Yul expression simplifier: Don't substitute out of scope variables

In some circumstances (in particular if not the AST is not in SSA form), it could happen that the ExpressionSimplifier would substitute out-of-scope variables.

Author: clonker

Changelog.md

@@ -7,6 +7,7 @@ Compiler Features:
 
 Bugfixes:
 * Assembler: Fix not using a fixed-width type for IDs being assigned to subassemblies nested more than one level away, resulting in inconsistent `--asm-json` output between target architectures.
+* Yul Optimizer: Fix edge case in which invalid Yul code is produced by ExpressionSimplifier due to expressions being substituted that contain out-of-scope variables.
 
 ### 0.8.30 (2025-05-07)
 

libyul/optimiser/ExpressionSimplifier.cpp

@@ -25,6 +25,7 @@
 #include <libyul/optimiser/SimplificationRules.h>
 #include <libyul/optimiser/OptimiserStep.h>
 #include <libyul/optimiser/OptimizerUtilities.h>
+#include <libyul/optimiser/Semantics.h>
 #include <libyul/AST.h>
 #include <libyul/Utilities.h>
 
@@ -45,7 +46,19 @@ void ExpressionSimplifier::visit(Expression& _expression)
 	while (auto const* match = SimplificationRules::findFirstMatch(
 		_expression,
 		m_dialect,
-		[this](YulName _var) { return variableValue(_var); }
+		[this](YulName const& _var) -> AssignedValue const* {
+			AssignedValue const* value = variableValue(_var);
+			if (!value || !value->value)
+				return nullptr;
+
+			// check that all variables in the value expression are in current scope
+			MovableChecker const checker(m_dialect, *value->value);
+			for (YulName const& referencedVar: checker.referencedVariables())
+				if (!inScope(referencedVar))
+					return nullptr;  // don't substitute if any referenced var is out of scope
+
+			return value;
+		}
 	))
 	{
 		auto const* evmDialect = dynamic_cast<EVMDialect const*>(&m_dialect);

test/cmdlineTests/yul_optimizer_steps_expression_simplifier_with_invalid_match/args

@@ -0,0 +1 @@
+--strict-assembly --optimize --yul-optimizations T:ts --bin

test/cmdlineTests/yul_optimizer_steps_expression_simplifier_with_invalid_match/input.yul

@@ -0,0 +1,23 @@
+{
+    function identity(value) -> ret {
+        ret := value
+    }
+    function fun_fun_31() -> ret {
+        let x := 0x01
+        let expr_9 := identity(x)
+
+        let expr_10 := 0x01
+        let expr_25 := 0x05
+        switch expr_10
+        case 0 { }
+        default {
+            let expr_14 := 0x06
+            let expr_18 := identity(x)
+            expr_25 := or(expr_14, expr_18)
+        }
+        // expression simplifier spuriously gets a match on this: `expr_25 == or(0x06, expr_18)` where expr_18 is
+        // from the inner scope
+        ret := or(expr_9, expr_25)
+        leave
+    }
+}

test/cmdlineTests/yul_optimizer_steps_expression_simplifier_with_invalid_match/output

@@ -0,0 +1,5 @@
+
+======= input.yul (EVM) =======
+
+Binary representation:
+00