Merge pull request #5292 from ethereum/redundantAssignEliminator

[Yul] Redundant assign eliminator.

libyul/optimiser/RedundantAssignEliminator.cpp

@@ -0,0 +1,193 @@
+/*
+	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 removes assignments to variables that are not used
+ * until they go out of scope or are re-assigned.
+ */
+
+#include <libyul/optimiser/RedundantAssignEliminator.h>
+
+#include <libyul/optimiser/Semantics.h>
+
+#include <libsolidity/inlineasm/AsmData.h>
+
+#include <libdevcore/CommonData.h>
+
+#include <boost/range/algorithm_ext/erase.hpp>
+
+using namespace std;
+using namespace dev;
+using namespace dev::yul;
+using namespace dev::solidity;
+
+void RedundantAssignEliminator::operator()(Identifier const& _identifier)
+{
+	changeUndecidedTo(_identifier.name, State::Used);
+}
+
+void RedundantAssignEliminator::operator()(VariableDeclaration const& _variableDeclaration)
+{
+	ASTWalker::operator()(_variableDeclaration);
+
+	for (auto const& var: _variableDeclaration.variables)
+		m_declaredVariables.insert(var.name);
+}
+
+void RedundantAssignEliminator::operator()(Assignment const& _assignment)
+{
+	visit(*_assignment.value);
+	for (auto const& var: _assignment.variableNames)
+		changeUndecidedTo(var.name, State::Unused);
+
+	if (_assignment.variableNames.size() == 1)
+		// Default-construct it in "Undecided" state if it does not yet exist.
+		m_assignments[_assignment.variableNames.front().name][&_assignment];
+}
+
+void RedundantAssignEliminator::operator()(If const& _if)
+{
+	visit(*_if.condition);
+
+	RedundantAssignEliminator branch{*this};
+	branch(_if.body);
+
+	join(branch);
+}
+
+void RedundantAssignEliminator::operator()(Switch const& _switch)
+{
+	visit(*_switch.expression);
+
+	bool hasDefault = false;
+	vector<RedundantAssignEliminator> branches;
+	for (auto const& c: _switch.cases)
+	{
+		if (!c.value)
+			hasDefault = true;
+		branches.emplace_back(*this);
+		branches.back()(c.body);
+	}
+
+	if (hasDefault)
+	{
+		*this = std::move(branches.back());
+		branches.pop_back();
+	}
+	for (auto& branch: branches)
+		join(branch);
+}
+
+void RedundantAssignEliminator::operator()(FunctionDefinition const& _functionDefinition)
+{
+	(*this)(_functionDefinition.body);
+
+	for (auto const& param: _functionDefinition.parameters)
+		changeUndecidedTo(param.name, State::Unused);
+	for (auto const& retParam: _functionDefinition.returnVariables)
+		changeUndecidedTo(retParam.name, State::Used);
+}
+
+void RedundantAssignEliminator::operator()(ForLoop const& _forLoop)
+{
+	// This will set all variables that are declared in this
+	// block to "unused" when it is destroyed.
+	BlockScope scope(*this);
+
+	// We need to visit the statements directly because of the
+	// scoping rules.
+	walkVector(_forLoop.pre.statements);
+
+	// We just run the loop twice to account for the
+	// back edge.
+	// There need not be more runs because we only have three different states.
+
+	visit(*_forLoop.condition);
+
+	RedundantAssignEliminator zeroRuns{*this};
+
+	(*this)(_forLoop.body);
+	(*this)(_forLoop.post);
+
+	visit(*_forLoop.condition);
+
+	RedundantAssignEliminator oneRun{*this};
+
+	(*this)(_forLoop.body);
+	(*this)(_forLoop.post);
+
+	visit(*_forLoop.condition);
+
+	// Order does not matter because "max" is commutative and associative.
+	join(oneRun);
+	join(zeroRuns);
+}
+
+void RedundantAssignEliminator::operator()(Block const& _block)
+{
+	// This will set all variables that are declared in this
+	// block to "unused" when it is destroyed.
+	BlockScope scope(*this);
+
+	ASTWalker::operator()(_block);
+}
+
+void RedundantAssignEliminator::run(Block& _ast)
+{
+	RedundantAssignEliminator rae;
+	rae(_ast);
+
+	std::set<Assignment const*> assignmentsToRemove;
+	for (auto const& variables: rae.m_assignments)
+		for (auto const& assignment: variables.second)
+		{
+			assertThrow(assignment.second != State::Undecided, OptimizerException, "");
+			if (assignment.second == State::Unused && MovableChecker{*assignment.first->value}.movable())
+				assignmentsToRemove.insert(assignment.first);
+		}
+
+	AssignmentRemover remover{assignmentsToRemove};
+	remover(_ast);
+}
+
+void RedundantAssignEliminator::join(RedundantAssignEliminator& _other)
+{
+	for (auto& var: _other.m_assignments)
+		if (m_assignments.count(var.first))
+		{
+			map<Assignment const*, State>& assignmentsHere = m_assignments[var.first];
+			for (auto& assignment: var.second)
+				assignmentsHere[assignment.first].join(assignment.second);
+		}
+		else
+			m_assignments[var.first] = std::move(var.second);
+}
+
+void RedundantAssignEliminator::changeUndecidedTo(string const& _variable, RedundantAssignEliminator::State _newState)
+{
+	for (auto& assignment: m_assignments[_variable])
+		if (assignment.second == State{State::Undecided})
+			assignment.second = _newState;
+}
+
+void AssignmentRemover::operator()(Block& _block)
+{
+	boost::range::remove_erase_if(_block.statements, [=](Statement const& _statement) -> bool {
+		return _statement.type() == typeid(Assignment) && m_toRemove.count(&boost::get<Assignment>(_statement));
+	});
+
+	ASTModifier::operator()(_block);
+}

