From: Eli Zaretskii <eliz@gnu.org>
To: Eli Zaretskii <eliz@gnu.org>
Cc: fabrice.popineau@gmail.com, monnier@iro.umontreal.ca,
emacs-devel@gnu.org
Subject: Re: GC and stack marking
Date: Thu, 22 May 2014 18:20:04 +0300 [thread overview]
Message-ID: <83mwe9n8m3.fsf@gnu.org> (raw)
In-Reply-To: <83y4xvm2tg.fsf@gnu.org>
> Date: Wed, 21 May 2014 20:58:19 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: fabrice.popineau@gmail.com, emacs-devel@gnu.org
>
> > From: Stefan Monnier <monnier@iro.umontreal.ca>
> > Cc: emacs-devel@gnu.org, fabrice.popineau@gmail.com
> > Date: Wed, 21 May 2014 13:40:21 -0400
> >
> > > I already tried that before, and came up empty-handed. I tried again
> > > now; the address of that value on the stack does not correspond to any
> > > local variable in the corresponding stack frame, and I also cannot
> > > find that address in the disassembly of the function whose stack frame
> > > includes the value.
> >
> > It might simply be a slot that's unused by the current stack frame,
> > whose value comes from some stack frame that existed some time in
> > the past.
>
> That's probably what it is, yes.
That's definitely what it is. The value gets onto the stack when
loadup.el does this:
(when (hash-table-p purify-flag)
(let ((strings 0)
(vectors 0)
(bytecodes 0)
(conses 0)
(others 0))
(maphash (lambda (k v)
(cond
((stringp k) (setq strings (1+ strings)))
((vectorp k) (setq vectors (1+ vectors)))
((consp k) (setq conses (1+ conses)))
((byte-code-function-p v) (setq bytecodes (1+ bytecodes)))
(t (setq others (1+ others)))))
purify-flag)
(message "Pure-hashed: %d strings, %d vectors, %d conses, %d bytecodes, %d others"
strings vectors conses bytecodes others)))
The call to hash-table-p pushes the table address on the stack before
calling Fhash_table_p, and it remains there until the call to
mark_stack.
> > > Now, I have a question: mark_stack stops examining the stack when it
> > > gets to its own stack frame. That is certainly safe, but it sounds
> > > too conservative: it should stop at the stack frame of
> > > Fgarbage_collect, I think, because no live Lisp object can appear
> > > while Fgarbage_collect runs, right?
> >
> > Sounds right, yes.
>
> I will try that and see if that helps. Of course, if my reading of
> GDB data is correct, and the value was indeed in the
> Fgarbage_collect's stack frame, it must help.
It did help, at least in an unoptimized build. The suggested patch is
below. It just reshuffles the existing code: we now determine the
limit for searching the stack in Fgarbage_collect, and then call a
subroutine that does what Fgarbage_collect should actually do. This
way, none of the variables local to Fgarbage_collect or its stack will
be searched by mark_stack.
Is the patchy below OK for the trunk? Does anyone see anything
problematic with it?
--- src/alloc.c~ 2014-05-21 18:04:29 +0300
+++ src/alloc.c 2014-05-22 18:18:32 +0300
@@ -4880,61 +4880,8 @@ dump_zombies (void)
from the stack start. */
static void
-mark_stack (void)
+mark_stack (void *end)
{
- void *end;
-
-#ifdef HAVE___BUILTIN_UNWIND_INIT
- /* Force callee-saved registers and register windows onto the stack.
