Re: Emacs port to gcc -fcheck-pointer-bounds

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

[-- Attachment #1: Type: text/plain, Size: 516 bytes --]

Stefan Monnier wrote:
>> and once you have the fake pointer, --enable-check-lisp-object-type
>> doesn't buy much extra safety that is useful.
> It does give us some extra checking, but not very much, indeed.
> Maybe we can turn it into a no-op.

To get the ball rolling on that, I installed the attached into master. This 
doesn't turn --enable-check-lisp-object-type into a no-op; it merely goes back 
to disabling it by default. We can remove it later if in practice it's not that 
helpful to enable it.

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: 0001-Default-CHECK_LISP_OBJECT_TYPE-to-no.patch --]
[-- Type: text/x-patch; name="0001-Default-CHECK_LISP_OBJECT_TYPE-to-no.patch", Size: 2138 bytes --]

From 5959b48ece0abe4639667c023da6363859088676 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 17 Dec 2017 18:43:14 -0800
Subject: [PATCH] Default CHECK_LISP_OBJECT_TYPE to "no"

* configure.ac: Go back to not defining CHECK_LISP_OBJECT_TYPE by
default for developer builds, since it is no longer that useful.
We can make it a no-op entirely later, if in practice it's not
that helpful to enable it.
---
 configure.ac | 7 +++----
 etc/NEWS     | 7 +++++++
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/configure.ac b/configure.ac
index 562b19a..ec1418b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -900,10 +900,9 @@ AC_DEFUN
 
 AC_ARG_ENABLE([check-lisp-object-type],
   [AS_HELP_STRING([--enable-check-lisp-object-type],
-     [Enable compile-time checks for the Lisp_Object data type,
-      which can catch some bugs during development.
-      The default is "no" if --enable-gcc-warnings is "no".])])
-if test "${enable_check_lisp_object_type-$gl_gcc_warnings}" != "no"; then
+     [Enable compile time checks for the Lisp_Object data type,
+      which can catch some bugs during development.])])
+if test "$enable_check_lisp_object_type" = yes; then
   AC_DEFINE([CHECK_LISP_OBJECT_TYPE], 1,
     [Define to enable compile-time checks for the Lisp_Object data type.])
 fi
diff --git a/etc/NEWS b/etc/NEWS
index 1382f96..1ab1930 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -38,6 +38,13 @@ interpreter or modules that it uses.  If your platform supports it you
 can enable it when configuring, e.g., './configure CFLAGS="-g3 -O2
 -mmpx -fcheck-pointer-bounds"' on Intel MPX platforms.
 
+** Emacs now normally uses a pointer type instead of an integer type
+for the fundamental word in the Emacs Lisp interpreter, to help
+catch typos and support -fcheck-pointer-bounds.  The 'configure'
+option --enable-check-lisp-object-type is therefore no longer as
+useful and so is no longer enabled by default in developer builds,
+to reduce differences between developer and production builds.
+
 \f
 * Startup Changes in Emacs 27.1
 
-- 
2.7.4