Use atomic operations for updating syntax tree reference counts

Co-Authored-By: Rick Winfrey <rewinfrey@github.com>

src/runtime/atomic.h

@@ -0,0 +1,30 @@
+#ifndef RUNTIME_ATOMIC_H_
+#define RUNTIME_ATOMIC_H_
+
+#include <stdint.h>
+
+#ifdef _WIN32
+
+#include <windows.h>
+
+static inline uint32_t atomic_inc(volatile uint32_t *p) {
+  return InterlockedIncrement(p);
+}
+
+static inline uint32_t atomic_dec(volatile uint32_t *p) {
+  return InterlockedDecrement(p);
+}
+
+#else
+
+static inline uint32_t atomic_inc(volatile uint32_t *p) {
+  return __sync_add_and_fetch(p, 1u);
+}
+
+static inline uint32_t atomic_dec(volatile uint32_t *p) {
+  return __sync_sub_and_fetch(p, 1u);
+}
+
+#endif
+
+#endif  // RUNTIME_ATOMIC_H_

src/runtime/tree.c

@@ -5,6 +5,7 @@
 #include <string.h>
 #include <stdio.h>
 #include "runtime/alloc.h"
+#include "runtime/atomic.h"
 #include "runtime/tree.h"
 #include "runtime/length.h"
 #include "runtime/language.h"
@@ -356,7 +357,7 @@ Tree *ts_tree_make_missing_leaf(TreePool *pool, TSSymbol symbol, const TSLanguag
 
 void ts_tree_retain(Tree *self) {
   assert(self->ref_count > 0);
-  self->ref_count++;
+  atomic_inc(&self->ref_count);
   assert(self->ref_count != 0);
 }
 
@@ -366,8 +367,7 @@ void ts_tree_release(TreePool *pool, Tree *self) {
   while (pool->tree_stack.size > 0) {
     Tree *tree = array_pop(&pool->tree_stack);
     assert(tree->ref_count > 0);
-    tree->ref_count--;
-    if (tree->ref_count == 0) {
+    if (atomic_dec(&tree->ref_count) == 0) {
       if (tree->children.size > 0) {
         for (uint32_t i = 0; i < tree->children.size; i++) {
           array_push(&pool->tree_stack, tree->children.contents[i]);

src/runtime/tree.h

@@ -29,7 +29,7 @@ typedef Array(Tree *) TreeArray;
 struct Tree {
   Length padding;
   Length size;
-  uint32_t ref_count;
+  volatile uint32_t ref_count;
   uint32_t bytes_scanned;
   uint32_t error_cost;
   uint32_t node_count;