During major GC, scan the environment part of closures only

xavierleroy · xavierleroy · commit cd2ed2b1ec63 · 2020-05-29T17:09:09.000+02:00
Here we start reaping the benefits of the new closure representation.

The fields of a closure block that contain the code pointers need not
be scanned (in general) and must not be scanned (in no-naked-pointers mode).

Here, conservatively, we skip them in no-naked-pointers mode only,
but it would be sound to skip them unconditionally.
diff --git a/runtime/major_gc.c b/runtime/major_gc.c
@@ -239,12 +239,7 @@ Caml_inline value* mark_slice_darken(value *gray_vals_ptr,
 #ifdef NATIVE_CODE_AND_NO_NAKED_POINTERS
   if (Is_block (child)
         && ! Is_young (child)
-        && Wosize_val (child) > 0  /* Atoms never need to be marked. */
-        /* Closure blocks contain code pointers at offsets that cannot
-           be reliably determined, so we always use the page table when
-           marking such values. */
-        && (!(Tag_val (v) == Closure_tag || Tag_val (v) == Infix_tag) ||
-            Is_in_heap (child))) {
+        && Wosize_val (child) > 0) {  /* Atoms never need to be marked. */
 #else
   if (Is_block (child) && Is_in_heap (child)) {
 #endif
@@ -398,6 +393,13 @@ static void mark_slice (intnat work)
       CAMLassert (start == 0);
       v = *--gray_vals_ptr;
       CAMLassert (Is_gray_val (v));
+#ifdef NO_NAKED_POINTERS
+      if (Tag_val(v) == Closure_tag) {
+        /* Skip the code pointers and integers at beginning of closure;
+           start scanning at the first word of the environment part. */
+        start = Start_env_closinfo(Closinfo_val(v));
+      }
+#endif
     }
     if (v != 0){
       hd = Hd_val(v);
