Re: conservative stack scanning

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

On 01/11/2013 11:51 AM, Stefan Monnier wrote:
> Shouldn't we just enable conservative stack scanning for
> all architectures?

Yes, we should give it a try.  If it doesn't work we can revert.
If it does work, we can simplify Emacs and this will help prevent
future bugs like Bug#13408.

Here's a proposed patch.

=== modified file 'ChangeLog'
--- ChangeLog	2013-01-11 23:28:04 +0000
+++ ChangeLog	2013-01-12 01:49:51 +0000
@@ -1,3 +1,10 @@
+2013-01-12  Paul Eggert  <eggert@cs.ucla.edu>
+
+	Enable conservative stack scanning for all architectures.
+	Suggested by Stefan Monnier in
+	<http://lists.gnu.org/archive/html/emacs-devel/2013-01/msg00183.html>.
+	* configure.ac (GC_MARK_STACK): Remove.
+
 2013-01-11  Paul Eggert  <eggert@cs.ucla.edu>
 
 	* lib/getopt_.h: Remove trailing CRs that crept in.

=== modified file 'configure.ac'
--- configure.ac	2013-01-10 03:43:02 +0000
+++ configure.ac	2013-01-12 01:49:51 +0000
@@ -3777,22 +3777,13 @@
 esac
 
 
-dnl These won't be used automatically yet.  We also need to know, at least,
+dnl This won't be used automatically yet.  We also need to know, at least,
 dnl that the stack is continuous.
 AH_TEMPLATE(GC_SETJMP_WORKS, [Define if setjmp is known to save all
   registers relevant for conservative garbage collection in the jmp_buf.])
 
-AH_TEMPLATE(GC_MARK_STACK, [Define to GC_USE_GCPROS_AS_BEFORE if
-  conservative garbage collection is not known to work.])
-
 
 case $opsys in
-  aix4-2 | hpux* | unixware)
-    dnl Conservative garbage collection has not been tested, so for now
-    dnl play it safe and stick with the old-fashioned way of marking.
-    AC_DEFINE(GC_MARK_STACK, [GC_USE_GCPROS_AS_BEFORE])
-    ;;
-
   dnl Not all the architectures are tested, but there are Debian packages
   dnl for SCM and/or Guile on them, so the technique must work.  See also
   dnl comments in alloc.c concerning setjmp and gcc.
@@ -3807,8 +3798,7 @@
 #else
 # error "setjmp not known to work on this arch"
 #endif
-    ]], [[]])], AC_DEFINE(GC_SETJMP_WORKS, 1),
-      AC_DEFINE(GC_MARK_STACK, [GC_USE_GCPROS_AS_BEFORE]) )
+    ]], [[]])], AC_DEFINE(GC_SETJMP_WORKS, 1))
     ;;
 esac
 

=== modified file 'src/lisp.h'
--- src/lisp.h	2013-01-11 13:25:10 +0000
+++ src/lisp.h	2013-01-12 01:49:51 +0000
@@ -2217,7 +2217,14 @@
    2    Mark the stack, and check that everything GCPRO'd is
 	marked.
    3	Mark using GCPRO's, mark stack last, and count how many
-	dead objects are kept alive.  */
+	dead objects are kept alive.
+
+   Formerly, method 0 was used.  Currently, method 1 is used unless
+   specified by hand when building, e.g.,
+   "make CPPFLAGS='-DGC_MARK_STACK=GC_USE_GCPROS_AS_BEFORE'".
+   Methods 2 and 3 are present mainly to debug the transition from 0 to 1.
+   At some point we should simplify Emacs by removing support
+   for all methods other than method 1.  */
 
 
 #define GC_USE_GCPROS_AS_BEFORE		0