diff --git a/compiler/src/prog8/ast/AST.kt b/compiler/src/prog8/ast/AST.kt
index 4d2225b3c..efed132da 100644
--- a/compiler/src/prog8/ast/AST.kt
+++ b/compiler/src/prog8/ast/AST.kt
@@ -496,6 +496,8 @@ class Module(override val name: String,
              val source: Path) : Node, INameScope {
     override lateinit var parent: Node
     lateinit var program: Program
+    val imports = mutableSetOf<Module>()
+    val importedBy = mutableSetOf<Module>()
 
     override fun linkParents(parent: Node) {
         this.parent=parent
@@ -1625,7 +1627,7 @@ class FunctionCallStatement(override var target: IdentifierReference,
     override fun process(processor: IAstProcessor) = processor.process(this)
 
     override fun toString(): String {
-        return "FunctionCall(target=$target, pos=$position)"
+        return "FunctionCallStatement(target=$target, pos=$position)"
     }
 }
 
@@ -1690,6 +1692,9 @@ class Subroutine(override val name: String,
                  override var statements: MutableList<IStatement>,
                  override val position: Position) : IStatement, INameScope {
     override lateinit var parent: Node
+    val calledBy = mutableSetOf<Subroutine>()
+    val calls = mutableSetOf<Subroutine>()
+
     val scopedname: String by lazy { makeScopedName(name) }
 
     override fun linkParents(parent: Node) {
diff --git a/compiler/src/prog8/optimizing/CallGraphBuilder.kt b/compiler/src/prog8/optimizing/CallGraphBuilder.kt
new file mode 100644
index 000000000..07d09bae4
--- /dev/null
+++ b/compiler/src/prog8/optimizing/CallGraphBuilder.kt
@@ -0,0 +1,90 @@
+package prog8.optimizing
+
+import prog8.ast.*
+
+
+class CallGraphBuilder(private val program: Program): IAstProcessor {
+
+    private val modulesImporting = mutableMapOf<Module, Set<Module>>().withDefault { mutableSetOf() }
+    private val modulesImportedBy = mutableMapOf<Module, Set<Module>>().withDefault { mutableSetOf() }
+    private val subroutinesCalling = mutableMapOf<Subroutine, Set<Subroutine>>().withDefault { mutableSetOf() }
+    private val subroutinesCalledBy = mutableMapOf<Subroutine, Set<Subroutine>>().withDefault { mutableSetOf() }
+
+    private fun forAllSubroutines(scope: INameScope, sub: (s: Subroutine) -> Unit) {
+        fun findSubs(scope: INameScope) {
+            scope.statements.forEach {
+                if(it is Subroutine)
+                    sub(it)
+                if(it is INameScope)
+                    findSubs(it)
+            }
+        }
+        findSubs(scope)
+    }
+
+    override fun process(program: Program) {
+        super.process(program)
+
+        program.modules.forEach {
+            it.importedBy.clear()
+            it.imports.clear()
+
+            it.importedBy.addAll(modulesImportedBy.getValue(it))
+            it.imports.addAll(modulesImporting.getValue(it))
+
+            if(it.isLibraryModule && it.importedBy.isEmpty())
+                it.importedBy.add(it)       // don't discard auto-imported library module
+
+            forAllSubroutines(it) { sub ->
+                sub.calledBy.clear()
+                sub.calls.clear()
+
+                sub.calledBy.addAll(subroutinesCalledBy.getValue(sub))
+                sub.calls.addAll(subroutinesCalling.getValue(sub))
+            }
+        }
+
+        val rootmodule = program.modules.first()
+        rootmodule.importedBy.add(rootmodule)       // don't discard root module
+    }
+
+    override fun process(directive: Directive): IStatement {
+        if(directive.directive=="%import") {
+            val importedModule: Module = program.modules.single { it.name==directive.args[0].name }
+            val thisModule = directive.definingModule()
+            modulesImporting[thisModule] = modulesImporting.getValue(thisModule).plus(importedModule)
+            modulesImportedBy[importedModule] = modulesImportedBy.getValue(importedModule).plus(thisModule)
+        }
+        return super.process(directive)
+    }
+
+    override fun process(functionCall: FunctionCall): IExpression {
+        val otherSub = functionCall.target.targetSubroutine(program.namespace)
+        if(otherSub!=null) {
+            val thisSub = functionCall.definingScope() as Subroutine
+            subroutinesCalling[thisSub] = subroutinesCalling.getValue(thisSub).plus(otherSub)
+            subroutinesCalledBy[otherSub] = subroutinesCalledBy.getValue(otherSub).plus(thisSub)
+        }
+        return super.process(functionCall)
+    }
+
+    override fun process(functionCallStatement: FunctionCallStatement): IStatement {
+        val otherSub = functionCallStatement.target.targetSubroutine(program.namespace)
+        if(otherSub!=null) {
+            val thisSub = functionCallStatement.definingScope() as Subroutine
+            subroutinesCalling[thisSub] = subroutinesCalling.getValue(thisSub).plus(otherSub)
+            subroutinesCalledBy[otherSub] = subroutinesCalledBy.getValue(otherSub).plus(thisSub)
+        }
+        return super.process(functionCallStatement)
+    }
+
+    override fun process(jump: Jump): IStatement {
+        val otherSub = jump.identifier?.targetSubroutine(program.namespace)
+        if(otherSub!=null) {
+            val thisSub = jump.definingScope() as Subroutine
+            subroutinesCalling[thisSub] = subroutinesCalling.getValue(thisSub).plus(otherSub)
+            subroutinesCalledBy[otherSub] = subroutinesCalledBy.getValue(otherSub).plus(thisSub)
+        }
+        return super.process(jump)
+    }
+}
diff --git a/compiler/src/prog8/optimizing/StatementOptimizer.kt b/compiler/src/prog8/optimizing/StatementOptimizer.kt
index 519032a67..9e272316c 100644
--- a/compiler/src/prog8/optimizing/StatementOptimizer.kt
+++ b/compiler/src/prog8/optimizing/StatementOptimizer.kt
@@ -11,7 +11,7 @@ import kotlin.math.floor
 
