master 556cc72: Improve C-h C-h bug fix

[Paul Eggert]

branch: master
commit 556cc727e5076d590f8286406e4f46cff3cee41e
Author: Paul Eggert <address@hidden>
Commit: Paul Eggert <address@hidden>

    Improve C-h C-h bug fix
    
    * src/lread.c (read1): Guard against two 'struct Lisp_Vector *'
    pointers differing only in their most significant bit.  Problem
    reported by Pip Cet (Bug#39529#22).
---
 src/lread.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/lread.c b/src/lread.c
index 1613719..70984d3 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -2982,10 +2982,13 @@ read1 (Lisp_Object readcharfun, int *pch, bool 
first_in_list)
                 as 0.  This placeholder 0 would lead to accidental sharing in
                 purecopy's hash-consing, so replace it with a (hopefully)
                 unique integer placeholder, which is negative so that it is
-                not confused with a DOC file offset.  Eventually
-                Snarf-documentation should replace the placeholder with the
-                actual docstring.  */
-             EMACS_UINT hash = XHASH (tmp) | (INTMASK - INTMASK / 2);
+                not confused with a DOC file offset (the USE_LSB_TAG shift
+                relies on the fact that VALMASK is one bit narrower than
+                INTMASK).  Eventually Snarf-documentation should replace the
+                placeholder with the actual docstring.  */
+             verify (INTMASK & ~VALMASK);
+             EMACS_UINT hash = ((XHASH (tmp) >> USE_LSB_TAG)
+                                | (INTMASK - INTMASK / 2));
              ASET (tmp, COMPILED_DOC_STRING, make_ufixnum (hash));
            }