Add structural simplifier as optimization step for Yul.

libyul/CMakeLists.txt

@@ -36,6 +36,7 @@ add_library(yul
 	optimiser/SSAValueTracker.cpp
 	optimiser/Semantics.cpp
 	optimiser/SimplificationRules.cpp
+	optimiser/StructuralSimplifier.cpp
 	optimiser/Substitution.cpp
 	optimiser/Suite.cpp
 	optimiser/SyntacticalEquality.cpp

libyul/optimiser/SimplificationRules.cpp

@@ -208,10 +208,7 @@ Expression Pattern::toExpression(SourceLocation const& _location) const
 
 u256 Pattern::d() const
 {
-	Literal const& literal = boost::get<Literal>(matchGroupValue());
-	assertThrow(literal.kind == LiteralKind::Number, OptimizerException, "");
-	assertThrow(isValidDecimal(literal.value.str()) || isValidHex(literal.value.str()), OptimizerException, "");
-	return u256(literal.value.str());
+	return valueOfNumberLiteral(boost::get<Literal>(matchGroupValue()));
 }
 
 Expression const& Pattern::matchGroupValue() const

libyul/optimiser/StructuralSimplifier.cpp

@@ -0,0 +1,138 @@
+/*
+	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/>.
+*/
+#include <libyul/optimiser/StructuralSimplifier.h>
+#include <libyul/optimiser/Semantics.h>
+#include <libyul/optimiser/Utilities.h>
+#include <libyul/AsmData.h>
+#include <libdevcore/CommonData.h>
+#include <libdevcore/Visitor.h>
+
+using namespace std;
+using namespace dev;
+using namespace yul;
+
+namespace {
+
+ExpressionStatement makePopExpressionStatement(langutil::SourceLocation const& _location, Expression&& _expression)
+{
+	return {_location, FunctionalInstruction{
+		_location,
+		solidity::Instruction::POP,
+		{std::move(_expression)}
+	}};
+}
+
+}
+
+void StructuralSimplifier::operator()(Block& _block)
+{
+	pushScope(false);
+	simplify(_block.statements);
+	popScope();
+}
+
+void StructuralSimplifier::simplify(std::vector<yul::Statement>& _statements)
+{
+	using OptionalStatements = boost::optional<vector<Statement>>;
+	GenericFallbackReturnsVisitor<OptionalStatements, If, Switch, ForLoop> const visitor(
+		[&](If& _ifStmt) -> OptionalStatements {
+			if (_ifStmt.body.statements.empty())
+				return {{makePopExpressionStatement(_ifStmt.location, std::move(*_ifStmt.condition))}};
+			if (expressionAlwaysTrue(*_ifStmt.condition))
+				return {std::move(_ifStmt.body.statements)};
+			else if (expressionAlwaysFalse(*_ifStmt.condition))
+				return {{}};
+			return {};
+		},
+		[](Switch& _switchStmt) -> OptionalStatements {
+			if (_switchStmt.cases.size() == 1)
+			{
+				auto& switchCase = _switchStmt.cases.front();
+				auto loc = locationOf(*_switchStmt.expression);
+				if (switchCase.value)
+					return {{If{
+						std::move(_switchStmt.location),
+						make_shared<Expression>(FunctionalInstruction{
+								std::move(loc),
+								solidity::Instruction::EQ,
+								{std::move(*switchCase.value), std::move(*_switchStmt.expression)}
+						}), std::move(switchCase.body)}}};
+				else
+					return {{
+						makePopExpressionStatement(loc, std::move(*_switchStmt.expression)),
+						std::move(switchCase.body)
+					}};
+			}
+			else
+				return {};
+		},
+		[&](ForLoop& _forLoop) -> OptionalStatements {
+			if (expressionAlwaysFalse(*_forLoop.condition))
+				return {std::move(_forLoop.pre.statements)};
+			else
+				return {};
+		}
+	);
+
+	iterateReplacing(
+		_statements,
+		[&](Statement& _stmt) -> OptionalStatements
+		{
+			visit(_stmt);
+			OptionalStatements result = boost::apply_visitor(visitor, _stmt);
+			if (result)
+				simplify(*result);
+			return result;
+		}
+	);
+}
+
+bool StructuralSimplifier::expressionAlwaysTrue(Expression const& _expression)
+{
+	return boost::apply_visitor(GenericFallbackReturnsVisitor<bool, Identifier const, Literal const>(
+		[&](Identifier const& _identifier) -> bool {
+			if (auto expr = m_value[_identifier.name])
+				return expressionAlwaysTrue(*expr);
+			return false;
+		},
+		[](Literal const& _literal) -> bool {
+			static YulString const trueString("true");
+			return
+				(_literal.kind == LiteralKind::Boolean && _literal.value == trueString) ||
+				(_literal.kind == LiteralKind::Number && valueOfNumberLiteral(_literal) != u256(0))
+			;
+		}
+	), _expression);
+}
+
+bool StructuralSimplifier::expressionAlwaysFalse(Expression const& _expression)
+{
+	return boost::apply_visitor(GenericFallbackReturnsVisitor<bool, Identifier const, Literal const>(
+		[&](Identifier const& _identifier) -> bool {
+			if (auto expr = m_value[_identifier.name])
+				return expressionAlwaysFalse(*expr);
+			return false;
+		},
+		[](Literal const& _literal) -> bool {
+			static YulString const falseString("false");
+			return
+				(_literal.kind == LiteralKind::Boolean && _literal.value == falseString) ||
+				(_literal.kind == LiteralKind::Number && valueOfNumberLiteral(_literal) == u256(0))
+			;
+		}
+	), _expression);
+}

