diff --git a/compiler/test/TestCallgraph.kt b/compiler/test/TestCallgraph.kt
new file mode 100644
index 000000000..16e9bf6a1
--- /dev/null
+++ b/compiler/test/TestCallgraph.kt
@@ -0,0 +1,91 @@
+package prog8tests
+
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.TestInstance
+import prog8.ast.statements.Block
+import prog8.ast.statements.Subroutine
+import prog8.compiler.target.C64Target
+import prog8.optimizer.CallGraph
+import prog8tests.helpers.assertSuccess
+import prog8tests.helpers.compileText
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+@TestInstance(TestInstance.Lifecycle.PER_CLASS)
+class TestCallgraph {
+    @Test
+    fun testGraphForEmptySubs() {
+        val sourcecode = """
+            %import string
+            main {
+                sub start() {
+                }
+                sub empty() {
+                }
+            }
+        """
+        val result = compileText(C64Target, false, sourcecode).assertSuccess()
+        val graph = CallGraph(result.programAst)
+
+        assertEquals(1, graph.imports.size)
+        assertEquals(1, graph.importedBy.size)
+        val toplevelModule = result.programAst.toplevelModule
+        val importedModule = graph.imports.getValue(toplevelModule).single()
+        assertEquals("string", importedModule.name)
+        val importedBy = graph.importedBy.getValue(importedModule).single()
+        assertTrue(importedBy.name.startsWith("on_the_fly_test"))
+
+        assertFalse(graph.unused(toplevelModule))
+        assertFalse(graph.unused(importedModule))
+
+        val mainBlock = toplevelModule.statements.filterIsInstance<Block>().single()
+        for(stmt in mainBlock.statements) {
+            val sub = stmt as Subroutine
+            assertFalse(sub in graph.calls)
+            assertFalse(sub in graph.calledBy)
+
+            if(sub === result.programAst.entrypoint)
+                assertFalse(graph.unused(sub), "start() should always be marked as used to avoid having it removed")
+            else
+                assertTrue(graph.unused(sub))
+        }
+    }
+
+    @Test
+    fun testGraphForEmptyButReferencedSub() {
+        val sourcecode = """
+            %import string
+            main {
+                sub start() {
+                    uword xx = &empty
+                    xx++
+                }
+                sub empty() {
+                }
+            }
+        """
+        val result = compileText(C64Target, false, sourcecode).assertSuccess()
+        val graph = CallGraph(result.programAst)
+
+        assertEquals(1, graph.imports.size)
+        assertEquals(1, graph.importedBy.size)
+        val toplevelModule = result.programAst.toplevelModule
+        val importedModule = graph.imports.getValue(toplevelModule).single()
+        assertEquals("string", importedModule.name)
+        val importedBy = graph.importedBy.getValue(importedModule).single()
+        assertTrue(importedBy.name.startsWith("on_the_fly_test"))
+
+        assertFalse(graph.unused(toplevelModule))
+        assertFalse(graph.unused(importedModule))
+
+        val mainBlock = toplevelModule.statements.filterIsInstance<Block>().single()
+        val startSub = mainBlock.statements.filterIsInstance<Subroutine>().single{it.name=="start"}
+        val emptySub = mainBlock.statements.filterIsInstance<Subroutine>().single{it.name=="empty"}
+
+        assertTrue(startSub in graph.calls, "start 'calls' (references) empty")
+        assertFalse(emptySub in graph.calls, "empty doesn't call anything")
+        assertTrue(emptySub in graph.calledBy, "empty gets 'called'")
+        assertFalse(startSub in graph.calledBy, "start doesn't get called (except as entrypoint ofc.)")
+    }
+}
diff --git a/compiler/test/TestOptimization.kt b/compiler/test/TestOptimization.kt
new file mode 100644
index 000000000..f3df0da7d
--- /dev/null
+++ b/compiler/test/TestOptimization.kt
@@ -0,0 +1,59 @@
+package prog8tests
+
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.TestInstance
+import prog8.ast.statements.Block
+import prog8.ast.statements.Subroutine
+import prog8.compiler.target.C64Target
+import prog8tests.helpers.assertSuccess
+import prog8tests.helpers.compileText
+import kotlin.test.assertEquals
+import kotlin.test.assertSame
+
+@TestInstance(TestInstance.Lifecycle.PER_CLASS)
+class TestOptimization {
+    @Test
+    fun testRemoveEmptySubroutineExceptStart() {
+        val sourcecode = """
+            main {
+                sub start() {
+                }
+                sub empty() {
+                    ; going to be removed
+                }
+            }
+        """
+        val result = compileText(C64Target, true, sourcecode).assertSuccess()
+        val toplevelModule = result.programAst.toplevelModule
+        val mainBlock = toplevelModule.statements.single() as Block
+        assertEquals(1, mainBlock.statements.size)
+        val startSub = mainBlock.statements[0] as Subroutine
+        assertSame(result.programAst.entrypoint, startSub)
+        assertEquals("start", startSub.name)
+        assertEquals(0, startSub.statements.size)
+    }
+
+    @Test
+    fun testDontRemoveEmptySubroutineIfItsReferenced() {
+        val sourcecode = """
+            main {
+                sub start() {
+                    uword xx = &empty
+                    xx++
+                }
+                sub empty() {
+                    ; should not be removed
+                }
+            }
+        """
+        val result = compileText(C64Target, true, sourcecode).assertSuccess()
+        val toplevelModule = result.programAst.toplevelModule
+        val mainBlock = toplevelModule.statements.single() as Block
+        val startSub = mainBlock.statements[0] as Subroutine
+        val emptySub = mainBlock.statements[1] as Subroutine
+        assertSame(result.programAst.entrypoint, startSub)
+        assertEquals("start", startSub.name)
+        assertEquals("empty", emptySub.name)
+        assertEquals(0, emptySub.statements.size)
+    }
+}
diff --git a/examples/test.p8 b/examples/test.p8
index cfa7a4362..f5ba855fa 100644
--- a/examples/test.p8
+++ b/examples/test.p8
@@ -1,18 +1,10 @@
-%import textio
-
 main {
     sub start() {
-        ubyte xx
+        uword address = &irq
+        ; cx16.set_irq(&irq, false)
+        address++
+    }
 
-        when xx {
-            2 -> {
-            }
-            3 -> {
-            }
-            50 -> {
-            }
-            else -> {
-            }
-        }
+    sub irq() {
     }
 }