Fix pdumper-related GC bug

* src/alloc.c (mark_maybe_pointer): Also mark pointers
to pdumper objects, even when the pointers are tagged.
Add a FIXME saying why this isn’t enough.

src/alloc.c

@@ -4755,6 +4755,17 @@ mark_maybe_pointer (void *p)
      definitely _don't_ have an object.  */
   if (pdumper_object_p (p))
     {
+      /* FIXME: This code assumes that every reachable pdumper object
+	 is addressed either by a pointer to the object start, or by
+	 the same pointer with an LSB-style tag.  This assumption
+	 fails if a pdumper object is reachable only via machine
+	 addresses of non-initial object components.  Although such
+	 addressing is rare in machine code generated by C compilers
+	 from Emacs source code, it can occur in some cases.  To fix
+	 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);
       /* 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.  */