Make pdumper-marking pickier

Prevent some false-positives in conservative GC marking.
This doesn’t fix any correctness bugs; it’s merely to
reclaim some memory instead of keeping it unnecessarily.
* src/alloc.c (mark_maybe_pointer): New arg SYMBOL_ONLY.
All callers changed.  Check that the pointer’s tag, if any,
matches the pdumper-reported type.

src/alloc.c

@@ -4740,7 +4740,7 @@ live_small_vector_p (struct mem_node *m, void *p)
    marked.  */
 
 static void
-mark_maybe_pointer (void *p)
+mark_maybe_pointer (void *p, bool symbol_only)
 {
   struct mem_node *m;
 
@@ -4765,15 +4765,21 @@ mark_maybe_pointer (void *p)
 	 this problem, the pdumper code should grok non-initial
 	 addresses, as the non-pdumper code does.  */
       uintptr_t mask = VALMASK;
-      p = (void *) ((uintptr_t) p & mask);
+      void *po = (void *) ((uintptr_t) p & mask);
+      char *cp = p;
+      char *cpo = po;
       /* Don't use pdumper_object_p_precise here! It doesn't check the
          tag bits. OBJ here might be complete garbage, so we need to
          verify both the pointer and the tag.  */
-      int type = pdumper_find_object_type (p);
-      if (pdumper_valid_object_type_p (type))
-        mark_object (type == Lisp_Symbol
-                     ? make_lisp_symbol (p)
-                     : make_lisp_ptr (p, type));
+      int type = pdumper_find_object_type (po);
+      if (pdumper_valid_object_type_p (type)
+	  && (!USE_LSB_TAG || p == po || cp - cpo == type))
+	{
+	  if (type == Lisp_Symbol)
+	    mark_object (make_lisp_symbol (po));
+	  else if (!symbol_only)
+	    mark_object (make_lisp_ptr (po, type));
+	}
       return;
     }
 
@@ -4791,6 +4797,8 @@ mark_maybe_pointer (void *p)
 
 	case MEM_TYPE_CONS:
 	  {
+	    if (symbol_only)
+	      return;
 	    struct Lisp_Cons *h = live_cons_holding (m, p);
 	    if (!h)
 	      return;
@@ -4800,6 +4808,8 @@ mark_maybe_pointer (void *p)
 
 	case MEM_TYPE_STRING:
 	  {
+	    if (symbol_only)
+	      return;
 	    struct Lisp_String *h = live_string_holding (m, p);
 	    if (!h)
 	      return;
@@ -4818,6 +4828,8 @@ mark_maybe_pointer (void *p)
 
 	case MEM_TYPE_FLOAT:
 	  {
+	    if (symbol_only)
+	      return;
 	    struct Lisp_Float *h = live_float_holding (m, p);
 	    if (!h)
 	      return;
@@ -4827,6 +4839,8 @@ mark_maybe_pointer (void *p)
 
 	case MEM_TYPE_VECTORLIKE:
 	  {
+	    if (symbol_only)
+	      return;
 	    struct Lisp_Vector *h = live_large_vector_holding (m, p);
 	    if (!h)
 	      return;
@@ -4836,6 +4850,8 @@ mark_maybe_pointer (void *p)
 
 	case MEM_TYPE_VECTOR_BLOCK:
 	  {
+	    if (symbol_only)
+	      return;
 	    struct Lisp_Vector *h = live_small_vector_holding (m, p);
 	    if (!h)
 	      return;
@@ -4897,7 +4913,7 @@ mark_memory (void const *start, void const *end)
   for (pp = start; (void const *) pp < end; pp += GC_POINTER_ALIGNMENT)
     {
       void *p = *(void *const *) pp;
-      mark_maybe_pointer (p);
+      mark_maybe_pointer (p, false);
 
       /* Unmask any struct Lisp_Symbol pointer that make_lisp_symbol
 	 previously disguised by adding the address of 'lispsym'.
@@ -4906,7 +4922,7 @@ mark_memory (void const *start, void const *end)
 	 non-adjacent words and P might be the low-order word's value.  */
       intptr_t ip;
       INT_ADD_WRAPV ((intptr_t) p, (intptr_t) lispsym, &ip);
-      mark_maybe_pointer ((void *) ip);
+      mark_maybe_pointer ((void *) ip, true);
     }
 }