 
     todo: implement usage counters for blocks, variables, subroutines, heap variables. Then:
-        todo remove unused: subroutines, blocks, modules (in this order)
+        todo remove unused: subroutines, blocks (in this order)
         todo remove unused: variable declarations
         todo remove unused strings and arrays from the heap
         todo inline subroutines that are called exactly once (regardless of their size)
@@ -28,6 +28,24 @@ class StatementOptimizer(private val program: Program) : IAstProcessor {
         private set
     private val pureBuiltinFunctions = BuiltinFunctions.filter { it.value.pure }
 
+
+    override fun process(program: Program) {
+        val callgraph = CallGraphBuilder(program)
+        callgraph.process(program)
+
+        // remove modules that are not imported, or are empty
+        val removeModules = mutableSetOf<Module>()
+        program.modules.forEach {
+            if(it.importedBy.isEmpty() || it.isEmpty()) {
+                printWarning("discarding empty or unused module: ${it.name}")
+                removeModules.add(it)
+            }
+        }
+        program.modules.removeAll(removeModules)
+
+        super.process(program)
+    }
+
     override fun process(block: Block): IStatement {
         if(block.statements.isEmpty()) {
             // remove empty block
@@ -38,6 +56,7 @@ class StatementOptimizer(private val program: Program) : IAstProcessor {
     }
 
     override fun process(subroutine: Subroutine): IStatement {
+        println("STMT OPTIMIZE $subroutine   ${subroutine.calledBy} ${subroutine.calls}")
         super.process(subroutine)
 
         if(subroutine.asmAddress==null) {