From d51384db61ba40d0c48051a9a9e24e70fe4694a1 Mon Sep 17 00:00:00 2001
From: Yorick Hardy <yorickhardy@gmail.com>
Date: Fri, 3 Jan 2025 00:58:58 +0200
Subject: [PATCH] gc: sweep and free empty heaps for the primordial thread

The primordial thread may not have an opportunity to sweep
heap pages which have been merged from terminated threads.
So sweep any unswept pages during the cooperation phase.

Partial work towards addressing issue #534.
---
 gc.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/gc.c b/gc.c
index 558a4e07..0c1aad9b 100644
--- a/gc.c
+++ b/gc.c
@@ -1902,6 +1902,37 @@ void gc_mut_update(gc_thread_data * thd, object old_obj, object value)
   }
 }
 
+static void gc_sweep_primordial_thread_heap() {
+  int heap_type, must_free;
+  gc_heap *h, *prev, *next, *sweep;
+  pthread_mutex_lock(&(primordial_thread->lock));
+  for (heap_type = 0; heap_type < NUM_HEAP_TYPES; heap_type++) {
+    prev = primordial_thread->heap->heap[heap_type];
+    h = prev->next;
+    while(h != NULL) {
+      next = h->next;
+      must_free = 0;
+      if (h->is_unswept) {
+        if (h->type <= LAST_FIXED_SIZE_HEAP_TYPE) {
+          sweep = gc_sweep_fixed_size(h, primordial_thread);
+        } else {
+          sweep = gc_sweep(h, primordial_thread);
+        }
+        must_free = (sweep == NULL);
+      } else {
+        must_free = gc_is_heap_empty(h);
+      }
+      if (must_free) {
+        gc_heap_free(h, prev);
+      } else {
+        prev = h;
+      }
+      h = next;
+    }
+  }
+  pthread_mutex_unlock(&(primordial_thread->lock));
+}
+
 /**
  * @brief Called by a mutator to cooperate with the collector thread
  * @param thd Mutator's thread data
@@ -1912,11 +1943,22 @@ void gc_mut_update(gc_thread_data * thd, object old_obj, object value)
  */
 void gc_mut_cooperate(gc_thread_data * thd, int buf_len)
 {
-  int i, status_c, status_m;
+  int i, status_c, status_m, stage, merged;
 #if GC_DEBUG_VERBOSE
   int debug_print = 0;
 #endif
 
+  // Since terminated threads' heap pages are merged into
+  // the primordial thread's heap, it may be that a sweep
+  // for the primordeal thread is never triggered even though
+  // the heep keeps growing. Perform a sweep here if necessary.
+  stage = ck_pr_load_int(&gc_stage);
+  merged = ck_pr_load_int(&gc_threads_merged);
+  if ((thd == primordial_thread) && (merged == 1) && ((stage == STAGE_SWEEPING) || (stage == STAGE_RESTING))) {
+    gc_sweep_primordial_thread_heap();
+    ck_pr_cas_int(&gc_threads_merged, 1, 0);
+  }
+
   // Handle any pending marks from write barrier
   gc_sum_pending_writes(thd, 0);