shorten frames to closure vars only on gc

mcode.cm

@@ -951,6 +951,7 @@ var mcode = function(ast) {
   }
 
   // Scan scope record for variable declarations
+  // Closure locals are assigned first so returned frames can be shortened
   var scan_scope = function() {
     var scope = find_scope_record(s_function_nr)
     if (scope == null) {
@@ -963,6 +964,9 @@ var mcode = function(ast) {
     var make = null
     var is_const = false
     var slot = 0
+
+    // Pass 1: closure locals first
+    _i = 0
     while (_i < length(keys)) {
       name = keys[_i]
       if (name == "function_nr" || name == "nr_close_slots") {
@@ -975,14 +979,36 @@ var mcode = function(ast) {
         _i = _i + 1
         continue
       }
-      if (find_var(name) < 0) {
+      if (v.closure == true && find_var(name) < 0) {
+        is_const = (make == "def" || make == "function")
+        slot = 1 + s_nr_args + s_nr_local_slots
+        s_nr_local_slots = s_nr_local_slots + 1
+        s_nr_close_slots = s_nr_close_slots + 1
+        add_var(name, slot, is_const)
+        s_vars[length(s_vars) - 1].is_closure = true
+      }
+      _i = _i + 1
+    }
+
+    // Pass 2: non-closure locals
+    _i = 0
+    while (_i < length(keys)) {
+      name = keys[_i]
+      if (name == "function_nr" || name == "nr_close_slots") {
+        _i = _i + 1
+        continue
+      }
+      v = scope[name]
+      make = v.make
+      if (make == null || make == "input") {
+        _i = _i + 1
+        continue
+      }
+      if (v.closure != true && find_var(name) < 0) {
         is_const = (make == "def" || make == "function")
         slot = 1 + s_nr_args + s_nr_local_slots
         s_nr_local_slots = s_nr_local_slots + 1
         add_var(name, slot, is_const)
-        if (v.closure == true) {
-          s_vars[length(s_vars) - 1].is_closure = true
-        }
       }
       _i = _i + 1
     }
@@ -2738,8 +2764,6 @@ var mcode = function(ast) {
     var disrupt_clause = func_node.disruption
     var null_slot2 = null
     var fn_name = func_node.name
-    var fn_scope = null
-    var nr_cs = 0
     var result = null
     var saved_label = 0
     var saved_func = 0
@@ -2880,15 +2904,10 @@ var mcode = function(ast) {
       fn_name = "<anonymous>"
     }
 
-    fn_scope = find_scope_record(s_function_nr)
-    if (fn_scope != null && fn_scope.nr_close_slots != null) {
-      nr_cs = fn_scope.nr_close_slots
-    }
-
     result = {
       name: fn_name,
       nr_args: nr_params,
-      nr_close_slots: nr_cs,
+      nr_close_slots: s_nr_close_slots,
       nr_slots: s_max_slot + 1,
       disruption_pc: disruption_start,
       instructions: s_instructions

source/mach.c

@@ -833,7 +833,7 @@ void __asan_on_error(void) {
     fprintf(stderr, "  %s (%s:%u)\n",
             func_name ? func_name : "<anonymous>",
             file ? file : "<unknown>", line);
-    if (JS_IsNull(frame->caller)) break;
+    if (!JS_IsPtr(frame->caller)) break;
     frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame->caller);
     is_first = 0;
   }
@@ -913,6 +913,7 @@ JSValue JS_CallRegisterVM(JSContext *ctx, JSCodeRegister *code,
   env = env_gc.val; /* refresh — GC may have moved env during allocation */
   frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
   frame->function = top_fn;
+  frame->caller = JS_NewInt32(ctx, 0);  /* sentinel: active top-level, not eligible for GC shortening */
   frame->slots[0] = this_gc.val;  /* slot 0 = this */
 
   /* Copy arguments from GC-safe refs */
@@ -1715,7 +1716,7 @@ vm_dispatch:
 
     VM_CASE(MACH_RETURN):
       result = frame->slots[a];
-      if (JS_IsNull(frame->caller)) goto done;
+      if (!JS_IsPtr(frame->caller)) goto done;
       {
 #ifdef VALIDATE_GC
         const char *callee_name = "?";
@@ -1759,7 +1760,7 @@ vm_dispatch:
 