libyul/optimiser/RedundantAssignEliminator.h

@@ -0,0 +1,185 @@
+/*
+	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 removes assignments to variables that are not used
+ * until they go out of scope or are re-assigned.
+ */
+
+#pragma once
+
+#include <libyul/ASTDataForward.h>
+
+#include <libyul/optimiser/ASTWalker.h>
+
+#include <map>
+
+namespace dev
+{
+namespace yul
+{
+
+/**
+ * Optimiser component that removes assignments to variables that are not used
+ * until they go out of scope or are re-assigned. This component
+ * respects the control-flow and takes it into account for removal.
+ *
+ * Example:
+ *
+ * {
+ *   let a
+ *   a := 1
+ *   a := 2
+ *   b := 2
+ *   if calldataload(0)
+ *   {
+ *     b := mload(a)
+ *   }
+ *   a := b
+ * }
+ *
+ * In the example, "a := 1" can be removed because the value from this assignment
+ * is not used in any control-flow branch (it is replaced right away).
+ * The assignment "a := 2" is also overwritten by "a := b" at the end,
+ * but there is a control-flow path (through the condition body) which uses
+ * the value from "a := 2" and thus, this assignment cannot be removed.
+ *
+ * Detailed rules:
+ *
+ * The AST is traversed twice: in an information gathering step and in the
+ * actual removal step. During information gathering, we maintain a
+ * mapping from assignment statements to the three states
+ * "unused", "undecided" and "used".
+ * When an assignment is visited, it is added to the mapping in the "undecided" state
+ * (see remark about for loops below) and every other assignment to the same variable
+ * that is still in the "undecided" state is changed to "unused".
+ * When a variable is referenced, the state of any assignment to that variable still
+ * in the "undecided" state is changed to "used".
+ * At points where control flow splits, a copy
+ * of the mapping is handed over to each branch. At points where control flow
+ * joins, the two mappings coming from the two branches are combined in the following way:
+ * Statements that are only in one mapping or have the same state are used unchanged.
+ * Conflicting values are resolved in the following way:
+ * "unused", "undecided" -> "undecided"
+ * "unused", "used" -> "used"
+ * "undecided, "used" -> "used".
+ *
+ * For for-loops, the condition, body and post-part are visited twice, taking
+ * the joining control-flow at the condition into account.
+ * In other words, we create three control flow paths: Zero runs of the loop,
+ * one run and two runs and then combine them at the end.
+ * Running at most twice is enough because there are only three different states.
+ *
+ * For switch statements that have a "default"-case, there is no control-flow
+ * part that skips the switch.
+ *
+ * When a variable goes out of scope, all statements still in the "undecided"
+ * state are changed to "unused", unless the variable is the return
+ * parameter of a function - there, the state changes to "used".
+ *
+ * In the second traversal, all assignments that are in the "unused" state are removed.
+ *
+ *
+ * This step is usually run right after the SSA transform to complete
+ * the generation of the pseudo-SSA.
+ *
+ * Prerequisite: Disambiguator.
+ */
+class RedundantAssignEliminator: public ASTWalker
+{
+public:
+	RedundantAssignEliminator(RedundantAssignEliminator const&) = default;
+	RedundantAssignEliminator& operator=(RedundantAssignEliminator const&) = default;
