gc: fix count reset to preserve gen2 count for survivors

youknowone · youknowone · commit 36e912b809be · 2026-03-05T20:37:11.000+09:00
Don't reset the target generation's count when it's the oldest (gen2),
since promote_survivors returns early and survivors remain in-place.
Only reset younger generations whose objects were promoted away.
diff --git a/crates/vm/src/gc_state.rs b/crates/vm/src/gc_state.rs
@@ -411,7 +411,10 @@ impl GcState {
         }
 
         if collecting.is_empty() {
-            for i in 0..=generation {
+            // Reset counts for generations whose objects were promoted away.
+            // For gen2 (oldest), survivors stay in-place so don't reset gen2 count.
+            let reset_end = if generation >= 2 { 2 } else { generation + 1 };
+            for i in 0..reset_end {
                 self.generations[i].count.store(0, Ordering::SeqCst);
             }
             let duration = start_time.elapsed().as_secs_f64();
@@ -521,7 +524,7 @@ impl GcState {
         if unreachable.is_empty() {
             drop(gen_locks);
             self.promote_survivors(generation, &survivor_refs);
-            for i in 0..=generation {
+            for i in 0..generation {
                 self.generations[i].count.store(0, Ordering::SeqCst);
             }
             let duration = start_time.elapsed().as_secs_f64();
@@ -541,7 +544,7 @@ impl GcState {
 
         if unreachable_refs.is_empty() {
             self.promote_survivors(generation, &survivor_refs);
-            for i in 0..=generation {
+            for i in 0..generation {
                 self.generations[i].count.store(0, Ordering::SeqCst);
             }
             let duration = start_time.elapsed().as_secs_f64();
@@ -685,8 +688,10 @@ impl GcState {
             });
         }
 
-        // Reset counts for all collected generations
-        for i in 0..=generation {
+        // Reset counts for generations whose objects were promoted away.
+        // For gen2 (oldest), survivors stay in-place so don't reset gen2 count.
+        let reset_end = if generation >= 2 { 2 } else { generation + 1 };
+        for i in 0..reset_end {
             self.generations[i].count.store(0, Ordering::SeqCst);
         }
 
