser), m_variablesToReplace(_variablesToReplace)
+	{ }
+
+	NameDispenser& m_nameDispenser;
+	std::set<std::string> const& m_variablesToReplace;
+	std::map<std::string, std::string> m_currentVariableValues;
+};
+
+}
+}