+	RedundantAssignEliminator(RedundantAssignEliminator&&) = default;
+	RedundantAssignEliminator& operator=(RedundantAssignEliminator&&) = default;
+
+	void operator()(Identifier const& _identifier) override;
+	void operator()(VariableDeclaration const& _variableDeclaration) override;
+	void operator()(Assignment const& _assignment) override;
+	void operator()(If const& _if) override;
+	void operator()(Switch const& _switch) override;
+	void operator()(FunctionDefinition const&) override;
+	void operator()(ForLoop const&) override;
+	void operator()(Block const& _block) override;
+
+	static void run(Block& _ast);
+
+private:
+	RedundantAssignEliminator() {}
+
+	class State
+	{
+	public:
+		enum Value { Unused, Undecided, Used };
+		State(Value _value = Undecided): m_value(_value) {}
+		bool operator==(State _other) const { return m_value == _other.m_value; }
+		bool operator!=(State _other) const { return !operator==(_other); }
+		void join(State _other)
+		{
+			// Using "max" works here because of the order of the values in the enum.
+			m_value = Value(std::max(int(_other.m_value), int(m_value)));
+		}
+	private:
+		Value m_value = Undecided;
+	};
+
+	/**
+	 * Takes care about storing the list of declared variables and
+	 * sets them to "unused" when it is destroyed.
+	 */
+	class BlockScope
+	{
+	public:
+		explicit BlockScope(RedundantAssignEliminator& _rae): m_rae(_rae)
+		{
+			swap(m_rae.m_declaredVariables, m_outerDeclaredVariables);
+		}
+		~BlockScope()
+		{
+			for (auto const& var: m_rae.m_declaredVariables)
+				m_rae.changeUndecidedTo(var, State::Unused);
+			swap(m_rae.m_declaredVariables, m_outerDeclaredVariables);
+		}
+
+	private:
+		RedundantAssignEliminator& m_rae;
+		std::set<std::string> m_outerDeclaredVariables;
+	};
+
+	/// Joins the assignment mapping with @a _other according to the rules laid out
+	/// above.
+	/// Will destroy @a _other.
+	void join(RedundantAssignEliminator& _other);
+	void changeUndecidedTo(std::string const& _variable, State _newState);
+
+	std::set<std::string> m_declaredVariables;
+	std::map<std::string, std::map<Assignment const*, State>> m_assignments;
+};
+
+class AssignmentRemover: public ASTModifier
+{
+public:
+	explicit AssignmentRemover(std::set<Assignment const*> const& _toRemove):
+		m_toRemove(_toRemove)
+	{}
+	void operator()(Block& _block) override;
+
+private:
+	std::set<Assignment const*> const& m_toRemove;
+};
+
+}
+}

test/libyul/YulOptimizerTest.cpp

@@ -36,6 +36,7 @@
 #include <libyul/optimiser/UnusedPruner.h>
 #include <libyul/optimiser/ExpressionJoiner.h>
 #include <libyul/optimiser/SSATransform.h>
+#include <libyul/optimiser/RedundantAssignEliminator.h>
 
 #include <libsolidity/parsing/Scanner.h>
 #include <libsolidity/inlineasm/AsmPrinter.h>
@@ -178,6 +179,18 @@ bool YulOptimizerTest::run(ostream& _stream, string const& _linePrefix, bool con
 		NameDispenser nameDispenser(*m_ast);
 		SSATransform::run(*m_ast, nameDispenser);
 	}
+	else if (m_optimizerStep == "redundantAssignEliminator")
+	{
+		disambiguate();
+		RedundantAssignEliminator::run(*m_ast);
+	}
+	else if (m_optimizerStep == "ssaPlusCleanup")
+	{
+		disambiguate();
+		NameDispenser nameDispenser(*m_ast);
+		SSATransform::run(*m_ast, nameDispenser);
+		RedundantAssignEliminator::run(*m_ast);
+	}
 	else
 	{
 		FormattedScope(_stream, _formatted, {formatting::BOLD, formatting::RED}) << _linePrefix << "Invalid optimizer step: " << m_optimizerStep << endl;

