diff --git a/src/main/scala/millfork/compiler/BuiltIns.scala b/src/main/scala/millfork/compiler/BuiltIns.scala
index 7ba63955..fb99542a 100644
--- a/src/main/scala/millfork/compiler/BuiltIns.scala
+++ b/src/main/scala/millfork/compiler/BuiltIns.scala
@@ -290,6 +290,9 @@ object BuiltIns {
   def compileByteComparison(ctx: CompilationContext, compType: ComparisonType.Value, lhs: Expression, rhs: Expression, branches: BranchSpec): List[AssemblyLine] = {
     val env = ctx.env
     val b = env.get[Type]("byte")
+    if (simplicity(env, lhs) >= 'J' && simplicity(env, rhs) < 'J') {
+      return compileByteComparison(ctx, ComparisonType.flip(compType), rhs, lhs, branches)
+    }
     val firstParamCompiled = MlCompiler.compile(ctx, lhs, Some(b -> RegisterVariable(Register.A, b)), NoBranching)
     val maybeConstant = env.eval(rhs)
     maybeConstant match {
diff --git a/src/main/scala/millfork/compiler/MfCompiler.scala b/src/main/scala/millfork/compiler/MfCompiler.scala
index 67bd2429..fa930bd4 100644
--- a/src/main/scala/millfork/compiler/MfCompiler.scala
+++ b/src/main/scala/millfork/compiler/MfCompiler.scala
@@ -1157,6 +1157,24 @@ object MlCompiler {
       case List(_) | Nil =>
         ErrorReporting.fatal("")
       case _ =>
+        params.tail.init.foreach { e =>
+          if (ctx.env.eval(e).isEmpty) e match {
+            case VariableExpression(_) =>
+            case LiteralExpression(_, _) =>
+            case IndexedExpression(_, VariableExpression(_)) =>
+            case IndexedExpression(_, LiteralExpression(_, _)) =>
+            case IndexedExpression(_, SumExpression(List(
+            (_, LiteralExpression(_, _)),
+            (false, VariableExpression(_))
+            ), false)) =>
+            case IndexedExpression(_, SumExpression(List(
+            (false, VariableExpression(_)),
+            (_, LiteralExpression(_, _))
+            ), false)) =>
+            case _ =>
+              ErrorReporting.warn("A complex expression may be evaluated multiple times", ctx.options, e.position)
+          }
+        }
         val conjunction = params.init.zip(params.tail).map {
           case (l, r) => FunctionCallExpression(operator, List(l, r))
         }.reduceLeft((a, b) => FunctionCallExpression("&&", List(a, b)))
diff --git a/src/test/scala/millfork/test/ComparisonSuite.scala b/src/test/scala/millfork/test/ComparisonSuite.scala
index 43f5506c..806e4f8a 100644
--- a/src/test/scala/millfork/test/ComparisonSuite.scala
+++ b/src/test/scala/millfork/test/ComparisonSuite.scala
@@ -293,4 +293,23 @@ class ComparisonSuite extends FunSuite with Matchers {
         | }
       """.stripMargin)(_.readWord(0xc000) should equal(6))
   }
+
+  test("Warnings") {
+    EmuBenchmarkRun(
+      """
+        | byte output @$c000
+        | void main () {
+        |  output = 5
+        |  if 2 <= three() <= 4 {
+        |   output += 1
+        |  }
+        |  if 2 <= three() <= 2 {
+        |   output += 78
+        |  }
+        | }
+        | byte three() {
+        |   return 3
+        | }
+      """.stripMargin)(_.readWord(0xc000) should equal(6))
+  }
 }