Yul Optimizer: reduce switches with const arguments

Changelog.md

@@ -12,6 +12,7 @@ Compiler Features:
  * Optimizer: Add rule for shifts with constants for Constantinople.
  * Optimizer: Combine multiple shifts with constant shift-by values into one.
  * Optimizer: Support shifts in the constant optimiser for Constantinople.
+ * Yul Optimizer: Add rule to replace switch statements with const expr. with matching case body
 
 
 Bugfixes:

libyul/optimiser/StructuralSimplifier.cpp

@@ -45,6 +45,26 @@ void StructuralSimplifier::operator()(Block& _block)
 	popScope();
 }
 
+boost::optional<dev::u256> StructuralSimplifier::hasLiteralValue(Expression const& _expression) const
+{
+	Expression const* expr = &_expression;
+
+	if (expr->type() == typeid(Identifier))
+	{
+		Identifier const& ident = boost::get<Identifier>(*expr);
+		if (m_value.count(ident.name))
+			expr = m_value.at(ident.name);
+	}
+
+	if (expr && expr->type() == typeid(Literal))
+	{
+		Literal const& literal = boost::get<Literal>(*expr);
+		return valueOfNumberLiteral(literal);
+	}
+
+	return boost::optional<u256>();
+}
+
 void StructuralSimplifier::simplify(std::vector<yul::Statement>& _statements)
 {
 	using OptionalStatements = boost::optional<vector<Statement>>;
@@ -62,7 +82,7 @@ void StructuralSimplifier::simplify(std::vector<yul::Statement>& _statements)
 				return {vector<Statement>{}};
 			return {};
 		},
-		[](Switch& _switchStmt) -> OptionalStatements {
+		[&](Switch& _switchStmt) -> OptionalStatements {
 			if (_switchStmt.cases.size() == 1)
 			{
 				auto& switchCase = _switchStmt.cases.front();
@@ -87,6 +107,32 @@ void StructuralSimplifier::simplify(std::vector<yul::Statement>& _statements)
 					return s;
 				}
 			}
+			else if (boost::optional<u256> const constExprVal = hasLiteralValue(*_switchStmt.expression))
+			{
+				Block* matchingCaseBlock = nullptr;
+				Case* defaultCase = nullptr;
+
+				for (auto& _case: _switchStmt.cases)
+				{
+					if (_case.value && valueOfNumberLiteral(*_case.value) == constExprVal)
+					{
+						matchingCaseBlock = &_case.body;
+						break;
+					}
+					else if (!_case.value)
+						defaultCase = &_case;
+				}
+
+				if (!matchingCaseBlock && defaultCase)
+					matchingCaseBlock = &defaultCase->body;
+
+				OptionalStatements s = vector<Statement>{};
+
+				if (matchingCaseBlock)
+					s->emplace_back(std::move(*matchingCaseBlock));
+
+				return s;
+			}
 			else
 				return {};
 		},

libyul/optimiser/StructuralSimplifier.h

@@ -18,6 +18,7 @@
 
 #include <libyul/optimiser/ASTWalker.h>
 #include <libyul/optimiser/DataFlowAnalyzer.h>
+#include <libdevcore/Common.h>
 
 namespace yul
 {
@@ -29,6 +30,7 @@ namespace yul
  * - remove if with false condition
  * - turn switch with single case into if
  * - replace switch with only default case with pop(expression) and body
+ * - replace switch with const expr with matching case body
  * - remove for with false condition
  *
  * Prerequisites: Disambiguator
@@ -46,6 +48,7 @@ private:
 	void simplify(std::vector<Statement>& _statements);
 	bool expressionAlwaysTrue(Expression const& _expression);
 	bool expressionAlwaysFalse(Expression const& _expression);
+	boost::optional<dev::u256> hasLiteralValue(Expression const& _expression) const;
 };
 
 }

test/libyul/yulOptimizerTests/fullSuite/switch_inline.yul

@@ -0,0 +1,13 @@
+{
+	mstore(f(1), 0)
+	function f(x) -> y {
+		switch x
+		case 0 { y := 8 }
+		case 1 { y := 9 }
+	}
+}
+// ----
+// fullSuite
+// {
+//     mstore(9, 0)
+// }

test/libyul/yulOptimizerTests/fullSuite/switch_inline_match_default.yul

@@ -0,0 +1,14 @@
+{
+	mstore(f(3), 0)
+	function f(x) -> y {
+		switch x
+		case 0 { y := 8 }
+		case 1 { y := 9 }
+		default { y := 10 }
+	}
+}
+// ----
+// fullSuite
+// {
+//     mstore(10, 0)
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_inline.yul

@@ -0,0 +1,14 @@
+{
+	let y := 200
+	switch 1
+	case 0 { y := 8 }
+	case 1 { y := 9 }
+}
+// ----
+// structuralSimplifier
+// {
+//     let y := 200
+//     {
+//         y := 9
+//     }
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_inline_match_default.yul

@@ -0,0 +1,15 @@
+{
+	let y := 200
+	switch 3
+	case 0 { y := 8 }
+	case 1 { y := 9 }
+	default { y := 10 }
+}
+// ----
+// structuralSimplifier
+// {
+//     let y := 200
+//     {
+//         y := 10
+//     }
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_inline_no_match.yul

@@ -0,0 +1,11 @@
+{
+	let y := 200
+	switch 3
+	case 0 { y := 8 }
+	case 1 { y := 9 }
+}
+// ----
+// structuralSimplifier
+// {
+//     let y := 200
+// }