test/libyul/yulOptimizerTests/redundantAssignEliminator/for.yul

@@ -0,0 +1,26 @@
+{
+    for {
+        let a := 2
+        // Should not be removed, even though you might think
+        // it goes out of scope
+        a := 3
+    } a { a := add(a, 1) }
+    {
+        a := 7
+    }
+}
+// ----
+// redundantAssignEliminator
+// {
+//     for {
+//         let a := 2
+//         a := 3
+//     }
+//     a
+//     {
+//         a := add(a, 1)
+//     }
+//     {
+//         a := 7
+//     }
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/for_branch.yul

@@ -0,0 +1,31 @@
+{
+    let x
+    let y
+    // Cannot be removed, because we might skip the loop
+    x := 1
+    for { } calldataload(0) { }
+    {
+        // Cannot be removed
+        x := 2
+        // Can be removed
+        y := 3
+    }
+    y := 8
+    mstore(x, 0)
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let x
+//     let y
+//     x := 1
+//     for {
+//     }
+//     calldataload(0)
+//     {
+//     }
+//     {
+//         x := 2
+//     }
+//     mstore(x, 0)
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/for_rerun.yul

@@ -0,0 +1,27 @@
+{
+    let x
+    // Cannot be removed, because we might run the loop only once
+    x := 1
+    for { } calldataload(0) { }
+    {
+        mstore(x, 2)
+        // Cannot be removed because of the line above
+        x := 2
+    }
+    x := 3
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let x
+//     x := 1
+//     for {
+//     }
+//     calldataload(0)
+//     {
+//     }
+//     {
+//         mstore(x, 2)
+//         x := 2
+//     }
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/function.yul

@@ -0,0 +1,23 @@
+{
+    let r
+    r := 1
+    function f(x, y) -> a, b {
+        // Can be removed, is param
+        x := 1
+        y := 2
+        // Cannot be removed, is return param
+        a := 3
+        b := 4
+    }
+    r := 2
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let r
+//     function f(x, y) -> a, b
+//     {
+//         a := 3
+//         b := 4
+//     }
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/if.yul

@@ -0,0 +1,24 @@
+{
+    let c
+    let d
+    c := calldataload(0)
+    d := 1
+    if c {
+        d := 2
+    }
+    // This enforces that none of the assignments above can be removed.
+    mstore(0, d)
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let c
+//     let d
+//     c := calldataload(0)
+//     d := 1
+//     if c
+//     {
+//         d := 2
+//     }
+//     mstore(0, d)
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/if_overwrite_all_branches.yul

@@ -0,0 +1,24 @@
+{
+    let c
+    let d
+    c := calldataload(0)
+    // This assignment will be overwritten in all branches and thus can be removed.
+    d := 1
+    if c {
+        d := 2
+    }
+    d := 3
+    mstore(0, d)
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let c
+//     let d
+//     c := calldataload(0)
+//     if c
+//     {
+//     }
+//     d := 3
+//     mstore(0, d)
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/if_used_in_one_branch.yul

@@ -0,0 +1,25 @@
+{
+    let c
+    let d
+    c := calldataload(0)
+    d := 1
+    if c {
+        // Uses the assignment above
+        d := d
+    }
+    d := 3
+    mstore(0, d)
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let c
+//     let d
+//     c := calldataload(0)
+//     d := 1
+//     if c
+//     {
+//     }
+//     d := 3
+//     mstore(0, d)
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/multi_assign.yul

@@ -0,0 +1,19 @@
+{
+    function f() -> a, b {}
+    let x, y
+    x := 1
+    x := 2
+    // Will not be used, but is a multi-assign, so not removed.
+    x, y := f()
+    x := 3
+    y := 4
+}
+// ----
+// redundantAssignEliminator
+// {
+//     function f() -> a, b
+//     {
+//     }
+//     let x, y
+//     x, y := f()
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/multivar.yul

@@ -0,0 +1,15 @@
+{
+    let a := 2
+    a := 7
+    let b := 8
+    b := a
+    a := b
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let a := 2
+//     a := 7
+//     let b := 8
+//     b := a
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/non_movable.yul

@@ -0,0 +1,11 @@
+{
+    let a
+    a := 0
+    a := mload(0)
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let a
+//     a := mload(0)
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/scopes.yul

