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).

1 files changed, 7 insertions, 4 deletions

diff --git a/src/lread.c b/src/lread.c
index 1613719eb1d..70984d37e18 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));
             }