diff --git a/codeAst/src/prog8/code/ast/AstExpressions.kt b/codeAst/src/prog8/code/ast/AstExpressions.kt
index 7174f7363..a6647bb0c 100644
--- a/codeAst/src/prog8/code/ast/AstExpressions.kt
+++ b/codeAst/src/prog8/code/ast/AstExpressions.kt
@@ -38,7 +38,11 @@ class PtArray(type: DataType, position: Position): PtExpression(type, position)
 }
 
 
-class PtBuiltinFunctionCall(val name: String, val void: Boolean, type: DataType, position: Position) : PtExpression(type, position) {
+class PtBuiltinFunctionCall(val name: String,
+                            val void: Boolean,
+                            val hasNoSideEffects: Boolean,
+                            type: DataType,
+                            position: Position) : PtExpression(type, position) {
     init {
         if(!void)
             require(type!=DataType.UNDEFINED)
@@ -47,7 +51,7 @@ class PtBuiltinFunctionCall(val name: String, val void: Boolean, type: DataType,
     val args: List<PtExpression>
         get() = children.map { it as PtExpression }
     override fun printProperties() {
-        print("$name void=$void")
+        print("$name void=$void noSideFx=$hasNoSideEffects")
     }
 }
 
diff --git a/codeGenVirtual/src/prog8/codegen/virtual/ExpressionGen.kt b/codeGenVirtual/src/prog8/codegen/virtual/ExpressionGen.kt
index 4777e8ae8..07bd01aba 100644
--- a/codeGenVirtual/src/prog8/codegen/virtual/ExpressionGen.kt
+++ b/codeGenVirtual/src/prog8/codegen/virtual/ExpressionGen.kt
@@ -77,7 +77,7 @@ internal class ExpressionGen(private val codeGen: CodeGen) {
                     segWithArg
                 }
                 is PtBuiltinFunctionCall -> {
-                    val segWithArg = PtBuiltinFunctionCall(segment.name, segment.void, segment.type, segment.position)
+                    val segWithArg = PtBuiltinFunctionCall(segment.name, segment.void, segment.hasNoSideEffects, segment.type, segment.position)
                     segWithArg.children.add(0, PtIdentifier(listOf(":vmreg-$sourceReg"), listOf(":vmreg-$sourceReg"), sourceDt, segment.position))
                     segWithArg
                 }
diff --git a/compiler/src/prog8/compiler/IntermediateAstMaker.kt b/compiler/src/prog8/compiler/IntermediateAstMaker.kt
index 51e4d6187..265874ec6 100644
--- a/compiler/src/prog8/compiler/IntermediateAstMaker.kt
+++ b/compiler/src/prog8/compiler/IntermediateAstMaker.kt
@@ -149,7 +149,8 @@ class IntermediateAstMaker(val program: Program) {
 
     private fun transform(srcNode: BuiltinFunctionCallStatement): PtBuiltinFunctionCall {
         val type = builtinFunctionReturnType(srcNode.name, srcNode.args, program).getOr(DataType.UNDEFINED)
-        val call = PtBuiltinFunctionCall(srcNode.name, true, type, srcNode.position)
+        val noSideFx = BuiltinFunctions.getValue(srcNode.name).pure
+        val call = PtBuiltinFunctionCall(srcNode.name, true, noSideFx, type, srcNode.position)
         for (arg in srcNode.args)
             call.add(transformExpression(arg))
         return call
@@ -436,7 +437,8 @@ class IntermediateAstMaker(val program: Program) {
 
     private fun transform(srcCall: BuiltinFunctionCall): PtBuiltinFunctionCall {
         val type = srcCall.inferType(program).getOrElse { throw FatalAstException("unknown dt") }
-        val call = PtBuiltinFunctionCall(srcCall.name, false, type, srcCall.position)
+        val noSideFx = BuiltinFunctions.getValue(srcCall.name).pure
+        val call = PtBuiltinFunctionCall(srcCall.name, false, noSideFx, type, srcCall.position)
         for (arg in srcCall.args)
             call.add(transformExpression(arg))
         return call