Add structural simplifier as optimization step for Yul.

7 files changed, 205 insertions, 4 deletions

diff --git a/libyul/CMakeLists.txt b/libyul/CMakeLists.txt
index 4bc8200c..a75c5908 100644
--- a/libyul/CMakeLists.txt
+++ b/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
diff --git a/libyul/optimiser/SimplificationRules.cpp b/libyul/optimiser/SimplificationRules.cpp
index 8ed63fa8..45b0ca2c 100644
--- a/libyul/optimiser/SimplificationRules.cpp
+++ b/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
diff --git a/libyul/optimiser/StructuralSimplifier.cpp b/libyul/optimiser/StructuralSimplifier.cpp
new file mode 100644
index 00000000..5fcfdae4
--- /dev/null
+++ b/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);
+}
diff --git a/libyul/optimiser/StructuralSimplifier.h b/libyul/optimiser/StructuralSimplifier.h
new file mode 100644
index 00000000..bbd8e005
--- /dev/null
+++ b/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);
+};
+
+}
diff --git a/libyul/optimiser/Suite.cpp b/libyul/optimiser/Suite.cpp
index 36f0e1eb..ad22bfa3 100644
--- a/libyul/optimiser/Suite.cpp
+++ b/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);
diff --git a/libyul/optimiser/Utilities.cpp b/libyul/optimiser/Utilities.cpp
index b8cdd339..b3b580d5 100644
--- a/libyul/optimiser/Utilities.cpp
+++ b/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);
+}
diff --git a/libyul/optimiser/Utilities.h b/libyul/optimiser/Utilities.h
index c543b119..1cfff62b 100644
--- a/libyul/optimiser/Utilities.h
+++ b/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);
+
 }