libyul/optimiser/StructuralSimplifier.h

@@ -0,0 +1,49 @@
+/*
+	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/>.
+*/
+#pragma once
+
+#include <libyul/optimiser/ASTWalker.h>
+#include <libyul/optimiser/DataFlowAnalyzer.h>
+
+namespace yul
+{
+
+/**
+ * Structural simplifier. Performs the following simplification steps:
+ * - replace if with empty body with pop(condition)
+ * - replace if with true condition with its body
+ * - remove if with false condition
+ * - turn switch with single case into if
+ * - replace switch with only default case with pop(expression) and body
+ * - remove for with false condition
+ *
+ * Prerequisites: Disambiguator
+ *
+ * Important: Can only be used on EVM code.
+ */
+class StructuralSimplifier: public DataFlowAnalyzer
+{
+public:
+	using DataFlowAnalyzer::operator();
+	void operator()(Block& _block) override;
+private:
+	void simplify(std::vector<Statement>& _statements);
+	bool expressionAlwaysTrue(Expression const &_expression);
+	bool expressionAlwaysFalse(Expression const &_expression);
+};
+
+}

libyul/optimiser/Suite.cpp

@@ -33,6 +33,7 @@
 #include <libyul/optimiser/ExpressionSimplifier.h>
 #include <libyul/optimiser/CommonSubexpressionEliminator.h>
 #include <libyul/optimiser/SSATransform.h>
+#include <libyul/optimiser/StructuralSimplifier.h>
 #include <libyul/optimiser/RedundantAssignEliminator.h>
 #include <libyul/optimiser/VarDeclPropagator.h>
 #include <libyul/AsmAnalysisInfo.h>
@@ -58,6 +59,7 @@ void OptimiserSuite::run(
 	(FunctionHoister{})(ast);
 	(FunctionGrouper{})(ast);
 	(ForLoopInitRewriter{})(ast);
+	StructuralSimplifier{}(ast);
 
 	NameDispenser dispenser{ast};
 