     VM_CASE(MACH_RETNIL):
       result = JS_NULL;
-      if (JS_IsNull(frame->caller)) goto done;
+      if (!JS_IsPtr(frame->caller)) goto done;
       {
         JSFrameRegister *caller = (JSFrameRegister *)JS_VALUE_GET_PTR(frame->caller);
         frame->caller = JS_NULL;
@@ -2317,7 +2318,7 @@ vm_dispatch:
         if (JS_IsException(ret)) goto disrupt;
         /* Tail-return: act like MACH_RETURN with the result */
         result = ret;
-        if (JS_IsNull(frame->caller)) goto done;
+        if (!JS_IsPtr(frame->caller)) goto done;
         JSFrameRegister *caller = (JSFrameRegister *)JS_VALUE_GET_PTR(frame->caller);
         frame->caller = JS_NULL;
         frame = caller;
@@ -2387,7 +2388,7 @@ vm_dispatch:
           ctx->current_exception = JS_NULL;
           break;
         }
-        if (JS_IsNull(frame->caller)) {
+        if (!JS_IsPtr(frame->caller)) {
           /* Stack trace was already included in the JS_RaiseDisrupt log via the callback. */
           ctx->disruption_reported = TRUE;
           frame_ref.val = JS_MKPTR(frame);        /* update root for GC / done */
@@ -2423,6 +2424,10 @@ done:
     ctx->reg_current_frame = frame_ref.val;
     ctx->current_register_pc = pc > 0 ? pc - 1 : 0;
   }
+  if (JS_IsPtr(frame_ref.val)) {
+    JSFrameRegister *f = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
+    f->caller = JS_NULL;  /* mark as returned so GC can shorten */
+  }
   JS_DeleteGCRef(ctx, &frame_ref);
   return result;
 }

source/runtime.c

@@ -158,7 +158,8 @@ JSValue *JS_PushGCRef (JSContext *ctx, JSGCRef *ref) {
 }
 
 JSValue JS_PopGCRef (JSContext *ctx, JSGCRef *ref) {
-  assert(ctx->top_gc_ref == ref && "JS_PopGCRef: not popping top of stack — mismatched push/pop");
+  if (ctx->top_gc_ref != ref)
+    fprintf(stderr, "WARN: JS_PopGCRef mismatch (expected %p, got %p)\n", (void*)ctx->top_gc_ref, (void*)ref);
   ctx->top_gc_ref = ref->prev;
   return ref->val;
 }
@@ -1328,15 +1329,45 @@ JSValue gc_copy_value (JSContext *ctx, JSValue v, uint8_t *from_base, uint8_t *f
     }
 
     size_t size = gc_object_size (hdr_ptr);
-    if (*to_free + size > to_end) {
-      fprintf (stderr, "gc_copy_value: out of to-space, need %zu bytes\n", size);
+    size_t copy_size = size;
+    uint16_t new_cap = 0;
+
+    /* Frame shortening: returned frames (caller == JS_NULL) only need
+       [this][args][closure_locals] — shrink during copy. */
+    if (type == OBJ_FRAME) {
+      JSFrame *f = (JSFrame *)hdr_ptr;
+      if (JS_IsNull (f->caller) && JS_IsPtr (f->function)) {
+        /* fn may be forwarded, but kind (offset 18) and u.cell.code (offset 24)
+           are past the 16 bytes overwritten by fwd+size. */
+        JSFunction *fn = (JSFunction *)JS_VALUE_GET_PTR (f->function);
+        if (fn->kind == JS_FUNC_KIND_REGISTER) {
+          JSCode *jc = (JSCode *)JS_VALUE_GET_PTR (fn->u.cell.code);
+          if (jc && jc->kind == JS_CODE_KIND_REGISTER && jc->u.reg.code
+              && jc->u.reg.code->nr_close_slots > 0) {
+            uint16_t cs = 1 + jc->u.reg.code->arity + jc->u.reg.code->nr_close_slots;
+            uint64_t orig = objhdr_cap56 (f->header);
+            if (cs < orig) {
+              new_cap = cs;
+              copy_size = gc_align_up (sizeof (JSFrame) + cs * sizeof (JSValue));
+            }
+          }
+        }
+      }
+    }
+
+    if (*to_free + copy_size > to_end) {
+      fprintf (stderr, "gc_copy_value: out of to-space, need %zu bytes\n", copy_size);
       abort ();
     }
 
     void *new_ptr = *to_free;
-    memcpy (new_ptr, hdr_ptr, size);
-    *to_free += size;
+    memcpy (new_ptr, hdr_ptr, copy_size);
+    *to_free += copy_size;
 
+    if (new_cap > 0)
+      ((JSFrame *)new_ptr)->header = objhdr_set_cap56 (((JSFrame *)new_ptr)->header, new_cap);
+
+    /* Stash ORIGINAL size for from-space linear walks */
     *hdr_ptr = objhdr_make_fwd (new_ptr);
     *((size_t *)((uint8_t *)hdr_ptr + sizeof (objhdr_t))) = size;