From e093be94e1569e05a25c454c2c47a1863c679c56 Mon Sep 17 00:00:00 2001
From: Jake Hughes <jh@jakehughes.uk>
Date: Wed, 21 Aug 2024 21:17:22 +0100
Subject: [PATCH] Revert to using old GC_finalize mechanism

Using an extension of the disclaim API proved difficult because of the
need to re-mark objects pointed to by finalisable objects before a
finaliser is run. This would have led to complex changes to the
mark-phase for no real benefit.

Instead, by switching back to the GC_finalize mechanism but ensuring
that finalisers are run off of mutator threads, we get this desired
behaviour 'for free'.

This approach uses a simple condition variable to sleep the finaliser thread
when there is no finalisation work to do. This is then potentially
resumed at the end of a GC cycle if that cycle discovered finalisable
objects.
---
 include/private/gc_priv.h | 10 +++++-----
 reclaim.c                 | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 5 deletions(-)

diff --git a/include/private/gc_priv.h b/include/private/gc_priv.h
index 893c6372b..74aa27825 100644
--- a/include/private/gc_priv.h
+++ b/include/private/gc_priv.h
@@ -341,6 +341,7 @@ typedef struct hblkhdr hdr;
 
 #include "gc/gc_inline.h"
 
+<<<<<<< HEAD
 #ifdef BUFFERED_FINALIZATION
 
 typedef struct GC_finalization_buffer_hdr GC_finalization_buffer_hdr;
@@ -355,6 +356,9 @@ struct GC_current_buffer {
 };
 
 GC_INNER void GC_maybe_spawn_finalize_thread();
+=======
+GC_INNER void GC_maybe_wake_finalizer_thread();
+>>>>>>> b44b0a3e (Revert to using old GC_finalize mechanism)
 
 #endif
 
@@ -402,11 +406,7 @@ GC_INNER void GC_maybe_spawn_finalize_thread();
 EXTERN_C_BEGIN
 
 #ifndef GC_NO_FINALIZATION
-#ifdef BUFFERED_FINALIZATION
-# define GC_INVOKE_FINALIZERS() GC_maybe_spawn_finalize_thread()
-#else
-# define GC_INVOKE_FINALIZERS() GC_notify_or_invoke_finalizers()
-#endif
+# define GC_INVOKE_FINALIZERS() GC_maybe_wake_finalizer_thread()
   GC_INNER void GC_notify_or_invoke_finalizers(void);
                         /* If GC_finalize_on_demand is not set, invoke  */
                         /* eligible finalizers. Otherwise:              */
diff --git a/reclaim.c b/reclaim.c
index ae6a420c9..9675fa199 100644
--- a/reclaim.c
+++ b/reclaim.c
@@ -21,6 +21,13 @@
 # include "gc/gc_disclaim.h"
 #endif
 
+#ifdef GC_PTHREADS
+static pthread_mutex_t flzr_mtx = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t flzr_t_has_work = PTHREAD_COND_INITIALIZER;
+#elif
+#error "This fork of BDWGC only supports POSIX threads"
+#endif
+
 GC_INNER signed_word GC_bytes_found = 0;
                         /* Number of bytes of memory reclaimed     */
                         /* minus the number of bytes originally    */
@@ -876,3 +883,33 @@ GC_API void GC_CALL GC_enumerate_reachable_objects_inner(
   ed.client_data = client_data;
   GC_apply_to_all_blocks(GC_do_enumerate_reachable_objects, (word)&ed);
 }
+
+static void* init_finalize_thread(void *arg)
+{
+  while(1) {
+    pthread_mutex_lock(&flzr_mtx);
+    while (GC_should_invoke_finalizers() == 0) {
+      pthread_cond_wait(&flzr_t_has_work, &flzr_mtx);
+    }
+    pthread_mutex_unlock(&flzr_mtx);
+    GC_invoke_finalizers();
+  }
+  return arg;
+}
+
+GC_INNER void GC_maybe_wake_finalizer_thread()
+{
+  if (!GC_finalizer_thread_exists) {
+    pthread_t t;
+    pthread_create(&t, NULL, init_finalize_thread, NULL /* arg */);
+    GC_finalizer_thread_exists = 1;
+    return;
+  }
+
+  if (GC_should_invoke_finalizers() == 0)
+    return;
+
+  pthread_mutex_lock(&flzr_mtx);
+  pthread_cond_signal(&flzr_t_has_work);
+  pthread_mutex_unlock(&flzr_mtx);
+}