Re: /srv/bzr/emacs/trunk r103781: * src/bytecode.c (Fbyte_code): CAR and CDR can GC.

[Daniel Colascione]

On 3/30/2011 11:04 AM, Stefan Monnier wrote:
> ------------------------------------------------------------
> revno: 103781
> committer: Stefan Monnier<address@hidden>
> branch nick: trunk
> timestamp: Wed 2011-03-30 14:04:11 -0400
> message:
>    * src/bytecode.c (Fbyte_code): CAR and CDR can GC.


Wait, what?  How?  wrong_type_argument never returns.  If we run into a 
problem, we'll throw up to some higher context and never hit the 
AFTER_POTENTIAL_GC case. BEFORE_POTENTIAL_GC (if it's not a noop) sets 
the stack top field to point to the top of the stack, but because we 
never return from wrong_type_argument, we don't care whether the stack 
is lost.  v1 might be on the stack, so the only thing keeping it alive 
might be stack.top.

Now, Fsignal *does* run signal-hook-function, which of course might GC. 
 But that's done using call2, which will keep error_symbol and data 
alive even if we do GC, so a GC in this case shouldn't actually cause 
any problems.  Likewise for the debugging hooks.  So GC shouldn't 
actually cause a problem, IIUC.

What am I missing?

=== modified file 'src/ChangeLog'
--- a/src/ChangeLog     2011-03-30 13:35:37 +0000
+++ b/src/ChangeLog     2011-03-30 18:04:11 +0000
@@ -1,3 +1,7 @@
+2011-03-30  Stefan Monnier  <address@hidden>
+
+       * bytecode.c (Fbyte_code): CAR and CDR can GC.
+
 2011-03-30  Zachary Kanfer  <address@hidden>  (tiny change)

        * keyboard.c (Fexecute_extended_command): Do log the "suggest key

=== modified file 'src/bytecode.c'
--- a/src/bytecode.c    2011-03-17 02:18:00 +0000
+++ b/src/bytecode.c    2011-03-30 18:04:11 +0000
@@ -554,7 +554,16 @@
          {
            Lisp_Object v1;
            v1 = TOP;
-           TOP = CAR (v1);
+           if (CONSP (v1))
+             TOP = XCAR (v1);
+           else if (NILP (v1))
+             TOP = Qnil;
+           else
+             {
+               BEFORE_POTENTIAL_GC ();
+               wrong_type_argument (Qlistp, v1);
+               AFTER_POTENTIAL_GC ();
+             }
            break;
          }

@@ -580,7 +589,17 @@
          {
            Lisp_Object v1;
            v1 = TOP;
-           TOP = CDR (v1);
+           if (CONSP (v1))
+             TOP = XCDR (v1);
+           else if (NILP (v1))
+             TOP = Qnil;
+           else
+             {
+               BEFORE_POTENTIAL_GC ();
+               wrong_type_argument (Qlistp, v1);
+               AFTER_POTENTIAL_GC ();
+             }
+           break;
            break;
          }

@@ -911,13 +930,13 @@
            v1 = POP;
            v2 = TOP;
            CHECK_NUMBER (v2);
-           AFTER_POTENTIAL_GC ();
            op = XINT (v2);
            immediate_quit = 1;
            while (--op >= 0 && CONSP (v1))
              v1 = XCDR (v1);
            immediate_quit = 0;
            TOP = CAR (v1);
+           AFTER_POTENTIAL_GC ();
            break;
          }