Fix visitation order bug for structural simplifier.

Changelog.md

@@ -12,6 +12,7 @@ Compiler Features:
 
 Bugfixes:
  * Type system: Detect if a contract's base uses types that require the experimental abi encoder while the contract still uses the old encoder
+ * Yul Optimizer: Fix visitation order bug for the structural simplifier.
 
 
 Build System:

libyul/optimiser/StructuralSimplifier.cpp

@@ -104,10 +104,12 @@ OptionalStatements reduceSingleCaseSwitch(Switch& _switchStmt)
 		s->emplace_back(If{
 				std::move(_switchStmt.location),
 				make_unique<Expression>(FunctionalInstruction{
-						std::move(loc),
-						solidity::Instruction::EQ,
-						{std::move(*switchCase.value), std::move(*_switchStmt.expression)}
-						}), std::move(switchCase.body)});
+					std::move(loc),
+					solidity::Instruction::EQ,
+					{std::move(*switchCase.value), std::move(*_switchStmt.expression)}
+				}),
+				std::move(switchCase.body)
+		});
 		return s;
 	}
 	else
@@ -202,10 +204,11 @@ void StructuralSimplifier::simplify(std::vector<yul::Statement>& _statements)
 		_statements,
 		[&](Statement& _stmt) -> OptionalStatements
 		{
-			visit(_stmt);
 			OptionalStatements result = boost::apply_visitor(visitor, _stmt);
 			if (result)
 				simplify(*result);
+			else
+				visit(_stmt);
 			return result;
 		}
 	);

test/libyul/yulOptimizerTests/structuralSimplifier/bugfix_visit_after_change.yul

@@ -0,0 +1,13 @@
+{
+	// This tests that a bug is fixed where x := 1 was wrongfully
+	// taken into account before actually visiting the if statement.
+	let x := 0
+	if x {
+		x := 1
+	}
+}
+// ----
+// structuralSimplifier
+// {
+//     let x := 0
+// }

test/libyul/yulOptimizerTests/structuralSimplifier/switch_no_remove_empty_case.yul

@@ -1,6 +1,6 @@
 {
 	let y := 200
-	switch y
+	switch calldataload(0)
 	case 0 { }
 	case 1 { y := 9 }
 	default { y := 100 }
@@ -9,7 +9,7 @@
 // structuralSimplifier
 // {
 //     let y := 200
-//     switch y
+//     switch calldataload(0)
 //     case 0 {
 //     }
 //     case 1 {