- This is the preferred method if available, obviating the need for
- machine dependent methods. */
- __builtin_unwind_init ();
- end = &end;
-#else /* not HAVE___BUILTIN_UNWIND_INIT */
-#ifndef GC_SAVE_REGISTERS_ON_STACK
- /* jmp_buf may not be aligned enough on darwin-ppc64 */
- union aligned_jmpbuf {
- Lisp_Object o;
- sys_jmp_buf j;
- } j;
- volatile bool stack_grows_down_p = (char *) &j > (char *) stack_base;
-#endif
- /* This trick flushes the register windows so that all the state of
- the process is contained in the stack. */
- /* Fixme: Code in the Boehm GC suggests flushing (with `flushrs') is
- needed on ia64 too. See mach_dep.c, where it also says inline
- assembler doesn't work with relevant proprietary compilers. */
-#ifdef __sparc__
-#if defined (__sparc64__) && defined (__FreeBSD__)
- /* FreeBSD does not have a ta 3 handler. */
- asm ("flushw");
-#else
- asm ("ta 3");
-#endif
-#endif
-
- /* Save registers that we need to see on the stack. We need to see
- registers used to hold register variables and registers used to
- pass parameters. */
-#ifdef GC_SAVE_REGISTERS_ON_STACK
- GC_SAVE_REGISTERS_ON_STACK (end);
-#else /* not GC_SAVE_REGISTERS_ON_STACK */
-
-#ifndef GC_SETJMP_WORKS /* If it hasn't been checked yet that
- setjmp will definitely work, test it
- and print a message with the result
- of the test. */
- if (!setjmp_tested_p)
- {
- setjmp_tested_p = 1;
- test_setjmp ();
- }
-#endif /* GC_SETJMP_WORKS */
-
- sys_setjmp (j.j);
- end = stack_grows_down_p ? (char *) &j + sizeof j : (char *) &j;
-#endif /* not GC_SAVE_REGISTERS_ON_STACK */
-#endif /* not HAVE___BUILTIN_UNWIND_INIT */
/* This assumes that the stack is a contiguous region in memory. If
that's not the case, something has to be done here to iterate
@@ -5542,22 +5489,14 @@ mark_pinned_symbols (void)
}
}
-DEFUN ("garbage-collect", Fgarbage_collect, Sgarbage_collect, 0, 0, "",
- doc: /* Reclaim storage for Lisp objects no longer needed.
-Garbage collection happens automatically if you cons more than
-`gc-cons-threshold' bytes of Lisp data since previous garbage collection.
-`garbage-collect' normally returns a list with info on amount of space in use,
-where each entry has the form (NAME SIZE USED FREE), where:
-- NAME is a symbol describing the kind of objects this entry represents,
-- SIZE is the number of bytes used by each one,
-- USED is the number of those objects that were found live in the heap,
-- FREE is the number of those objects that are not live but that Emacs
- keeps around for future allocations (maybe because it does not know how
- to return them to the OS).
-However, if there was overflow in pure space, `garbage-collect'
-returns nil, because real GC can't be done.
-See Info node `(elisp)Garbage Collection'. */)
- (void)
+/* Subroutine of Fgarbage_collect that does most of the work. It is a
+ separate function so that we could limit mark_stack in searching
+ the stack frames below this function, thus avoiding the rare cases
+ where mark_stack finds values that look like live Lisp objects on
+ portions of stack that couldn't possibly contain such live
+ objects. */
+static Lisp_Object
+garbage_collect_1 (void *end)
{
struct buffer *nextb;
char stack_top_variable;
@@ -5655,7 +5594,7 @@ See Info node `(elisp)Garbage Collection
#if (GC_MARK_STACK == GC_MAKE_GCPROS_NOOPS \
|| GC_MARK_STACK == GC_MARK_STACK_CHECK_GCPROS)
- mark_stack ();
+ mark_stack (end);
#else
{
register struct gcpro *tail;
@@ -5678,7 +5617,7 @@ See Info node `(elisp)Garbage Collection
#endif
#if GC_MARK_STACK == GC_USE_GCPROS_CHECK_ZOMBIES
- mark_stack ();
+ mark_stack (end);
#endif
/* Everything is now marked, except for the data in font caches
@@ -5838,6 +5777,82 @@ See Info node `(elisp)Garbage Collection
return retval;
}
+DEFUN ("garbage-collect", Fgarbage_collect, Sgarbage_collect, 0, 0, "",
+ doc: /* Reclaim storage for Lisp objects no longer needed.
+Garbage collection happens automatically if you cons more than
+`gc-cons-threshold' bytes of Lisp data since previous garbage collection.
+`garbage-collect' normally returns a list with info on amount of space in use,
+where each entry has the form (NAME SIZE USED FREE), where:
+- NAME is a symbol describing the kind of objects this entry represents,
+- SIZE is the number of bytes used by each one,
+- USED is the number of those objects that were found live in the heap,
+- FREE is the number of those objects that are not live but that Emacs
+ keeps around for future allocations (maybe because it does not know how
+ to return them to the OS).
+However, if there was overflow in pure space, `garbage-collect'
+returns nil, because real GC can't be done.
+See Info node `(elisp)Garbage Collection'. */)
+ (void)
+{
+#if (GC_MARK_STACK == GC_MAKE_GCPROS_NOOPS \
+ || GC_MARK_STACK == GC_MARK_STACK_CHECK_GCPROS \
+ || GC_MARK_STACK == GC_USE_GCPROS_CHECK_ZOMBIES)
+ void *end;
+
+#ifdef HAVE___BUILTIN_UNWIND_INIT
+ /* Force callee-saved registers and register windows onto the stack.
