diff --git a/compiler/src/prog8/compiler/Compiler.kt b/compiler/src/prog8/compiler/Compiler.kt
index 956bcf0e4..b9710d20c 100644
--- a/compiler/src/prog8/compiler/Compiler.kt
+++ b/compiler/src/prog8/compiler/Compiler.kt
@@ -91,7 +91,7 @@ fun compileProgram(filepath: Path,
             importedFiles = imported
             processAst(programAst, errors, compilationOptions)
             if (compilationOptions.optimize)
-                optimizeAst(programAst, errors)
+                optimizeAst(programAst, errors, BuiltinFunctionsFacade(BuiltinFunctions))
             postprocessAst(programAst, errors, compilationOptions)
 
             // printAst(programAst)
@@ -134,6 +134,8 @@ private class BuiltinFunctionsFacade(functions: Map<String, FSignature>): IBuilt
     lateinit var program: Program
 
     override val names = functions.keys
+    override val purefunctionNames = functions.filter { it.value.pure }.map { it.key }.toSet()
+
     override fun constValue(name: String, args: List<Expression>, position: Position): NumericLiteralValue? {
         val func = BuiltinFunctions[name]
         if(func!=null) {
@@ -252,14 +254,14 @@ private fun processAst(programAst: Program, errors: ErrorReporter, compilerOptio
     errors.handle()
 }
 
-private fun optimizeAst(programAst: Program, errors: ErrorReporter) {
+private fun optimizeAst(programAst: Program, errors: ErrorReporter, functions: IBuiltinFunctions) {
     // optimize the parse tree
     println("Optimizing...")
     while (true) {
         // keep optimizing expressions and statements until no more steps remain
         val optsDone1 = programAst.simplifyExpressions()
         val optsDone2 = programAst.splitBinaryExpressions()
-        val optsDone3 = programAst.optimizeStatements(errors)
+        val optsDone3 = programAst.optimizeStatements(errors, functions)
         programAst.constantFold(errors) // because simplified statements and expressions can result in more constants that can be folded away
         errors.handle()
         if (optsDone1 + optsDone2 + optsDone3 == 0)
diff --git a/compiler/src/prog8/optimizer/Extensions.kt b/compiler/src/prog8/optimizer/Extensions.kt
index 9adb188a7..b4e0f1a8e 100644
--- a/compiler/src/prog8/optimizer/Extensions.kt
+++ b/compiler/src/prog8/optimizer/Extensions.kt
@@ -1,5 +1,6 @@
 package prog8.optimizer
 
+import prog8.ast.IBuiltinFunctions
 import prog8.ast.Program
 import prog8.compiler.ErrorReporter
 
@@ -38,8 +39,8 @@ internal fun Program.constantFold(errors: ErrorReporter) {
 }
 
 
-internal fun Program.optimizeStatements(errors: ErrorReporter): Int {
-    val optimizer = StatementOptimizer(this, errors)
+internal fun Program.optimizeStatements(errors: ErrorReporter, functions: IBuiltinFunctions): Int {
+    val optimizer = StatementOptimizer(this, errors, functions)
     optimizer.visit(this)
     val optimizationCount = optimizer.applyModifications()
 
diff --git a/compiler/src/prog8/optimizer/StatementOptimizer.kt b/compiler/src/prog8/optimizer/StatementOptimizer.kt
index ba62fe856..988e047d2 100644
--- a/compiler/src/prog8/optimizer/StatementOptimizer.kt
+++ b/compiler/src/prog8/optimizer/StatementOptimizer.kt
@@ -1,5 +1,6 @@
 package prog8.optimizer
 
+import prog8.ast.IBuiltinFunctions
 import prog8.ast.INameScope
 import prog8.ast.Node
 import prog8.ast.Program
@@ -10,18 +11,17 @@ import prog8.ast.walk.AstWalker
 import prog8.ast.walk.IAstModification
 import prog8.ast.walk.IAstVisitor
 import prog8.compiler.ErrorReporter
-import prog8.compiler.functions.BuiltinFunctions
 import prog8.compiler.target.CompilationTarget
 import kotlin.math.floor
 
 
 internal class StatementOptimizer(private val program: Program,
-                                  private val errors: ErrorReporter
+                                  private val errors: ErrorReporter,
+                                  private val functions: IBuiltinFunctions
 ) : AstWalker() {
 
     private val noModifications = emptyList<IAstModification>()
     private val callgraph = CallGraph(program)
-    private val pureBuiltinFunctions = BuiltinFunctions.filter { it.value.pure }
 
     override fun after(block: Block, parent: Node): Iterable<IAstModification> {
         if("force_output" !in block.options()) {
@@ -72,9 +72,9 @@ internal class StatementOptimizer(private val program: Program,
     }
 
     override fun after(functionCallStatement: FunctionCallStatement, parent: Node): Iterable<IAstModification> {
-        if(functionCallStatement.target.nameInSource.size==1 && functionCallStatement.target.nameInSource[0] in BuiltinFunctions) {
+        if(functionCallStatement.target.nameInSource.size==1 && functionCallStatement.target.nameInSource[0] in functions.names) {
             val functionName = functionCallStatement.target.nameInSource[0]
-            if (functionName in pureBuiltinFunctions) {
+            if (functionName in functions.purefunctionNames) {
                 errors.warn("statement has no effect (function return value is discarded)", functionCallStatement.position)
                 return listOf(IAstModification.Remove(functionCallStatement, functionCallStatement.definingScope()))
             }
diff --git a/compiler/test/UnitTests.kt b/compiler/test/UnitTests.kt
index e9b745bda..d7b8beefc 100644
--- a/compiler/test/UnitTests.kt
+++ b/compiler/test/UnitTests.kt
@@ -408,6 +408,7 @@ class TestPetscii {
 class TestMemory {
     private class DummyFunctions: IBuiltinFunctions {
         override val names: Set<String> = emptySet()
+        override val purefunctionNames: Set<String> = emptySet()
         override fun constValue(name: String, args: List<Expression>, position: Position): NumericLiteralValue? = null
         override fun returnType(name: String, args: MutableList<Expression>) = InferredTypes.InferredType.unknown()
     }
diff --git a/compilerAst/src/prog8/ast/AstToplevel.kt b/compilerAst/src/prog8/ast/AstToplevel.kt
index cb05704b3..5c1132834 100644
--- a/compilerAst/src/prog8/ast/AstToplevel.kt
+++ b/compilerAst/src/prog8/ast/AstToplevel.kt
@@ -246,6 +246,7 @@ interface IAssignable {
 
 interface IBuiltinFunctions {
     val names: Set<String>
+    val purefunctionNames: Set<String>
     fun constValue(name: String, args: List<Expression>, position: Position): NumericLiteralValue?
     fun returnType(name: String, args: MutableList<Expression>): InferredTypes.InferredType
 }