@@ -0,0 +1,16 @@
+{
+    let a
+    {
+        let b
+        b := 2
+        a := 2
+    }
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let a
+//     {
+//         let b
+//     }
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/simple.yul

@@ -0,0 +1,10 @@
+{
+    let a
+    a := 1
+    a := 2
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let a
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/switch_overwrite_in_all.yul

@@ -0,0 +1,22 @@
+{
+    let x
+    // Will be overwritten in all branches
+    x := 1
+    switch calldataload(0)
+    case 0 { x := 2 }
+    default { x := 3 }
+    mstore(x, 0)
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let x
+//     switch calldataload(0)
+//     case 0 {
+//         x := 2
+//     }
+//     default {
+//         x := 3
+//     }
+//     mstore(x, 0)
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/switch_overwrite_in_one.yul

@@ -0,0 +1,19 @@
+{
+    let x
+    // Will NOT be overwritten in all branches
+    x := 1
+    switch calldataload(0)
+    case 0 { x := 2 }
+    mstore(x, 0)
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let x
+//     x := 1
+//     switch calldataload(0)
+//     case 0 {
+//         x := 2
+//     }
+//     mstore(x, 0)
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/switch_overwrite_use_combination.yul

@@ -0,0 +1,23 @@
+{
+    let x
+    // Will be used in some and overwritten in others
+    x := 1
+    switch calldataload(0)
+    case 0 { x := 2 }
+    default { mstore(x, 1) }
+    mstore(x, 0)
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let x
+//     x := 1
+//     switch calldataload(0)
+//     case 0 {
+//         x := 2
+//     }
+//     default {
+//         mstore(x, 1)
+//     }
+//     mstore(x, 0)
+// }

test/libyul/yulOptimizerTests/redundantAssignEliminator/switch_unused.yul

@@ -0,0 +1,16 @@
+{
+    let x
+    // Not referenced anywhere.
+    x := 1
+    switch calldataload(0)
+    case 0 { mstore(0, 1) }
+}
+// ----
+// redundantAssignEliminator
+// {
+//     let x
+//     switch calldataload(0)
+//     case 0 {
+//         mstore(0, 1)
+//     }
+// }

test/libyul/yulOptimizerTests/ssaPlusCleanup/control_structures.yul

@@ -0,0 +1,35 @@
+{
+    function copy(from, to) -> length {
+        length := mload(from)
+        mstore(to, length)
+        from := add(from, 0x20)
+        to := add(to, 0x20)
+        for { let x := 1 } lt(x, length) { x := add(x, 0x20) } {
+            mstore(add(to, x), mload(add(from, x)))
+        }
+    }
+}
+// ----
+// ssaPlusCleanup
+// {
+//     function copy(from, to) -> length
+//     {
+//         let length_1 := mload(from)
+//         length := length_1
+//         mstore(to, length_1)
+//         let from_1 := add(from, 0x20)
+//         let to_1 := add(to, 0x20)
+//         for {
+//             let x_1 := 1
+//             let x := x_1
+//         }
+//         lt(x, length_1)
+//         {
+//             let x_2 := add(x, 0x20)
+//             x := x_2
+//         }
+//         {
+//             mstore(add(to_1, x), mload(add(from_1, x)))
+//         }
+//     }
+// }

test/libyul/yulOptimizerTests/ssaPlusCleanup/multi_reassign.yul

@@ -0,0 +1,17 @@
+{
+    let a := 1
+    a := 2
+    a := 3
+    a := 4
+    mstore(0, a)
+}
+// ----
+// ssaPlusCleanup
+// {
+//     let a_1 := 1
+//     let a := a_1
+//     let a_2 := 2
+//     let a_3 := 3
+//     let a_4 := 4
+//     mstore(0, a_4)
+// }

test/libyul/yulOptimizerTests/ssaPlusCleanup/multi_reassign_with_use.yul

@@ -0,0 +1,17 @@
+{
+    let a := 1
+    a := add(a, 2)
+    a := add(a, 3)
+    a := mload(add(a, 4))
+    mstore(0, a)
+}
+// ----
+// ssaPlusCleanup
+// {
+//     let a_1 := 1
+//     let a := a_1
+//     let a_2 := add(a_1, 2)
+//     let a_3 := add(a_2, 3)
+//     let a_4 := mload(add(a_3, 4))
+//     mstore(0, a_4)
+// }