+ This is the preferred method if available, obviating the need for
+ machine dependent methods. */
+ __builtin_unwind_init ();
+ end = &end;
+#else /* not HAVE___BUILTIN_UNWIND_INIT */
+#ifndef GC_SAVE_REGISTERS_ON_STACK
+ /* jmp_buf may not be aligned enough on darwin-ppc64 */
+ union aligned_jmpbuf {
+ Lisp_Object o;
+ sys_jmp_buf j;
+ } j;
+ volatile bool stack_grows_down_p = (char *) &j > (char *) stack_base;
+#endif
+ /* This trick flushes the register windows so that all the state of
+ the process is contained in the stack. */
+ /* Fixme: Code in the Boehm GC suggests flushing (with `flushrs') is
+ needed on ia64 too. See mach_dep.c, where it also says inline
+ assembler doesn't work with relevant proprietary compilers. */
+#ifdef __sparc__
+#if defined (__sparc64__) && defined (__FreeBSD__)
+ /* FreeBSD does not have a ta 3 handler. */
+ asm ("flushw");
+#else
+ asm ("ta 3");
+#endif
+#endif
+
+ /* Save registers that we need to see on the stack. We need to see
+ registers used to hold register variables and registers used to
+ pass parameters. */
+#ifdef GC_SAVE_REGISTERS_ON_STACK
+ GC_SAVE_REGISTERS_ON_STACK (end);
+#else /* not GC_SAVE_REGISTERS_ON_STACK */
+
+#ifndef GC_SETJMP_WORKS /* If it hasn't been checked yet that
+ setjmp will definitely work, test it
+ and print a message with the result
+ of the test. */
+ if (!setjmp_tested_p)
+ {
+ setjmp_tested_p = 1;
+ test_setjmp ();
+ }
+#endif /* GC_SETJMP_WORKS */
+
+ sys_setjmp (j.j);
+ end = stack_grows_down_p ? (char *) &j + sizeof j : (char *) &j;
+#endif /* not GC_SAVE_REGISTERS_ON_STACK */
+#endif /* not HAVE___BUILTIN_UNWIND_INIT */
+#endif /* GC_MARK_STACK */
+ return garbage_collect_1 (end);
+}
/* Mark Lisp objects in glyph matrix MATRIX. Currently the
only interesting objects referenced from glyphs are strings. */
next prev parent reply other threads:[~2014-05-22 15:20 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-19 16:31 GC and stack marking Eli Zaretskii
2014-05-19 18:47 ` Paul Eggert
2014-05-19 19:14 ` Eli Zaretskii
2014-05-19 19:58 ` Paul Eggert
2014-05-19 20:03 ` Eli Zaretskii
2014-05-19 20:17 ` Paul Eggert
2014-05-20 16:37 ` Eli Zaretskii
2014-05-20 13:44 ` Stefan Monnier
2014-05-20 16:57 ` Eli Zaretskii
2014-05-20 17:54 ` Stefan Monnier
2014-05-20 19:28 ` Eli Zaretskii
2014-05-20 22:01 ` Stefan Monnier
2014-05-21 2:48 ` Eli Zaretskii
2014-05-21 3:01 ` Stefan Monnier
2014-05-21 15:39 ` Eli Zaretskii
2014-05-21 15:57 ` Dmitry Antipov
2014-05-21 16:06 ` Dmitry Antipov
2014-05-21 16:55 ` Eli Zaretskii
2014-05-21 16:53 ` Eli Zaretskii
2014-05-21 17:40 ` Stefan Monnier
2014-05-21 17:58 ` Eli Zaretskii
2014-05-22 15:20 ` Eli Zaretskii [this message]
2014-05-22 16:14 ` Stefan Monnier
2014-05-24 12:03 ` Eli Zaretskii
2014-05-20 19:12 ` Daniel Colascione
2014-05-20 19:43 ` Eli Zaretskii
2014-05-20 22:03 ` Stefan Monnier
2014-05-21 2:51 ` Eli Zaretskii
2014-05-31 6:31 ` Florian Weimer
2014-05-31 14:24 ` Stefan Monnier
-- strict thread matches above, loose matches on Subject: below --
2014-05-21 19:31 Barry OReilly
2014-05-21 20:13 ` Eli Zaretskii
2014-05-21 20:49 ` Barry OReilly
2014-05-22 2:43 ` Eli Zaretskii
2014-05-22 3:12 ` Daniel Colascione
2014-05-22 5:37 ` David Kastrup
2014-05-22 13:57 ` Stefan Monnier
2014-05-22 15:49 ` Eli Zaretskii
2014-05-22 14:59 ` Barry OReilly
2014-05-22 17:03 ` Eli Zaretskii
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