From ed5db9bc3620971c768524ee46eb50d3d2a1044c Mon Sep 17 00:00:00 2001
From: Andy Wingo <wingo@igalia.com>
Date: Wed, 5 Mar 2025 17:17:55 +0100
Subject: [PATCH] nofl: Reclaim another mark bit if concurrent tracing is
 disabled

---
 api/gc-config.h  |  4 ++++
 src/nofl-space.h | 22 +++++++++++++---------
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/api/gc-config.h b/api/gc-config.h
index ca1b38d14..867af63d2 100644
--- a/api/gc-config.h
+++ b/api/gc-config.h
@@ -33,4 +33,8 @@
 #define GC_CONSERVATIVE_TRACE 0
 #endif
 
+#ifndef GC_CONCURRENT_TRACE
+#define GC_CONCURRENT_TRACE 0
+#endif
+
 #endif // GC_CONFIG_H
diff --git a/src/nofl-space.h b/src/nofl-space.h
index e13572653..9e7245ebf 100644
--- a/src/nofl-space.h
+++ b/src/nofl-space.h
@@ -235,12 +235,16 @@ enum nofl_metadata_byte {
   NOFL_METADATA_BYTE_NONE = 0,
   NOFL_METADATA_BYTE_YOUNG = 1,
   NOFL_METADATA_BYTE_MARK_0 = 2,
-  NOFL_METADATA_BYTE_MARK_1 = 4,
-  NOFL_METADATA_BYTE_MARK_2 = 8,
-  NOFL_METADATA_BYTE_MARK_MASK = (NOFL_METADATA_BYTE_YOUNG |
-                                  NOFL_METADATA_BYTE_MARK_0 |
-                                  NOFL_METADATA_BYTE_MARK_1 |
-                                  NOFL_METADATA_BYTE_MARK_2),
+  NOFL_METADATA_BYTE_MARK_1 = 3,
+#if GC_CONCURRENT_TRACE
+  NOFL_METADATA_BYTE_MARK_2 = 4,
+  NOFL_METADATA_BYTE_MARK_MASK = 7,
+  /* NOFL_METADATA_BYTE_UNUSED_0 = 8, */
+#else
+  NOFL_METADATA_BYTE_MARK_MASK = 3,
+  /* NOFL_METADATA_BYTE_UNUSED_0 = 4, */
+  /* NOFL_METADATA_BYTE_UNUSED_1 = 8, */
+#endif
   NOFL_METADATA_BYTE_END = 16,
   NOFL_METADATA_BYTE_PINNED = 32,
   NOFL_METADATA_BYTE_LOGGED_0 = 64,
@@ -254,8 +258,10 @@ nofl_advance_current_mark(uint8_t mark) {
     case NOFL_METADATA_BYTE_MARK_0:
       return NOFL_METADATA_BYTE_MARK_1;
     case NOFL_METADATA_BYTE_MARK_1:
+#if GC_CONCURRENT_TRACE
       return NOFL_METADATA_BYTE_MARK_2;
     case NOFL_METADATA_BYTE_MARK_2:
+#endif
       return NOFL_METADATA_BYTE_MARK_0;
     default:
       GC_CRASH();
@@ -955,10 +961,8 @@ nofl_space_contains_edge(struct nofl_space *space, struct gc_edge edge) {
 static inline int
 nofl_space_is_survivor(struct nofl_space *space, struct gc_ref ref) {
   uint8_t *metadata = nofl_metadata_byte_for_object(ref);
-  uint8_t mask = NOFL_METADATA_BYTE_MARK_0
-    | NOFL_METADATA_BYTE_MARK_1 | NOFL_METADATA_BYTE_MARK_2;
   uint8_t byte = atomic_load_explicit(metadata, memory_order_relaxed);
-  return byte & mask;
+  return nofl_metadata_byte_has_mark(byte, space->survivor_mark);
 }
 
 static uint8_t*