cleaned up the way return statements are added to avoid code falling through in/out of subroutines

compiler/src/prog8/ast/processing/AstChecker.kt

@@ -1058,12 +1058,12 @@ internal class AstChecker(private val program: Program,
                     && stmt.target.nameInSource.last()=="exit"
                     && index < statements.lastIndex) {
                 println("STMT AFTER EXIT ${statements[index+1]}") // TODO fix message if next stmt is not a regular stmt
-                errors.warn("unreachable code, exit call above never returns", statements[index + 1].position)
+                errors.warn("unreachable code, preceding exit call will never return", statements[index + 1].position)
             }
 
             if(stmt is Return && index < statements.lastIndex) {
                 println("STMT AFTER RETURN ${statements[index+1]}") // TODO fix message if next stmt is not a regular stmt
-                errors.warn("unreachable code, return statement above", statements[index + 1].position)
+                errors.warn("unreachable code, preceding return statement", statements[index + 1].position)
             }
 
             stmt.accept(this)

compiler/src/prog8/ast/processing/AstWalker.kt

@@ -30,10 +30,21 @@ interface IAstModification {
         }
     }
 
-    class Insert(val after: Statement?, val stmt: Statement, val parent: Node) : IAstModification {
+    class InsertFirst(val stmt: Statement, val parent: Node) : IAstModification {
         override fun perform() {
             if(parent is INameScope) {
-                val idx = if(after==null) 0 else parent.statements.indexOf(after)
+                parent.statements.add(0, stmt)
+                stmt.linkParents(parent)
+            } else {
+                throw FatalAstException("parent of an insert modification is not an INameScope")
+            }
+        }
+    }
+
+    class InsertAfter(val after: Statement, val stmt: Statement, val parent: Node) : IAstModification {
+        override fun perform() {
+            if(parent is INameScope) {
+                val idx = parent.statements.indexOf(after)+1
                 parent.statements.add(idx, stmt)
                 stmt.linkParents(parent)
             } else {

compiler/src/prog8/compiler/target/AsmVariableAndReturnsPreparer.kt

@@ -41,17 +41,40 @@ class AsmVariableAndReturnsPreparer(val program: Program, val errors: ErrorRepor
                             it.value = null     // make sure no value init assignment for this vardecl will be created later (would be superfluous)
                             val target = AssignTarget(null, IdentifierReference(listOf(it.name), it.position), null, null, it.position)
                             val assign = Assignment(target, null, initValue, it.position)
-                            IAstModification.Insert(null, assign, scope)
+                            IAstModification.InsertFirst(assign, scope)
                         } +
                         decls.map { IAstModification.ReplaceNode(it, NopStatement(it.position), scope) } +
-                        decls.map { IAstModification.Insert(null, it, sub) }    // move it up to the subroutine
+                        decls.map { IAstModification.InsertFirst(it, sub) }    // move it up to the subroutine
             }
         }
         return emptyList()
     }
 
     override fun after(subroutine: Subroutine, parent: Node): Iterable<IAstModification> {
-        TODO("insert Return statements at the required places such as at the end of a subroutine if they're missing")
-        return emptyList()
+        // add the implicit return statement at the end (if it's not there yet), but only if it's not a kernel routine.
+        // and if an assembly block doesn't contain a rts/rti, and some other situations.
+        val mods = mutableListOf<IAstModification>()
+        val returnStmt = Return(null, subroutine.position)
+        if(subroutine.asmAddress==null
+                && subroutine.statements.isNotEmpty()
+                && subroutine.amountOfRtsInAsm()==0
+                && subroutine.statements.lastOrNull {it !is VarDecl } !is Return
+                && subroutine.statements.last() !is Subroutine) {
+            mods += IAstModification.InsertAfter(subroutine.statements.last(), returnStmt, subroutine)
+        }
+
+        // precede a subroutine with a return to avoid falling through into the subroutine from code above it
+        val outerScope = subroutine.definingScope()
+        val outerStatements = outerScope.statements
+        val subroutineStmtIdx = outerStatements.indexOf(subroutine)
+        if(subroutineStmtIdx>0
+                && outerStatements[subroutineStmtIdx-1] !is Jump
+                && outerStatements[subroutineStmtIdx-1] !is Subroutine
+                && outerStatements[subroutineStmtIdx-1] !is Return
+                && outerScope !is Block) {
+            mods += IAstModification.InsertAfter(outerStatements[subroutineStmtIdx-1], returnStmt, outerScope as Node)
+        }
+
+        return mods
     }
 }

examples/test.p8

@@ -14,12 +14,30 @@ main {
         v1 = 100
         v2 = 127
         A=5
-        return
 
         sub bla () {
-        A=99
+            A=99
+            bla2()
+
+
+            sub bla2 () {
+                A=100
+                foo.ding()
+                foo.ding2()
+            }
+
         }
 
 
     }
 }
+
+foo {
+    ubyte derp=99
+    sub ding() {
+        A=derp
+    }
+    sub ding2() {
+        A=derp
+    }
+}