YulOpt: Remove empty cases

Changelog.md

@@ -5,6 +5,9 @@ Language Features:
 
 Compiler Features:
  * SMTChecker: Support one-dimensional arrays.
+ * Yul Optimizer: Add rule to remove empty default switch cases
+ * Yul Optimizer: Add rule to remove empty cases if no default exists
+ * Yul Optimizer: Add rule to replace a switch with no cases with pop(expression)
 
 
 Bugfixes:

libyul/optimiser/StructuralSimplifier.h

@@ -28,6 +28,9 @@ namespace yul
  * - replace if with empty body with pop(condition)
  * - replace if with true condition with its body
  * - remove if with false condition
+ * - remove empty default switch case
+ * - remove empty switch case if no default case exists
+ * - replace switch with no cases with pop(expression)
  * - 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
@@ -49,6 +52,7 @@ private:
 	bool expressionAlwaysTrue(Expression const& _expression);
 	bool expressionAlwaysFalse(Expression const& _expression);
 	boost::optional<dev::u256> hasLiteralValue(Expression const& _expression) const;
+	boost::optional<std::vector<Statement>> reduceNoCaseSwitch(Switch& _switchStmt) const;
 };
 
 }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_remove_empty_all.yul

@@ -0,0 +1,19 @@
+{
+	let y := 200
+	switch add(y, 4)
+	case 0 { }
+	case 1 { }
+	default { }
+
+	switch 4
+	case 0 { }
+	case 1 { }
+	default { }
+}
+// ----
+// structuralSimplifier
+// {
+//     let y := 200
+//     pop(add(y, 4))
+//     pop(4)
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_remove_empty_case.yul

@@ -8,8 +8,8 @@
 // structuralSimplifier
 // {
 //     let y := 200
-//     switch y
+//     if eq(1, y)
-//     case 1 {
+//     {
 //         y := 9
 //     }
 // }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_remove_empty_cases.yul

@@ -9,8 +9,8 @@
 // structuralSimplifier
 // {
 //     let y := 200
-//     switch y
+//     if eq(1, y)
-//     case 1 {
+//     {
 //         y := 9
 //     }
 // }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_remove_empty_default_case.yul

@@ -8,8 +8,8 @@
 // structuralSimplifier
 // {
 //     let y := 200
-//     switch y
+//     if eq(1, y)
-//     case 1 {
+//     {
 //         y := 9
 //     }
 // }