Re: [Emacs-diffs] master bc78ff2 2/2: Use 0 for Qnil

[Paul Eggert <eggert@cs.ucla.edu>]

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YAMAMOTO Mitsuharu wrote:
> With the above macros, XUNTAG (a, type) is expanded to:
>
>    ((void *) ((char *) (0) - (type) + (intptr_t) XLI (a)))
>
> It is relative to the NULL pointer and clang seems to treat its
> dereference as an undefined behavior.

Thanks for diagnosing that.  This is arguably a bug in clang, but it's easy 
enough to work around as you suggest.  I reproduced the problem with Fedora 21 
x86-64 (clang 3.5.0) and installed the attached patch, which fixed it for me.


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From 8d025fa15493be4079d05629e9ca8b7ae325fc58 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 7 Jan 2015 23:02:01 -0800
Subject: [PATCH] Port Qnil==0 XUNTAG to clang

clang has undefined behavior if the program subtracts an integer
from (char *) 0.  Problem reported by YAMAMOTO Mitsuharu in:
http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00132.html
* lisp.h (lisp_h_XUNTAG) [USE_LSB_TAG]:
(XUNTAG) [!USE_LSB_TAG]: Port to clang 3.5.0.
---
 src/ChangeLog | 7 +++++++
 src/lisp.h    | 5 +++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/ChangeLog b/src/ChangeLog
index eae16c2..817483b 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,12 @@
 2015-01-08  Paul Eggert  <eggert@cs.ucla.edu>
 
+	Port Qnil==0 XUNTAG to clang
+	clang has undefined behavior if the program subtracts an integer
+	from (char *) 0.  Problem reported by YAMAMOTO Mitsuharu in:
+	http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00132.html
+	* lisp.h (lisp_h_XUNTAG) [USE_LSB_TAG]:
+	(XUNTAG) [!USE_LSB_TAG]: Port to clang 3.5.0.
+
 	Port GFileMonitor * hack to Qnil==0 platforms
 	Reported by Glenn Morris in: http://bugs.gnu.org/15880#112
 	* gfilenotify.c (monitor_to_lisp, lisp_to_monitor): New functions.
diff --git a/src/lisp.h b/src/lisp.h
index 1f18b5e..b9263f8 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -368,7 +368,7 @@ error !;
 # define lisp_h_XFASTINT(a) XINT (a)
 # define lisp_h_XINT(a) (XLI (a) >> INTTYPEBITS)
 # define lisp_h_XTYPE(a) ((enum Lisp_Type) (XLI (a) & ~VALMASK))
-# define lisp_h_XUNTAG(a, type) XUNTAGBASE (a, type, 0)
+# define lisp_h_XUNTAG(a, type) ((void *) (intptr_t) (XLI (a) - (type)))
 # define lisp_h_XUNTAGBASE(a, type, base) \
     ((void *) ((char *) (base) - (type) + (intptr_t) XLI (a)))
 #endif
@@ -905,7 +905,8 @@ XUNTAGBASE (Lisp_Object a, int type, void *base)
 INLINE void *
 XUNTAG (Lisp_Object a, int type)
 {
-  return XUNTAGBASE (a, type, 0);
+  intptr_t i = USE_LSB_TAG ? XLI (a) - type : XLI (a) & VALMASK;
+  return (void *) i;
 }
 
 #endif /* ! USE_LSB_TAG */
-- 
2.1.0