default declared variables to uninitialized

This is a special value that should only be used to reserve stack slots
for locals.  Loading this value is a runtime error.

Author: annacrombie

include/lang/vm.h

@@ -89,6 +89,7 @@ enum compile_time_constant_objects {
 	/* obj_meson = 2, */
 	obj_bool_true = 3,
 	obj_bool_false = 4,
+	obj_uninitialized = 5,
 	compile_time_constant_objects_end,
 };
 

src/lang/compiler.c

@@ -424,7 +424,7 @@ static void
 vm_comp_reserve_local_stack_slot(struct workspace *wk, obj id)
 {
 	push_code(wk, op_constant);
-	push_constant(wk, id);
+	push_constant(wk, obj_uninitialized);
 }
 
 static void

src/lang/object.c

@@ -7,6 +7,7 @@
 
 #include "compat.h"
 
+#include "lang/vm.h"
 #include <inttypes.h>
 #include <stdlib.h>
 
@@ -106,6 +107,9 @@ make_default_objects(struct workspace *wk)
 	assert(id == obj_bool_false);
 	*(bool *)get_obj_internal(wk, id, obj_bool) = false;
 
+	id = make_obj(wk, obj_null);
+	assert(id == obj_uninitialized);
+
 	making_default_objects = false;
 }
 
@@ -1890,6 +1894,11 @@ obj_to_s_opts(struct workspace *wk, obj o, struct tstr *sb, struct obj_to_s_opts
 	struct obj_to_s_ctx ctx = { .sb = sb, .opts = opts };
 	enum obj_type t = get_obj_type(wk, o);
 
+	if (o == obj_uninitialized) {
+		tstr_pushs(wk, sb, "<uninit>");
+		return;
+	}
+
 	switch (t) {
 	case obj_include_directory: {
 		struct obj_include_directory *inc = get_obj_include_directory(wk, o);

src/lang/vm.c

@@ -2050,6 +2050,15 @@ vm_op_add_store_m(struct workspace *wk)
 	object_stack_push(wk, *member_target);
 }
 
+#define vm_op_store_uninit_check(__val)                    \
+	if (__val == obj_uninitialized)                    \
+	vm_diagnostic(wk,                                  \
+		0,                                         \
+		wk->vm.in_analyzer ? log_warn : log_error, \
+		0,                                         \
+		"load of %suninitialized variable",        \
+		wk->vm.in_analyzer ? "potentially " : "")
+
 #define vm_op_store_load_l_common() \
 	const struct call_frame *frame = arr_get(&wk->vm.call_stack, wk->vm.call_stack.len - 1); \
 	obj slot_idx = vm_get_constant(wk->vm.code.e, &wk->vm.ip) + frame->stack_base; \
@@ -2059,6 +2068,8 @@ static void
 vm_op_load_l(struct workspace *wk)
 {
 	vm_op_store_load_l_common();
+	vm_op_store_uninit_check(slot->o);
+
 	object_stack_push(wk, slot->o);
 }
 
@@ -2076,6 +2087,7 @@ static void
 vm_op_add_store_l(struct workspace *wk)
 {
 	vm_op_store_load_l_common();
+	vm_op_store_uninit_check(slot->o);
 
 	const struct obj_stack_entry *src = object_stack_pop_entry(&wk->vm.stack);
 	slot->o = vm_perform_add_store_mutations(wk, src->o, slot->o);
@@ -2086,12 +2098,13 @@ vm_op_add_store_l(struct workspace *wk)
 #define vm_op_store_load_u_common() \
 	const struct call_frame *frame = arr_get(&wk->vm.call_stack, wk->vm.call_stack.len - 1); \
 	obj slot_idx = vm_get_constant(wk->vm.code.e, &wk->vm.ip); \
-	obj *slot = frame->closure->upvalues[slot_idx]->location;
+	obj *slot = frame->closure->upvalues[slot_idx]->location \
 
 static void
 vm_op_load_u(struct workspace *wk)
 {
 	vm_op_store_load_u_common();
+	vm_op_store_uninit_check(*slot);
 	object_stack_push(wk, *slot);
 }
 
@@ -2107,6 +2120,7 @@ static void
 vm_op_add_store_u(struct workspace *wk)
 {
 	vm_op_store_load_u_common();
+	vm_op_store_uninit_check(*slot);
 	obj val = object_stack_pop(&wk->vm.stack);
 	*slot = vm_perform_add_store_mutations(wk, val, *slot);
 	object_stack_push(wk, *slot);