build fix for Apple M1 (issue #354 and pr #356)

1 files changed, 6 insertions, 2 deletions

diff --git a/include/mimalloc-internal.h b/include/mimalloc-internal.h
index cf4d80e..7160bc4 100644
--- a/include/mimalloc-internal.h
+++ b/include/mimalloc-internal.h
@@ -324,10 +324,14 @@ static inline mi_heap_t** mi_tls_pthread_heap_slot(void) {
 #elif defined(MI_TLS_PTHREAD)
 #include <pthread.h>
 extern pthread_key_t _mi_heap_default_key;
-#else
-extern mi_decl_thread mi_heap_t* _mi_heap_default;  // default heap to allocate from
 #endif
 
+// Default heap to allocate from (if not using TLS- or pthread slots).
+// Do not use this directly but use through `mi_heap_get_default()` (or the unchecked `mi_get_default_heap`).
+// This thread local variable is only used when neither MI_TLS_SLOT, MI_TLS_PTHREAD, or MI_TLS_PTHREAD_SLOT_OFS are defined.
+// However, on the Apple M1 we do use the address of this variable as the unique thread-id (issue #356).
+extern mi_decl_thread mi_heap_t* _mi_heap_default;  // default heap to allocate from
+
 static inline mi_heap_t* mi_get_default_heap(void) {
 #if defined(MI_TLS_SLOT)
   mi_heap_t* heap = (mi_heap_t*)mi_tls_slot(MI_TLS_SLOT);