emacs-27 68b6dad: Be more aggressive in marking objects during GC

[Paul Eggert]

branch: emacs-27
commit 68b6dad1d8e22fe700871c9a5a18da3dd496cc8a
Author: Paul Eggert <eggert@cs.ucla.edu>
Commit: Paul Eggert <eggert@cs.ucla.edu>

    Be more aggressive in marking objects during GC
    
    Simplified version of a patch from Pip Cet (Bug#41321#299).
    * src/alloc.c (maybe_lisp_pointer): Remove.  All uses removed.
    (mark_memory): Also look at the pointer offset by ‘lispsym’,
    for symbols.
---
 src/alloc.c | 26 ++++++++++----------------
 1 file changed, 10 insertions(+), 16 deletions(-)

diff --git a/src/alloc.c b/src/alloc.c
index 1c6b664..568fee6 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4585,18 +4585,6 @@ mark_maybe_objects (Lisp_Object const *array, ptrdiff_t 
nelts)
     mark_maybe_object (*array);
 }
 
-/* Return true if P might point to Lisp data that can be garbage
-   collected, and false otherwise (i.e., false if it is easy to see
-   that P cannot point to Lisp data that can be garbage collected).
-   Symbols are implemented via offsets not pointers, but the offsets
-   are also multiples of LISP_ALIGNMENT.  */
-
-static bool
-maybe_lisp_pointer (void *p)
-{
-  return (uintptr_t) p % LISP_ALIGNMENT == 0;
-}
-
 /* If P points to Lisp data, mark that as live if it isn't already
    marked.  */
 
@@ -4609,9 +4597,6 @@ mark_maybe_pointer (void *p)
   VALGRIND_MAKE_MEM_DEFINED (&p, sizeof (p));
 #endif
 
-  if (!maybe_lisp_pointer (p))
-    return;
-
   if (pdumper_object_p (p))
     {
       int type = pdumper_find_object_type (p);
@@ -4715,7 +4700,16 @@ mark_memory (void const *start, void const *end)
 
   for (pp = start; (void const *) pp < end; pp += GC_POINTER_ALIGNMENT)
     {
-      mark_maybe_pointer (*(void *const *) pp);
+      char *p = *(char *const *) pp;
+      mark_maybe_pointer (p);
+
+      /* Unmask any struct Lisp_Symbol pointer that make_lisp_symbol
+        previously disguised by adding the address of 'lispsym'.
+        On a host with 32-bit pointers and 64-bit Lisp_Objects,
+        a Lisp_Object might be split into registers saved into
+        non-adjacent words and P might be the low-order word's value.  */
+      p += (intptr_t) lispsym;
+      mark_maybe_pointer (p);
 
       verify (alignof (Lisp_Object) % GC_POINTER_ALIGNMENT == 0);
       if (alignof (Lisp_Object) == GC_POINTER_ALIGNMENT