From 4c6f45fa8eef1a15d5790c1f3d3e608b548015db Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Engdeg=C3=A5rd?= <mattiase@acm.org>
Date: Tue, 17 Sep 2024 13:07:01 +0200
Subject: [PATCH] Re-enable GC mark trace buffer by default

Enable GC_REMEMBER_LAST_MARKED by default (it was disabled in Emacs 29)
to make it easier to debug difficult-to-reproduce GC problems
encountered by users.  This increases GC costs by about 5 %, which can
be avoided by turning the mark trace buffer back off using the new
--disable-gc-mark-trace option.

See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2024-09/msg00240.html

* configure.ac (--disable-gc-mark-trace): New config option.
* etc/NEWS: Mention it.
* src/alloc.c: Enable it by default and avoid a compiler warning.
---
 configure.ac | 11 +++++++++++
 etc/NEWS     |  7 +++++++
 src/alloc.c  |  5 ++---
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/configure.ac b/configure.ac
index f63ef180c5b..5eaa1c7f962 100644
--- a/configure.ac
+++ b/configure.ac
@@ -776,6 +776,17 @@ AS_IF([test "$with_android" = no || test -n "$XCONFIGURE"],[
     [android_enable_checking=yes
      export android_enable_checking])])
 
+AC_ARG_ENABLE([gc-mark-trace],
+ [AS_HELP_STRING([--disable-gc-mark-trace],
+                 [disable the mark trace buffer used for debugging the Emacs
+                 garbage collector])],
+ [ac_enable_gc_mark_trace="${enableval}"],
+ [ac_enable_gc_mark_trace=yes])
+if test "x$ac_enable_gc_mark_trace" = xyes ; then
+   AC_DEFINE([GC_REMEMBER_LAST_MARKED], [1],
+   [Define to 1 to enable GC mark trace buffer.])
+fi
+
 dnl The name of this option is unfortunate.  It predates, and has no
 dnl relation to, the "sampling-based elisp profiler" added in 24.3.
 dnl Actually, it stops it working.
diff --git a/etc/NEWS b/etc/NEWS
index 9b66e67c49a..2bb419ea129 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -55,6 +55,13 @@ and to resolve potential incompatibilities between GNU/Linux and *BSD
 versions of ALSA.  Use '--with-sound=alsa' to build with ALSA on these
 operating systems instead.
 
+---
+** New configuration option '--disable-gc-mark-trace'.
+This disables the GC mark trace buffer for about 5 % better garbage
+collection performance.  Doing so may make it more difficult for Emacs
+developers to help finding GC-related bugs that you run into, which is
+why it the mark trace buffer is enabled by default.
+
 
 * Startup Changes in Emacs 30.1
 
diff --git a/src/alloc.c b/src/alloc.c
index 666f77bfce1..258a3d1aaf2 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -6849,11 +6849,10 @@ mark_glyph_matrix (struct glyph_matrix *matrix)
       }
 }
 
-/* Whether to remember a few of the last marked values for debugging.  */
-#define GC_REMEMBER_LAST_MARKED 0
-
 #if GC_REMEMBER_LAST_MARKED
+/* Remember a few of the last marked values for debugging purposes.  */
 enum { LAST_MARKED_SIZE = 1 << 9 }; /* Must be a power of 2.  */
+extern Lisp_Object last_marked[LAST_MARKED_SIZE];
 Lisp_Object last_marked[LAST_MARKED_SIZE] EXTERNALLY_VISIBLE;
 static int last_marked_index;
 #endif