From: Philipp Stephani <phst@google.com>
Date: Sat, 1 Aug 2020 19:01:24 +0000 (+0200)
Subject: Improve offset calculation in wide int builds
X-Git-Tag: emacs-28.0.90~6873
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=fce47c93252fe0be14e8c169f6bab8b0fd512cab;p=emacs.git

Improve offset calculation in wide int builds

* src/alloc.c (mark_maybe_object): Make sure that OFFSET isn’t widened
during subtraction.
---

diff --git a/src/alloc.c b/src/alloc.c
index f203061161b..e556fc86a3b 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