@@ -71,6 +73,7 @@ void OptimiserSuite::run(
 
 		CommonSubexpressionEliminator{}(ast);
 		ExpressionSimplifier::run(ast);
+		StructuralSimplifier{}(ast);
 		SSATransform::run(ast, dispenser);
 		RedundantAssignEliminator::run(ast);
 		RedundantAssignEliminator::run(ast);
@@ -98,6 +101,7 @@ void OptimiserSuite::run(
 		VarDeclPropagator{}(ast);
 		RedundantAssignEliminator::run(ast);
 		ExpressionSimplifier::run(ast);
+		StructuralSimplifier{}(ast);
 		CommonSubexpressionEliminator{}(ast);
 		SSATransform::run(ast, dispenser);
 		RedundantAssignEliminator::run(ast);

libyul/optimiser/Utilities.cpp

@@ -21,6 +21,7 @@
 #include <libyul/optimiser/Utilities.h>
 
 #include <libyul/AsmData.h>
+#include <libyul/Exceptions.h>
 
 #include <libdevcore/CommonData.h>
 
@@ -37,3 +38,11 @@ void yul::removeEmptyBlocks(Block& _block)
 	};
 	boost::range::remove_erase_if(_block.statements, isEmptyBlock);
 }
+
+u256 yul::valueOfNumberLiteral(Literal const& _literal)
+{
+	assertThrow(_literal.kind == LiteralKind::Number, OptimizerException, "");
+	std::string const& literalString = _literal.value.str();
+	assertThrow(isValidDecimal(literalString) || isValidHex(literalString), OptimizerException, "");
+	return u256(literalString);
+}

libyul/optimiser/Utilities.h

@@ -20,6 +20,7 @@
 
 #pragma once
 
+#include <libdevcore/Common.h>
 #include <libyul/AsmDataForward.h>
 
 namespace yul
@@ -28,4 +29,6 @@ namespace yul
 /// Removes statements that are just empty blocks (non-recursive).
 void removeEmptyBlocks(Block& _block);
 
+dev::u256 valueOfNumberLiteral(Literal const& _literal);
+
 }

test/libyul/YulOptimizerTest.cpp

@@ -39,6 +39,7 @@
 #include <libyul/optimiser/ExpressionJoiner.h>
 #include <libyul/optimiser/SSATransform.h>
 #include <libyul/optimiser/RedundantAssignEliminator.h>
+#include <libyul/optimiser/StructuralSimplifier.h>
 #include <libyul/optimiser/Suite.h>
 #include <libyul/AsmPrinter.h>
 #include <libyul/AsmParser.h>
@@ -213,6 +214,11 @@ bool YulOptimizerTest::run(ostream& _stream, string const& _linePrefix, bool con
 		SSATransform::run(*m_ast, nameDispenser);
 		RedundantAssignEliminator::run(*m_ast);
 	}
+	else if (m_optimizerStep == "structuralSimplifier")
+	{
+		disambiguate();
+		StructuralSimplifier{}(*m_ast);
+	}
 	else if (m_optimizerStep == "fullSuite")
 		OptimiserSuite::run(*m_ast, *m_analysisInfo);
 	else

test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul

@@ -599,10 +599,6 @@
 //                     revert(_2, _2)
 //                 }
 //                 let abi_decode_abi_decode_length_14_1069 := 0x2
-//                 if _2
-//                 {
-//                     revert(_2, _2)
-//                 }
 //                 let allocateMe_memPtr_315 := mload(abi_encode_pos_590)
 //                 let allocateMe_newFreePtr := add(allocateMe_memPtr_315, abi_encode_pos_590)
 //                 if or(gt(allocateMe_newFreePtr, _945), lt(allocateMe_newFreePtr, allocateMe_memPtr_315))

test/libyul/yulOptimizerTests/fullSuite/medium.yul

@@ -9,16 +9,23 @@
     pop(allocate(0x20))
     let x := allocate(0x40)
     mstore(array_index_access(x, 3), 2)
+    if 0 {
+        mstore(0x40, 0x20)
+    }
+    if sub(2,1) {
+        for { switch mul(1,2) case 2 { mstore(0x40, 0x20) } } sub(1,1) {} { mstore(0x80, 0x40) }
+    }
 }
 // ----
 // fullSuite
 // {
 //     {
-//         let _18 := 0x20
-//         let allocate__7 := 0x40
-//         mstore(allocate__7, add(mload(allocate__7), _18))
-//         let allocate_p_12_31 := mload(allocate__7)
-//         mstore(allocate__7, add(allocate_p_12_31, allocate__7))
-//         mstore(add(allocate_p_12_31, 96), 2)
+//         let _1 := 0x20
+//         let allocate__19 := 0x40
+//         mstore(allocate__19, add(mload(allocate__19), _1))
+//         let allocate_p_24_41 := mload(allocate__19)
+//         mstore(allocate__19, add(allocate_p_24_41, allocate__19))
+//         mstore(add(allocate_p_24_41, 96), 2)
+//         mstore(allocate__19, _1)
 //     }
 // }

