From 9bbc9fb8a4666e7c084f56916e993cb07ea4dabd Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sat, 16 Dec 2023 14:39:10 +0000
Subject: Add mark for GC objects

---
 runtime/src/gc.c | 18 ++++++++++++++++++
 runtime/src/gc.h |  3 +++
 2 files changed, 21 insertions(+)
 create mode 100644 runtime/src/gc.c

(limited to 'runtime/src')

diff --git a/runtime/src/gc.c b/runtime/src/gc.c
new file mode 100644
index 00000000..5c485b26
--- /dev/null
+++ b/runtime/src/gc.c
@@ -0,0 +1,18 @@
+#include <stdbool.h>
+
+#include "./patio/patio.h"
+#include "./gc.h"
+
+// The mark value for each object is initialized to 0. Each mark phase
+// alternates between using 0b010 and 0b100 as the mark value. We check only for
+// the bit set to 1, meaning static values can have a mark value of 0b110 to be
+// considered always marked.
+PatioInt current_mark_value = 0b010;
+
+void alternate_mark_value() {
+    current_mark_value ^= 0b110;
+}
+
+bool is_marked(struct ShedGcObjectHeader* object) {
+    return (object->mark & current_mark_value) == current_mark_value;
+}
diff --git a/runtime/src/gc.h b/runtime/src/gc.h
index d7f5f9af..f8da17d3 100644
--- a/runtime/src/gc.h
+++ b/runtime/src/gc.h
@@ -1,8 +1,11 @@
 #ifndef SHED_GC_H
 #define SHED_GC_H
 
+#include "./patio/patio.h"
+
 struct ShedGcObjectHeader {
     struct ShedGcObjectHeader* next;
+    PatioInt mark;
 };
 
 #endif
-- 
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