master fce47c9 2/2: Improve offset calculation in wide int builds

[Philipp Stephani]

branch: master
commit fce47c93252fe0be14e8c169f6bab8b0fd512cab
Author: Philipp Stephani <phst@google.com>
Commit: Philipp Stephani <phst@google.com>

    Improve offset calculation in wide int builds
    
    * src/alloc.c (mark_maybe_object): Make sure that OFFSET isn’t widened
    during subtraction.
---
 src/alloc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/alloc.c b/src/alloc.c
index f203061..e556fc8 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4641,8 +4641,10 @@ mark_maybe_object (Lisp_Object obj)
       break;
     }
 
-  void *po = (char *) ((intptr_t) (char *) XLP (obj)
-                       + (offset - LISP_WORD_TAG (type_tag)));
+  bool overflow
+    = INT_SUBTRACT_WRAPV (offset, LISP_WORD_TAG (type_tag), &offset);
+  eassert (!overflow);
+  void *po = (char *) ((intptr_t) (char *) XLP (obj) + offset);
 
   /* If the pointer is in the dump image and the dump has a record
      of the object starting at the place where the pointer points, we