From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 16 Feb 2020 19:36:19 +0000 (-0800)
Subject: Improve C-h C-h bug fix
X-Git-Tag: emacs-28.0.90~7874
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=556cc727e5076d590f8286406e4f46cff3cee41e;p=emacs.git

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).
---

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));
 	    }