test/libyul/yulOptimizerTests/structuralSimplifier/empty_if_movable_condition.yul

@@ -0,0 +1,7 @@
+{ let a := mload(0) if a {} }
+// ----
+// structuralSimplifier
+// {
+//     let a := mload(0)
+//     pop(a)
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/empty_if_non_movable_condition.yul

@@ -0,0 +1,6 @@
+{ if mload(0) {} }
+// ----
+// structuralSimplifier
+// {
+//     pop(mload(0))
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/for_false_condition.sol

@@ -0,0 +1,10 @@
+{
+    for { let a := 42 } 0 { a := a } {
+        let b := a
+    }
+}
+// ----
+// structuralSimplifier
+// {
+//     let a := 42
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/if_false_condition.yul

@@ -0,0 +1,5 @@
+{ if 0 { mstore(0, 0) } }
+// ----
+// structuralSimplifier
+// {
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/if_true_condition.yul

@@ -0,0 +1,6 @@
+{ if 1 { mstore(0, 0) } }
+// ----
+// structuralSimplifier
+// {
+//     mstore(0, 0)
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/nested.yul

@@ -0,0 +1,6 @@
+{ if 1 { if 1 { for { mstore(0, 0) } 0 {} { mstore(2, 3) }  if 0 { mstore(1, 2) } } } }
+// ----
+// structuralSimplifier
+// {
+//     mstore(0, 0)
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_only_default.yul

@@ -0,0 +1,11 @@
+{
+	switch mload(0) default { mstore(1, 2) }
+}
+// ----
+// structuralSimplifier
+// {
+//     pop(mload(0))
+//     {
+//         mstore(1, 2)
+//     }
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_to_if.yul

@@ -0,0 +1,11 @@
+{
+	switch 1 case 2 { mstore(0, 0) }
+}
+// ----
+// structuralSimplifier
+// {
+//     if eq(2, 1)
+//     {
+//         mstore(0, 0)
+//     }
+// }

test/tools/yulopti.cpp

@@ -46,6 +46,7 @@
 #include <libyul/optimiser/ExpressionJoiner.h>
 #include <libyul/optimiser/RedundantAssignEliminator.h>
 #include <libyul/optimiser/SSATransform.h>
+#include <libyul/optimiser/StructuralSimplifier.h>
 #include <libyul/optimiser/VarDeclPropagator.h>
 
 #include <libdevcore/JSON.h>
@@ -124,7 +125,8 @@ public:
 			}
 			cout << "(q)quit/(f)flatten/(c)se/propagate var(d)ecls/(x)plit/(j)oin/(g)rouper/(h)oister/" << endl;
 			cout << "  (e)xpr inline/(i)nline/(s)implify/(u)nusedprune/ss(a) transform/" << endl;
-			cout << "  (r)edundant assign elim./re(m)aterializer/f(o)r-loop-pre-rewriter? ";
+			cout << "  (r)edundant assign elim./re(m)aterializer/f(o)r-loop-pre-rewriter/" << endl;
+			cout << "  s(t)ructural simplifier? " << endl;
 			cout.flush();
 			int option = readStandardInputChar();
 			cout << ' ' << char(option) << endl;
@@ -165,6 +167,9 @@ public:
 			case 's':
 				ExpressionSimplifier::run(*m_ast);
 				break;
+			case 't':
+				(StructuralSimplifier{})(*m_ast);
+				break;
 			case 'u':
 				UnusedPruner::runUntilStabilised(*m_ast);
 				break;